Plaster of Paris Mixing Guide...

7/16/2013

Video Below:
This video is also available on YouTube here:

If you only want the instructions for mixing Plaster of Paris please go directly to the video. If you would like to understand more about the chemistry, manufacture, physical characteristics, and some tips for re-enforcing, coloring, dangers, and proper disposal, please read on.
The basis for this post is that I have checked several sites for the instructions and found them wrong, misleading, or lacking in good information about this versatile casting material. Plaster of Paris has been in use for well over 5 thousand years and is still misunderstood. So, while I am not an expert, I have used this material for years with good success and continue to find new uses.
Plaster of Paris is made from calcium sulfate dihydrate, (CaSO4.2H2O), frequently called gypsum. This simply means that there are two molecules of water with each calcium sulfate. It is a naturally occurring mineral found in many locations around the world but was originally named after a large deposit in Montmartre near Paris. The dihydrate is ground and roasted at 150 C (300F) to drive off 1 and ½ molecules of water as steam. The modified material is now calcium sulfate hemihydrate, Plaster of Paris, and has only ½ a molecule of water, 2CaSO4·2H2O + Heat → 2CaSO4·½H2O + steam. Since it takes in energy as heat it is an endothermic reaction. So, when we buy Plaster of Paris, we are buying the hemihydrate of calcium sulfate. In order to make it a castable solid we add water so that it becomes the dihydrate again! The Plaster of Paris gives off the energy that it has stored and when setting into a solid provides an exothermic reaction. It can become very hot and can burn exposed skin. So, it is not used as a casting medium for human parts casting. When it is used as a cast for broken limbs it is used over bandage material and not in direct contact.
Plaster of Paris is a very soft mineral although it can be relatively strong when it is used as a cast. On the Mohs scale of mineral hardness, with talc being 1 and diamond being 10, Plaster of Paris is a 2. This allows it to be sanded, trimmed, and damaged easily. Once set, it is not water soluble, and it has a very low rate of expansion, about 0.1 %. It sets or hardens in a monoclinic crystal lattice which means that the particles are all facing the same direction. This provides a dense cast, but there are still interstitial areas and it is porous.
Plaster of Paris sets in a very short period of time depending on the temperature of the mix and the air temperature. Typically, it can be de-molded in about 30 to 45 minutes. If it is cool to the touch it has at least set. But, set time is not the same as cure time. In order to fully cure, the cast will usually take between 48 and 72 hours with good ventilation to allow any excess water to escape. There is a great deal of speculation on the web about increasing set time. I have seen everything mentioned from baking powder to vinegar. But, adding other chemicals will change the material characteristics and may lead to failure due to interruption of the crystal lattice. The best methods to increase the working time is to use cold water, use short mixing times, or by using an excess of water. It is also possible to purchase Plaster of Paris with retardants that are designed to increase working time. But, by using cool water, and short mixing times, I have had material that had a working time of 30 minutes.
Plaster of Paris can be strengthened by using glass fiber, gauze bandage, or other suitable material. In fact, there is a product called modrock that is coated with plaster and simply moistened to use. It is also easy to color the plaster with poster paint or tempera. But, use caution as some colors can cause an almost instant set time. Food color would be a poor choice as it is not color fast.
Plaster of Paris is a fine powder and should not be inhaled. Pour material slowly and use a mask if you feel the need. But, it is a safe material with great casting potential. One serious caution is to dispose it as the solid and not put it in a drain as it will clog the pipes.
If there are any facts that I failed to include please use the comments section to ask questions or add any other useful information. Now, let get mixing!

196 Comments

Shane

10/10/2013 04:10:43 pm

Can you use acrylic admix with plaster of paris strengthen bonding properties and decrease moisture absorption? I have tested the mix using 100% acrylic admix in place of water and also a 50/50 mixture. Both solidify appropriately but I am not sure of long term affects yet.
Thanks,
Shane

Ken

10/10/2013 09:17:24 pm

Hi Shane,
Interesting question that depends on the application. Normally used in thin sets and mortar, the acrylic admix is filling the interstitial voids of naturally porous plaster, and the water portion is actually reacting to make the dihydrate. Moisture resistance will be better but I am uncertain about shear strength. I have used several polymers including Elmer's glue, (Polyvinyl Acetate) with good results. My best guess is that 100% may be too high, but if it works then test it over time. Without knowing the application I am a little in the dark. Ken.

Ankit raval

5/15/2015 06:48:57 am

What is ration of polyvinyl accetate

Mike

10/26/2013 09:46:09 pm

Thanks for the clip. Nice.

We cast small objects for use as targets for pellet gun practice. What's the best way to brittle or weaken the plaster so that
it tends to shatter / pulverize dramatically when hit? Thanks.

Ken

10/27/2013 02:22:23 pm

Hi Mike,
The inherent softness of POP on the mohs scale makes "shattering" difficult but I would think that a polymer would increase stiffness and shatter potential. The acrylic mentioned in the prior comment, Elmer's glue, (Polyvinyl Acetate), or DAP resin glue might be good candidates. But I would not rule out the possibility of incorporating larger pore size by adding detergent to cause a foam, or baking powder to incorporate CO2. Looks like you have some experiments to do! Thanks for the question. Ken.

sky

4/14/2014 06:03:27 am

Add lots of hot water. It works for me.

Pete

5/12/2016 06:25:58 am

Bake it in the oven on low heat for half an hour or so. It's turns into a brittle chalk.

Mike

10/27/2013 04:27:59 pm

Thanks, Ken.

For instance, a pool cue chalk which is slightly under 1" cube will fracture into 2-3 pieces when hit with a 5.5mm (.22) pellet at 460 fps (and less) I would like one just to go "poof." I'll make some samples and report back. Have you any ideas on ratios of water to plaster to Elmers?

Ken

10/28/2013 09:57:38 pm

Hey Mike,
I would try a 10 % Elmer's glue as a first try. Elmer's glue is about 30 % solids (PVA), and the rest water. So, 10 grams of glue contains 3 grams of PVA. But, you could make the standard 2 parts POP to one water and just add the glue and disregard the extra water as it will evaporate. One thing to remember is that POP can be de-molded in just a short time, but the full cure will take 48 to 72 hours for the best shatter possibilities. I think that the key to your problem is to disrupt the way the crystals form in the monoclinic structure. You may want to read up on crystal structure of plaster. Just a thought. Ken.

Michael

12/19/2013 12:42:10 pm

Very useful article.
I have a question about coloring plaster of paris. Is it better to use powdered or liquid tempera paint?

Ken

12/22/2013 02:06:57 pm

Hey Michael,
My preference is to use powder as the liquid has binders that may cause gelling. But, it is possible to use acrylic paints, and any water based latex paints as well. Pigments for coloring cement are also available. Use a trial small mix to make sure that it does not cause a problem before committing to a larger batch. Ken.

Gaylene

5/14/2014 03:41:10 pm

I would like to use POP with powdered tempera paint as the mortar for mosaics at a YMCA camp this year. Do you have any advice for mixing these. How far in advance should I mix it? How much can be maDE At one time? etc.

Caitlin

10/18/2015 10:52:33 pm

Do you have a recommendation for the ratio of powdered pigment in the mix? And should it be added to the dry plaster or the water (or does it matter)?

Adam

1/15/2014 11:58:34 am

Hi Ken

I am just starting off with using plaster of paris for small models and sculptures and was wondering if fillers for stone or metal effects work ok with it and if so at what ratios?

Thanks in advance

Ken

1/15/2014 02:15:37 pm

Hey Adam,
As far as fillers go, almost any material that does minimal structural change will work. But, it depends on the strength needed and the overall use of the model. I have used sand, small stone, metal filings, and glass fiber. Ratios can only be determined by experimentation.It might be helpful to check some model railroad sites to see how they make their filled models. Thanks for the question, Ken.

ahmed

3/17/2014 04:37:18 pm

hi ken,
i am using gypsum powder from oman,the material is good but it get set very quickly due to which lots of gypsum got wasted while applying on wall,how can we increase the setting time of it

Ken

3/17/2014 09:23:36 pm

Ahmad,
There are only three ways to lengthen the set time without buying a product with retarders already in the gypsum. Using cold water will allow a slower set, shorter mixing times, and an excess of water. If you are using on a vertical wall a thin mix might not work. But cold water should give you about a 30 minute work time. Ken.

mohamnad Naseem Khan link

7/22/2015 12:51:26 am

For mor time you can add citric acid 0.2...0.5%

Adam

6/30/2014 07:32:20 pm

Hi Ken

Do you know if adding a small amount of cement to plaster of Paris would strengthen it at all?

Regards
Adam

Ken

7/1/2014 09:20:01 pm

Hey Adam,
Although the cure mechanism of Portland cement and POP are similar, cement gains its strength with sand and aggregate as concrete. It is not really going to add much strength as a simple addition. If you are using fiberglass as a reenforcing material you should be fine. If you want ti try a small amount of cement, use no more than 5 %. Thanks, Ken.

Diana

8/15/2014 05:12:10 pm

I've seen/heard several warnings about certain colours resulting in instant setting -- but no-one appears to know/share what those colours are. Do you know? And, is it the colour, or the type of colourant used that makes a difference?
Thanks for you post, too. Has been helpful.

Ken

8/16/2014 03:04:57 pm

Hi Diana,
Adding color to POP is very much an art and requires some experimentation. Generally speaking, it is the specific pigment used that causes the set time or integrity of the casting. There are two problems when considering adding color. First is the choice of pigment and the second is saturation. Almost any color added will be more of a pastel rather than a fully saturated color. But, the interaction of the pigment seems to be the key to instant setting problems. I have not done the research on pigment interaction to advise which ones to avoid. Sorry to offer no easy solutions but experiment or paint after the casting. Ken.

Diana

8/16/2014 05:06:20 pm

Thanks so much for your quick response! I will experiment, as suggested.

Diana

9/25/2014 03:33:26 pm

Hi Ken,
I did as you suggested, and tested some of the colour in a small batch first,, and it worked out really well. However, when it came time for the actual casting, I forgot !! to add the colour. sigh.... So now I have a couple more questions:
1. I've decided NOT to just paint my item (which is half a hand) with the colour I'd prepped, because the colour was pretty lifeless and I am not a great painter. Instead I'm considering getting some spray colour and going for a bronze (I think). Should I be protecting the PoP first? I read on the box that I should shellac it before painting. One of your notes suggested some elmer's glue? I'd like the hand to last as long as possible (it will be in a shadow box so theoretically will be somewhat protected). Whatever it is I use I want to be thin enough that it doesn't take away any of the natural wrinkles and pores of the hand.
2. any suggestions for what kind of spray paint/stain/etc would be good?
Thanks again for taking the time to answer questions. Your advice has been invaluable, and easy to understand. Very much appreciated.

Ken

9/25/2014 09:02:01 pm

Hey Diana,
I had to place this answer on your original post as the "reply" button failed to show up on the new question. First, the Elmer's glue will not be appropriate here as it is an emulsion in water and the trapped water may cause problems with subsequent paint. I would suggest several light coats of Krylon acrylic spray, available at all hardware stores, to fill the pores and protect the POP. Also, it is thin and will not bother the detail. One caution however is that the POP hand has to be really cured and dry. Perhaps Debbie's tissue paper would be a good method. If it is several days since casting it should be fine. Let me know if you received this answer and how it works for you, Ken.

Diana

9/26/2014 06:33:17 am

Thanks again!
The POP hand is very dry! Has been "drying" for a couple of weeks now, so I'm pretty sure that's not a problem. I found a glossy clear Krylon acrylic online, but am hoping that the krylon acrylic paints will also work, in part because I don't want gloss. Would you recommend clear protective layers first or would it be okay to go directly to a colour spray, if I can find one I like?

Ken

9/26/2014 03:10:18 pm

Diana,
Absolutely! You can spray paint directly on the cast but I would suggest that you use several thin coats rather than one heavy coat. Both Krylon and Rust-Oleum sell high gloss, satin, and flat in many colors and textures. The cast should last a long time and can always be re-painted if you choose. Good luck and thanks for the questions, Ken.

Diana

11/20/2014 03:47:54 am

Hi Ken,
Thought I'd let you know that the project is finished! Sprayed the hand with a couple of light coats of Krylon, then a layer of brown tempera, followed by another couple of light coats of Krylon. The funny thing is that I was looking for a pewter colour for the spray and they didn't have one, so I bought gold. And no matter how long I shook it, or how careful I was with the spraying -- when I sprayed my sample pieces, they all sprayed gold. When I sprayed the hand, it always always always ended up silver. So I guess I got close to pewter after all. And I (and those who have seen the finished product) LOVE the final result. Your help has been invaluable. Thank you!!

Pat

9/15/2014 02:38:44 am

I have some gray metal molded bookends that are sometimes partially filled with plaster to add weight.

I plan to fill the statue, let it cure, and then epoxy a piece of sheet metal to the bottom that is then cover with felt.

Sometime I see bookends where the plaster of paris has fractured and the object now has a rattle to it.

Any advice as to how to avoid the fracturing during the cure period. Can I leave them outside to dry on a 90+ degree day. (not in the sun)

Thanks

Ken

9/15/2014 03:08:32 pm

Hi Pat,
Plaster of Paris is a very soft material and does suffer from some sloughing off of small amounts, but the cure time/temperature is not responsible for the fracturing. Two possible solutions can be considered: The first would be to use gauze bandage strips to reinforce the plaster. The second is to make a very dilute solution of Elmer's glue and add it to the cured bookend. Since the plaster is porous, the glue (polyvinyl acetate), will fill the pores and protect the plaster. Try a small experiment to test it out. Good luck and thanks for the question. Ken.

Moxie

9/23/2014 08:27:23 pm

Thanks for the info.pls can u explain why cold water is preferred to warm water whn dihydrating a pop.

Ken

9/24/2014 02:50:43 pm

Moxie,
The use of cold water slows down the rate at which the POP reacts to form a solid. The cold water allows for a more controlled release of heat (exotherm). Hot water decreases the set time but both provide the same material when set. Thanks for the question, Ken.

Artistwings

9/24/2014 08:32:51 pm

Ken
I want to create a negative mold of an actual leaf from plaster of paris. It needs to be applied thick on this dimensional plane so it won't run off the sides.
Do you have any ideas how to initially mix the plaster make the plaster thick (yogurt like) with the longest working possible?

Ken

9/25/2014 03:10:11 pm

Artistwings,
I hope that I understand the problem correctly and provide useful information. It seems that you have to cast the mold of the leaf in a vertical position and need a more viscous mix that is slow setting. Unfortunately, these two conditions are mutually exclusive. The possible solution is to wait until the mixed POP is almost set and apply at that time. I think that I would probably use a thinner mix and brush on a thin coat and let it sit and then add another coat until the cast was thick enough. Perhaps even adding some gauze bandage to reinforce would also is an option. Another more remote solution would be to use some citric acid (sold as sour salt in markets) as a retarder and make a mix with less water to provide a thicker mix. However, that would require some experimentation. Hope that this helps, Ken.

debbie callow

9/25/2014 08:36:59 am

Hi If you wrap tissue paper around the solid cast you will notice the paper becomes wet (even after 24, 48hrs hrs it will continue to exude water.) Keep replacing it until it is no longer wet. This is a good indicator as to when it is completely dry as it can look dry but isnt always. I make sweet trees and use this method in my pots otherwise the sweets would go soggy.

Ken

9/25/2014 03:17:29 pm

Debbie,
Good point and as I mentioned in the writeup there is a difference between set and cure. This is a cool way to test water retention. Thanks for the info, Ken.

debbie callow

9/25/2014 09:14:01 am

Hi, Sorry a question I forgot to ask in my previous message. I have searched the internet to try and find the answer but to no avail. Would you know if plaster of paris will crack a glass jar if poured into it. Thank you so much. Debbie

Ken

9/25/2014 03:26:41 pm

Hey again Debbie,
Since the rate of expansion is so low, less that 1/10 0f 1 percent, I would guess that it would not break glass. However, there is also a thermal consideration. POP heats up as it sets and that may induce fracture but again, I think it unlikely. Of course, it may depend on the glass. Also, there will be a long cure time as the excess water has less surface area to escape. I think that the best course of action is to try a small experiment with the kind of glass that you plan to use. And, let us know what you discover. Let's add to the information, thanks, Ken.

Brandon

10/2/2014 01:06:23 pm

I am trying to strengthen my POP for use in fine detail silicone mold castings (specifically hisrt arts molds). I need to retain all the fine detail and not damage the molds. As a kicker I need to due this on a limited budget. I'd like to use dental stone, however, no place near me sells it and it is too expensive to ship a 50lbs. box. What would you suggest?

Ken

10/2/2014 08:50:09 pm

Hi Brandon,
Although Dental Stone or Hydrocal would be the best choices, if money is a problem, then I have only one reasonable alternative. Take one cup of cool water and add 1/4 cup of Elmer's glue. Mix well and add 2 cups POP slowly and mix as in the video. The polyvinyl acetate polymer in Elmer's will add compressive strength and provide more density. Try it and let us know how it works, Ken.

dhaval

3/2/2015 07:38:54 am

Hi ken;
Can I use ethelene vinyl acetate to increase pop strength.and can I use hydroxy methelen cellulose as a retarder.this two chemical give me result in wall putty.also suggest other chemical if any

manoj

10/7/2014 04:24:36 pm

Hi ken
I have bought commercial POP. It is used in our country to protect newly laid marble n stones on d floor. The existing floor is vitrified tiles with 0.01% porosity. The commercial pop that I bought is not setting even after 36 hours of mixing. What could be the reason. Secondly what economical method I can use to set it. If suppose the existing material has no strength left , if i add another fresh or good pop to it will it set. What should be the mix in that case. Pls educate me on this urgently.
Regards
Manoj

Ken

10/7/2014 09:24:01 pm

Manoj,
I am not sure exactly what you mean by "protect the tiles". Is it placed on top or used as an adhesive to secure the tiles? Are you sure that it is calcium sulfate hemihydrate and not the dihydrate? Are the ingredients listed on the package. If you mix 2 parts POP with one part of water does it set. I need more information please to assist. Thanks, Ken.

dhaval

3/2/2015 07:45:26 am

Manoj.
You can use joint color for vitrified.also white adhesive available.you decrease setting time to adding magnasium cloride

Al Mansur

11/20/2014 12:19:01 am

Hi Ken,

Can you tell me the resulting volume of POP mixed 2:1?
Is it the same volume as the POP or does the water increase the volume?

Ken

11/20/2014 02:27:09 pm

Hi Al,
Adding the water to plaster does not substantially increase the volume as the water is absorbed by the plaster and the air in the plaster is displaced. Hope that this helps, Ken.

LaTryce

11/27/2014 12:15:15 pm

Can plaster of paris be used to fill a hollow statue?

Ken

12/4/2014 09:12:36 pm

LaTryce,
The only consideration might be the fact that the weight will go up depending on the size but POP makes a good filler. Ken.

Rosemary

12/4/2014 02:43:09 am

I'm having trouble getting a POP plus PVA glue mixture to set, and when it does, to be strong. My first experiment (using DAP internal use POP and Elmer's white glue) took a long time to set and was very soft and weak even after a couple of weeks. The ratio was 2 cups POP to 1 cup water (cold) to 1/4 cup white glue mixed into the water before the POP was sprinkled on top. I have no trouble at all when using the same ratio of POP to cold water without the addition of glue.

My second experiment was 8 parts POP to 2 parts fine perlite to 1/2 part green concrete dye mixed together. Added to a premixture of 5 parts cold water plus 2 parts of PVA external use glue, listed as gluing plaster and wood, among other things. The mixture was very runny (which is good for pouring), the green color was wonderful but the mixture doesn't look like it is interested in setting. I mixed it over an hour ago. Is there something in the DAP POP that is combining with the PVA to stop the mixture from setting properly? I live in the State of California which has weird restrictions on a lot of building material and paint items so I am wondering if the POP is a special Califorrnia blend. Of course, the actual ingredients are not listed on the carton.

Rosemary

12/4/2014 03:08:49 am

Update. My second mixture has set. It is still soft and will need to cure for a day or two but it demolded without incident. There are zero bubbles and no gaps, probably because the mixture was a lot runnier than my usual viscose pours. Yay! The color continues to look great.

I made a test pour into a mold that was two small square clear plastic plates with a sunken square in the middle and with one edge cut off (to make a pouring hole). I stuck them together with masking tape and held the mold upright sandwiched between a springy double wall of Lego blocks. The mixture didn't quite fill the mold which is OK. When it has cured I will clamp the plate and see how much force I can exert before it snaps.

My final molded pieces are modular sliders for drawers that build upwards on flat shelving. The slider parts need to be thick enough and strong enough to take the weight of the up-cycled cardboard Costco boxes that will be used as "drawers".

Ken

12/4/2014 09:36:22 pm

Hi Rosemary,
My first thought is that there are several products that are made by DAP and some have additives to retard drying. I use a 5 # bucket clearly marked Plaster of Paris and the ingredients are listed: Calcium Sulfate Hemihydrate, Calcium Sulfate, and Silica (to keep product dry). As far as I know, the only applicable California requirement is the proposition 65 which applies only to sanding the dry product. (Inhalation). The other thought is that the amount of PVA glues may be too high for the formation of a strong crystal structure. Thanks for the comments and update, Ken.

Adeel

1/24/2015 10:26:20 am

Ken ,
First of All , i am very thankful of ur article. And all the contributors in comment section. I am POP MANUFACTURER in pakistan . I was facing problems because of quick setting of POP.Cold water worked very good it increased the time from 5 minutes to 12 minutes for the material that was Termed heated' n was of no use. I m viewing your website on mobile cant see the video. Can u elaborate short mixes?it will be helping .By the way we sale POP to false ceiling guys . I am thinking on the line to cast my own tiles n sale them on bulk

Ken

1/24/2015 09:24:12 pm

Hi Adeel,
I am not sure what you mean by "short mixes". Please be more specific and I will try to assist. Ken.

Adeel

1/25/2015 11:36:22 pm

By short mixes you meant Small amounts of POP to be mixed with water? If that is what you meant we have to cast 2 feet x 2 feet plates of 4 sooter (1.27cm) thickness .The plate should be prepared in one mix other wise it will not be of same strength and can break easily.what quantity of POP u consider is a 'short mix'?

dhaval

3/3/2015 06:06:28 am

Adeel,
First I think that you can increase setting time of pop by adding some other filler like calcite,dolomide in lower percentage.it reduce small strength of your pop. you can also add cellulose ether to pop in very smaller percentage .

Yvonne Grant

2/25/2015 12:20:34 am

I want to use POP to permanently set 15" silk flower wired-stems arranged in a short 4" DEEP DISH so that it does not easily tip over. Will POP do this job? Advice and comments welcome! Thanks!

Ken

2/25/2015 03:06:13 pm

Hi Yvonne,
Generally speaking, POP is good as a base material due to its density and weight. However, in your particular requirement, it will depend on the diameter of the bottom. If it is like a tea cup it may want to tip. If the diameter is flatter, it will work just fine. Thanks, Ken.

Lawrence Toms link

2/26/2015 12:39:14 pm

hi,

Can I ask if there is any way of achieving two things with plaster of paris please?

1. get it to foam so that it sets lighter and more thermally insulative
2. improve its resistance to hi temperatures in general in terms of
a. insulating/conductive properties
b. material durability/integrity over time

I'm thinking of mixing in a little gorilla glue?

thanks so much for your time :-)

Ken

2/26/2015 10:10:28 pm

Hi Lawrence,
I have successfully made POP foam with baking soda and citric acid but the resulting product is very weak due to the cell structure. The formation of the hardened POP is temperature dependent and increasing the insulating properties would be difficult at best. Check some of the previous posts for additives used to strengthen and increase functionality of POP. But consider that everything added tends to alter the monoclinic crystal structure. Gorilla glue is a water cured polyurethane and may foam somewhat but the result may be a much weaker product. Thanks, Ken.

allan

6/30/2016 02:02:51 pm

try mixing in some light material - polymer foam particles, paper pulp, sawdust.
POP and paper mix is used to plaster walls.

Brenda

3/15/2015 11:48:37 am

Hi Ken,
Can you please advise on an expected curing time for plaster of Paris versus thickness. I know it will vary and really am after an indication.
I have a cast piece that is a variation in size 8" x3" x 3"
Thank you in advance
Brenda
Thanks in advance

Ken

3/15/2015 02:26:43 pm

Hi Brenda,
The rule that I use is 48 to 72 hours for complete cure even for fairly large castings. POP is porous and the excess water can escape over this time. Another way to test cure is to weigh the casting when demolded and periodically thereafter until the weight stabilizes. Thanks for the question, Ken.

Jackie

5/23/2015 10:43:26 pm

II produce casts of baby's hands which I stick to mount board with PVA glue. I have had a couple of instances where after about six months a pink hue appears on the front where the glue is on the other side. I allow two days fir the glue to dry before putting the cast into a frame. I am just wondering whether some sort of reaction us occurring between the glue and the plaster. Any ideas? Thanks for any help you can give.

Ken

5/24/2015 03:34:18 pm

Hi Jackie,
The first question: Is this phenomena from the same batch of plaster? If so, I would guess that there is an impurity in the plaster that reacts with either the oxygen or CO2 in the air. For example, if it had some iron it may form a rust-like color. If it is different batches of plaster it may be gypsum starting material that is adulterated. I have used PVA many times and have never seen a reaction causing any color so I would guess that it has some material in the plaster or in the water used to make the mix. Sounds like some small experiments are in order but unfortunately it takes time to see the change. Hope that this helps, Ken.

Taiwo

6/4/2015 09:56:27 am

Please, I will like to start a new impression business in my country and will also like to use pop for my molds. Please how can I get a best colour mixture. Like obtaining gold Nd silver? Thanks

Ken Quast

6/4/2015 10:05:28 pm

Taiwo,
Coloring POP can be a difficult task as mentioned in the video and writeup but I have had some luck with poster paint and acrylic paints. But, I believe that both silver and gold would be difficult to achieve easily. It would be easier to paint after molding but that may not be possible in your situation. Do a search for cement or concrete dyes and you may find something that could work. Sorry I can't provide specific recommendations. Ken.

KARAN

6/11/2015 06:54:02 am

Can we mix 50% normal cement with 50% POP for increasing strength of POP. Its just for experiment purpose to make some models. Actually cement takes time to set. I need cement like strength (can take 10 days for full strength) but time of setting should be like POP when I am making some craft structure.

Ken

6/12/2015 03:44:53 pm

Hi Karan,
Adding 50 % cement will actually weaken it significantly as it interferes with the crystal structure. I would suggest no more than 5 % may work. But, the better alternative would be dental plaster like Hydrocal or Hydrostone. With this there is greater compression strength. Thanks for the question, Ken.

Frank

6/12/2015 09:14:32 pm

Ken. Very good info for the hobbyist. I am casting large lead weights in plaster mold but concerned about the plaster being dry enough so it doesn't explode the lead in my face due to the water in the plaster turning into steam. How long should I dry the plaster?

Ken

6/13/2015 02:49:31 pm

Hi Frank,
The two most important variables are temperature and humidity so at a given temperature there may be some level of moisture. Here are some thoughts: After demolding (say an hour or two depending on size), weigh of the mold. Check the mold periodically and when the weight has stabilized, the majority of water is gone. However, before pouring the lead, you could heat the mold to a low temperature ( 100 to 150 F). But even with a small amount of moisture, I would guess that the mold would simply crack and not explode but I am not certain. Perhaps you can confirm with a small experiment with very small molds and see what happens. Good luck and thanks for the question, Ken.

Renee

6/26/2015 05:59:22 am

Hi Ken

I'm from South Africa and found your article and the following q&a section to be very helpful. I do however have a question of my own. I make very small casts of baroque elements for use in scrapbook and altered art. I want to use a antique gold rub on some of the casts and I'm worried that I will just rub away fine sharp detail. Would thin layers of light gesso strengthen the cured casts? Thanks for your help in advance.

Ken

6/26/2015 04:34:46 pm

Hi Renee,
I am not sure that traditional gesso would help as it is made from calcium carbonate which is chalk and is as soft as POP. It would probably depend on the binder used. An acrylic gesso might be better and also consider an acrylic spray lacquer. Another good choice would be a white glue like Elmer's glue which has an emulsion of polyvinyl acetate. Hope that this helps, Ken.

Andy Cook

7/20/2015 11:26:35 pm

Hi Ken!

I just want to say thanks for doing this page! I'm just about to start some PoP experiments and the video, info and your answers to people's questions has been really useful and interesting. I have a sheet of notes I've been making.

Cheers mate!

All the best,
Andy (UK)

Ray

8/11/2015 08:06:32 pm

we use POP to make dig block for kids How can I make it softer. Talc?

Ken

8/11/2015 09:28:44 pm

Hi Ray,
POP is already almost as soft as talk on the mohs scale but when cast it is friable but very dense and pretty difficult to get into. But, I get your point and I have two thoughts that are both somewhat speculative. The first is to add some dish washing detergent like Dawn to weaken the bonds. The second is to add baking soda to the dry mix and citric acid (sold at grocery stores) to the water portion. When the two are mixed there will be CO2 generated that should cause some foam to form. This would also weaken the POP. I have done some of these experiments but you will have to try small amounts to test yourself. If you are successful please post the results as we can all learn something new. If anyone else has an idea please post! Thanks Ray, Ken.

Laura

8/16/2015 03:20:44 pm

hi. thank you for your insightful page. i want to seal the surface of my pop prototype shapes, in order to have a non porous surface to make a polyurethane rubber mould from it. is there a material or product that can be applied to the surface that could be absorbed into the plaster surface and still maintain the surface texture. an additive to the mix could be possible as long as it does not alter it's workability (to be carved a rasped once set) spraying a paint onto the powder porous surface is problematic! thanks in advance. Laura

Ken

8/16/2015 09:36:21 pm

Hi Laura,
It sounds as if your necessary restrictions may preclude some solutions but I have some ideas. First, you could use Elmer's glue (which is polyvinyl acetate, a good polymer to use). Reduce the viscosity with water and brush on enough to seal the pores. Allow the excess water to evaporate and you should have a good surface. I would suggest a silicone release spray for the polyurethane. Another possibility would be Krylon clear lacquer in spray form to fill the pores. If other ideas come to mind I will respond again, Thanks, Ken.

akta

8/18/2015 06:01:42 am

Hi Ken, wanted to know if I cover blue plastic water pool that kids play in, in summer with plaster of paris decorate it with pebble stone while it's setting, would it work? And can I fill that pool with water. Would it stay as is after it sets.

Ken

8/18/2015 09:07:40 pm

Hi Akta,
The simple answer is that it would work but I believe that there is a better alternative. I would suggest Quikrete, which is a cement product that has sand, small gravel, and Portland cement. It is much less expensive than POP and will last longer. It is available at Home Depot, Lowes, and most hardware stores in various size bags,and you just have to add water and mix. Simple and cheap! Two words that I like. Thanks, Ken.

Jay

9/15/2015 06:43:59 pm

Hello, I had mixed POP and Sand, roughly in equal parts and added water to make a slurry and it began to set too quickly for me to complete add a pair of U-bolts, so I added a LOT more water and began breaking back up to turn it back into a slurry. From what I am reading, I broke up the lattice and it is never going to set back up and hard since there is no longer going to be a proper reaction. Does that sound about right?

Jay

9/15/2015 06:56:44 pm

My apologies for not proof reading prior to hitting send! Here it is again...hopefully with a more intelligible question.

Hello, I recently mixed PoP and Sand, roughly in equal parts and added water to make into a slurry; but it began to set too quickly for me to complete adding a pair of U-bolts. I added a LOT more water and began breaking up the now hardening mixture and remixing with the water, to turn it back into a slurry.
From what I am reading, I made a bad situation worse because apparently I broke up the lattice and it is now never going to set up and harden since there is no longer an exothermic reaction. Does that sound about right?

Ken

9/15/2015 10:06:03 pm

Hi Jay,
You are correct that the hemihydrate has converted to the dihydrate and will no longer be reactive to water and will remain the dihydrate until a lot of heat converts it back to the hemihydrate. The lattice is discontinuous and is forming small particles. Time to redo! I am curious as to the reason for such a high ratio of sand. I would appreciate your observation about the finished cast strength with the sand. Thanks, Ken.

banumathi.kalyanam

9/27/2015 10:48:42 am

I need to melt pop hardended idol..can you help me

Ken

9/27/2015 10:11:15 pm

Plaster of Paris cannot be melted easily as the temperatures required would be very high. If you want to dissolve it you could try very warm water with Sodium Bicarbonate (baking soda). It may be slow and may not work but it is a thought. Ken.

Tamoor

10/18/2015 07:24:03 am

Aoa
Ken
I want to make pop roof ceilling sheets ,but strengthen problem.
Plz tell me . How sheets strengthen
And reduce wait?
Thanks alot

Ken

10/19/2015 04:21:05 am

Hi Tamoor,
Strengthen with Fiberglas window screen or regular Fiberglas. You could also use cloth like used cotton. Use hot water and the set time will be very short. Ken.

Hans Monsma

10/23/2015 01:28:44 am

Hail the reader. I cast solid concrete pieces using a very dry mix (like the hollow building block manufacturers use), hoping to remove the mould directly after casting, allowing me to make 8 or 10 castings daily. My moulds are steel. My mix is 3:1 sand cement, I add polyprop strands - very short to get a good binding as well. At this stage it seems a bit of a hit and miss process for me, some stay beautifully in the cast shape - others, using the same mix simply fall apart when removing the mould. It looks that I need a more clay like mix - or a very fast drying mix which will allow me to remove the product from the mould within say 30 mins of casting. It need not be dry at that stage, but sturdy enough to be self supporting- not quite so crumbly. I have thought of using some POP (say 3 sand 1 cement and 1 POP. Alternatively using 1part tile adhesive into the mix. Any ideas please - I can submit a photo with sizes of the casting if you allow me excess to your email.
thank you
Hans

Ken

10/23/2015 10:40:25 pm

Hans,
This is certainly an interesting problem with no clear answer. The first thought that I have is that some are fine and others not. So, the question is why are some casts correct and others failing? I would suggest that there are two distinct possibilities. The first is the distribution of whatever water is in the mix. The reaction for curing concrete is dependent on sufficient wetting. Second, is there some reason that the mold is inhibiting the cure? I would guess that adding POP is not going to help due to the fact that there is not enough and it would simply act as a filler. Similarly, the tile adhesive would probably cause more problems. I think that the water distribution is the most likely problem. Sorry that I can't be of more help. Ken.

Maggie

10/29/2015 07:56:11 pm

Hi....I'm wanting to make some sweet trees and they've to be in glass jars. Will the plaster of Paris when it expands break the glass? or could I use building plaster.Its to fix the dowelling into the jar. Thank you.

Ken

10/29/2015 10:19:04 pm

Hi Maggie,
I had previously answered this question for Debbie and I suspect that it applies to your application:
Since the rate of expansion is so low, less that 1/10 0f 1 percent, I would guess that it would not break glass. However, there is also a thermal consideration. POP heats up as it sets and that may induce fracture but again, I think it unlikely. Of course, it may depend on the glass. Also, there will be a long cure time as the excess water has less surface area to escape. I think that the best course of action is to try a small experiment with the kind of glass that you plan to use. And, let us know what you discover. Let's add to the information, thanks, Ken.

Lawrence Toms link

11/3/2015 04:42:08 pm

When using glass fibres to reinforce the finished set, what is the recommended recipe/ratio please?

Ken

11/3/2015 11:45:06 pm

Hi Lawrence,
To my knowledge there are no specific ratios for re-enforcement as there are variables to consider. The most important is the length of the fibers. Long glass fiber would be less than short pieces. My own recommendation would be around 5 % of short pieces and perhaps 2 % of long strands. But, the fiber should be oil and lubricant free so a quick rinse in isopropyl alcohol would be appropriate. Thanks for the question, Ken.

Motanache

11/6/2015 11:53:11 am

Dear Ken,
You are a magician and a scientist. I try to make some stiff molds that also do not set too quick and the information you posted is invaluable. Thank you so much for educating me.

arun

11/20/2015 01:38:56 pm

what is the time require to heat the gypsum (120 degree celcius) to pop

Ken

11/20/2015 11:38:23 pm

Arun,
Based on my experiments it takes about 1 hour at 120 C (248 F) to remove approximately 3/4 of the water which is what commercial POP contains. It will become water-free (anhydrous) over several hours. POP made from the anhydrous sets more slowly but is a bit stronger. Ken.

Bronwyn

11/26/2015 10:41:58 pm

Hi Ken. I'm a teacher creating school project which is a large stone /Boulder replica to go outdoors. Around the mold I am going to use fibreglass tape then plaster of Paris Because it's for the outdoors I want a layer of concrete. Will this combination work? Many thanks

Ken

11/27/2015 05:07:14 am

Hi Bronwyn,
If I understand correctly the question is, will concrete adhere to POP and the answer is yes but Portland cement can also be incorporated into the POP mix. I assume that you have considered simply over-coating with a suitable color tint epoxy or other external protection. If I am going down a rabbit hole just let me know! Thanks, Ken.

Rubi

12/3/2015 05:01:05 pm

I want to color the plaster of Paris with acrylic paints paste it on the wall and push pebbles into it then wen dry coat with acrylic sealer will this work?

Ken

12/4/2015 05:11:35 am

Hi Rubi,
It should work just fine but pre-test the acrylic paint to make sure that it does not cause the POP to set too quickly. Try a small amount to be sure. Good luck and thanks, Ken.

Lourens

12/4/2015 06:45:20 pm

Hi,
I want to cast a pizza stone form gypsum (plaster of paris) Will it work and will it be safe to use as a pizza stone?

Thanks

Ken

12/4/2015 11:04:12 pm

Lourens,
POP would be a poor choice for a pizza stone as the high temperature will cause the plaster to lose water and begin to crumble and decompose. Thanks for the question, Ken.

rafeeq ahmed

12/15/2015 08:23:15 pm

Can POP used as coarse aggregate in concrete instead of jelly, if yes can pop attend more strength than usual concrete??

Ken

12/15/2015 11:23:58 pm

My best thought is that POP would not be a very good aggregate it is very soft ( 1 on the mohs scale). This would be detrimental to the bond formed in the setting concrete. Ken.

Emily

12/16/2015 08:47:28 pm

Hello, I am following instructions for a kids fresco art project using POP. Instructions call for pouring POP into a styrofoam tray(like a meat tray) waiting for it to set and then flipping it onto a flat surface to paint the bottom side of the plaster while it's still damp. The bottom side being curved and therefore more aesthetically pleasing to hang for display with the flat (top side) against the wall. No where can I find information as to how long it would take for the POP to set up in order for it to be handled this way. Can you give me an estimate please?

Ken

12/16/2015 10:39:38 pm

Hi Emily,
As mentioned in the write up there is a difference between set and cure. You should be able to flip the cast in about 30 minutes or so. The POP will begin to feel cooler and is then set. But, full cure should be considered at 24 hours. Here is the way I would suggest the process: After 30 minutes put a flat piece over the top pf the tray and turn over and remove the tray. This will minimize the stress to the cast. Good luck, Ken.

Emily

12/16/2015 09:06:27 pm

My other thought...could you make the molds ahead of time, and then spray them with water when you are ready to paint them? Or does it readily accept paint without it being wet?

Ken

12/16/2015 10:43:57 pm

Hi again Emily,
The answer is yes to both questions. Since POP casts are not water soluble you can safely re-wet the surface and then paint. Or, the cast will be acceptable to paint unless extremely dry. Thanks, Ken.

SKA

12/30/2015 11:46:56 am

what is the formulation for gypsum investment materials

Ken

12/31/2015 04:45:58 am

SKA,
I am not able to provide and one particular formula for investment casting as it varies with the casting process and one binder may not work well with your intended process. I would suggest a search of the investment casting suppliers for some thoughts, Ken.

Alistair Phillips link

12/31/2015 02:49:19 pm

I'm using a POP mold to cast a wax bust preparatory to casting in bronze using the lost wax process. The problem is that because the mold is so hard and rigid I cannot separate the wax from the mold without breakage. I have thought of immersing the POP mold complete with wax into water, would this work or is there something I could add to the POP before mixing so that it would break down in water.

Ken

12/31/2015 11:14:23 pm

Hi Alistair,
Since POP is not water soluble, and the cure actually requires hydration, it will not re-dissolve. However I do have a thought. POP is porous and will retain water in the pores so what if you soaked in water to saturate and then placed in the freezer. It may cause the POP to fracture but would not harm the wax. It is a long-shot but perhaps worth a try. There are materials that will weaken the crystal structure but it will take some experiments and some research to find the right combination. I would think that this might be tough to do easily. Keep us posted and thanks, Ken.

Curt

1/13/2016 08:06:59 pm

I have been working on an art project and mistakenly used a 1:1 ratio of POP and water. The set time was the same but the cure time seams longer. Should I scape it and start over?

Ken

1/13/2016 10:51:42 pm

Hi Curt,
The cure time is longer because it is more difficult for the cast to release the excess of water. The only downside I can think of is that the crystal structure may be less than optimal and may be slightly more fragile. Depending on the end use and material used I would probably re-do. But, that is your call based on the desired result. Ken.

rinzin

1/18/2016 10:47:15 am

Hi sir
Thanks for your post formally...i heard and have seen in some source that acids are been used for the setting property...and currently in some of the PoP plant, I
have seen citric acid has been using wherby tartaric acid is also recommented...what will be the difference in using those two acids

Ken

1/18/2016 10:59:59 pm

Rinzin,
Both citric and tartaric acid are organic acids that slow the cure time. But, the addition may cause the crystal structure to be changed and result in a weaker cast. To slow down the set time I use cold water and that works fairly well. Thanks for the comment and question, Ken.

Chris

1/23/2016 07:35:58 am

Hi Ken,
Thanks for answering everyone's questions!
I would like to use pop or pop with waterbased latex paint to paint over the old fashioned and unsightly 12x12 tiles on my zero-clearance fireplace surround.
My questions are...
Will pop stick to clean smooth tiles?
Can I put it on thick. In order to cover the spaced between the tiles so that it looks like one piece?
Although I never use the fireplace, except for once to test it...the surround did not get very hot at all. The tiles are on the outside and never exposed to flames. I am assuming it safe to use on the tiles? Looking forward to your reply. Thanks so much!

Ken

1/24/2016 12:52:39 am

Hi Chris,
POP is a very poor adhesive and particularly on vertical surfaces it will typically fail. But, for that matter, latex paint has the same problem. POP adheres best to porous surfaces where it can get a bite and hang on. But in a vertical position the weight and crystal structure makes adhesion difficult. My best suggestion is to do a search for "tile surface adhesives" and then perhaps a color can be added. Sorry for the bad news but the chemistry and physics are hard to defy. Thanks for the question and I hope that this helps, Ken.

Chris

1/24/2016 01:18:29 am

Hi Ken!
Thank you so much for that info. Makes sense...chemistry will always trump.
The reason I came up with pop for the tile surround is because apparently chalk paint (not chalkboard paint) which has become so popular recently, is made with pop and paint already pre-mixed. Chalk paint, according to to many, can adhere to any surface without priming. Would it help if the tile surface were to be primed with something like Kilz primer, which is supposed to help adhesion? Thanks again!

allan

6/30/2016 02:26:04 pm

can't you go to store and buy special primer that is used to prime tiles before plasteting with proper plaster or stucco.
there are also tile glues that stick strongly to old tiles so you can re-tile the surface.
or you can covers the tiles with thin layer of the tile-glue and then replaster the surface

Suresh VG link

2/9/2016 10:28:08 am

Hi Ken,

Can you name the retarder that can be mixed with POP and which is available in India. Also the method to use the retarder.

Your answers to questions are so much informative it makes the use of POP very simpler.

Thanks Suresh

Ken

2/10/2016 12:29:45 am

Hi Suresh,
There are easy to find chemicals like vinegar and citric acid that can be used but they can also change the finished product by causing crystallization problems. My suggestion would be to use very cold water or an excess of water to slow down the set. But the best way to do it would be to find a product there that already had a retarder in it as it would be more likely to be successful. Good luck in your search, Ken.

suresh vg link

2/10/2016 12:47:05 pm

Hi Ken,
Thank you very much. I shall try the cold water.

Suresh

jjensen

2/24/2016 12:12:39 am

Hi, im a newbe here. Im making a forge to melt gold ore. Ive been using plaster of paris as a heat insulator and after a couple of heats it breaks apart. My question is will sand make it stronger and more heat resistant. I need it to stand up to around 2, 200º F or do you know of anything that would work. Thank you much

Ken

2/24/2016 06:01:37 am

jjensen,
Unfortunately in the process of heating POP you are removing the water that makes it the dihydrate and going back to the hemihydrate and there is no way to avoid the change. You should try a refractory cement available in most hardware stores and Home Depot, Lowes, etc. Hope that this helps, Ken.

karin

2/24/2016 12:05:37 pm

Hi, i am trying to cast scrolls that i want to then stick onto my dresser and then paint. but i am srugling to get it out of the molds without pieces breaking. i have used castning rubber , and also candle wax to form the molds. some are 10 cm in lengh x 1cm in thichness,and others are 30 x 1 cm .please help.i have tried spraying cook and spray before i pour the plaster of paris into the molds ,and tried less water as well as more water

Ken

2/25/2016 04:59:05 am

Hi karin,
Wax should be fine but you may want to chill the mold before pouring the POP as it is exothermic and may be warming the wax too much. I believe that the breakage is due simply to the fact that the piece is thin and somewhat fragile. I would suggest using Fiberglas threads that are sold in auto stores and hardware stores as re-enforcement. Gauze pieces could also work but most of all, make certain that the piece is cured completely and not just set. This means 24 hours or more to let all excess water leave. Good luck, Ken.

karin

2/25/2016 12:40:03 pm

thx Ken , i realised that i have been mixing tht pop wrong and from now on will do it your way. have a nice day

suresh VG link

2/24/2016 04:06:56 pm

Hi Ken,
I am planning to make a block of POP having 1000 mm length, 300 mm height and 150 mm width. This will have inside designs also.
(1) How long the POP should be kept in the mould to properly set ?.
(2) Any precaution to be taken before taking out of the mould ?.
(3) Also I am expecting the moulding process will take 30 minutes to complete. Is it possible to complete the
process with cold water alone ?. Can I use ice cubes to make the water more colder than normal temperature so that I will get more setting time?

Please advice
Thanks
Suresh

Ken

2/26/2016 05:14:35 am

suresh VG,
You can use ice water to slow the set time but the remainder of the questions are dependent on many variables. The time to de-mold will depend on the air temperature and humidity. Dry, hot temps will be shorter. But, this is a large cast so I would let it set as long as you can allow. The cast will be weak until cured so handle it carefully so as not to break it before it is fully cured which could take at least 24 hours or more. Ken.

suresh VG link

2/26/2016 10:15:23 am

Hi Ken,
Thank you very much.

Suresh

Bonnie

3/26/2016 06:59:52 pm

Hi Ken,
Thank you for the information. I'm trying to find a way to make a mold out of an impression in cement. My siblings and I all made handprints in the cement at my mom's house and she's moving soon. I'd like to make a mold of it and give it to her as a gift.

I've been told by some people that Plaster of Paris would work for this, but I'm hesitant to do this as the concrete is porous and I'm concerned I wouldn't be able to get the cast out once it is set. Vaseline has been suggested as a base prior to laying the plaster, but I wanted to know your opinion. Do you have any information about this?

Thank you in advance!

Ken

3/26/2016 10:24:52 pm

Hey Bonnie,
You are absolutely correct that the porous cement would not release the POP. Depending on several factors including sentimentality, cost, and time, there are at least two good approaches. The first would be to use a two-part Silicone molding material (RTV) and cast a positive mold, remove, and make a POP negative mold as the original is and no release material would be needed for either step. The second would be to use a vegetable oil like Wesson or other cooking oil and lubricate the concrete with it to fill the pores and make the surface hydrophobic. But, there are some caveats here. If the pores are large and relatively deep and dry, it may take more than one application of oil to work. I would suggest that you try a small sample in the concrete nearby. A third possibility would be to use something like Play Doh clay and make a positive with that and then cast the negative with POP. A fourth alternative would be to use the reusable casting material that I show how to make. Just type "mold" in the search box on any of my pages and the post will be listed. The Vaseline may work as it is hydrophobic. but it may obscure detail and is difficult to clean up. Hope this helps, Ken. PS: Let us know what you did and how it worked to expand our information.

Marty

3/28/2016 04:18:31 am

Ken,
Thanks for you informative video and answers.
Is the POP structurally and chemically done before the full curing? I know that the curing gets rid of the excess water, but is the material done changing? I ask because I have an unusual application. I attach wires to 1/4" grids like in your video and embed them in POP. I place this in the soil and measure the electrical resistance between grids in order to gauge soil water content (for irrigation). The POP isolates the grids from the salts in the soil that would uncontrollably skew measurements. My procedure is to cure the sensor, then soak it in water and place it in the soil. After a couple of weeks it will dry to the soil water level and track it. I am wondering if I can skip the full curing step.

Ken

3/29/2016 04:47:06 am

Hi Marty,
The cure time is a bit more than just clearing excess water and is more about the crystal structure becoming monoclinic. The problem is knowing when the interstitial areas have been filled and the crystalline structure locked in. One way to approximate knowing this is to take your electrode when set (~30 minutes) and weigh it. Then weigh it again every hour or so until the weight reaches a plateau. All of any remaining water will be truly excess but the cure is complete. I guess that my best recommendation is to give it at least 12 hours or until clearly white to be certain before adding the water and putting in the soil. Good luck, Ken.

Marty

3/29/2016 06:40:24 am

Thanks, Ken, that's exactly what I was curious about. Also your explanation makes clearer why a slower cure is better than, for example, putting it out in the sun.

jyoti

3/30/2016 02:33:57 pm

dear sir, plz tell me asap if i can use pop for making sculpture,can it be knead like dough or will solidify before work,

Ken

3/30/2016 09:56:40 pm

Jyoti,
No, Plaster of Paris is either wet or it is set and can not be kneaded like dough. Thanks, Ken.

Anamika

4/3/2016 09:05:52 pm

Hi. My questions were already answered here. I am happy to see that a POP discussion has been going on for over 2 years!

JenA

4/8/2016 06:57:19 pm

Hi Ken, I appreciate your taking the time to answer people's questions! Anyhow, I am looking into the best way to affix a lot of shells to a wooden end table that I have already sanded. I was thinking of applying a thinned out layer of POP to the entire table, letting that dry, and then eventually adding more layers of POP, mixed with the normal ratios, when adding the shells, so as to hold them in place. Or, I could just prime the table and only use the POP to affix the shells in place. Any advice? Thanks!

Ken

4/8/2016 11:19:24 pm

Hi JenA,
In order to answer this question it would help me to know how you will prime the table before adding any POP. The adhesion of POP to wood is virtually non-existent unless you are depending on just the weight of the plaster. I have some ideas about how I would do it but please let me know what you have planned. Ken.

JenA

4/9/2016 03:33:39 am

Hi Ken, Thanks for the response. I was thinking of just priming the table (legs and all) with white colored primer paint. After it dries I could then possibly apply the POP and embed shells in it before it dries? Perhaps this is not the right approach - I just have seen some pretty neat tables where POP seems like it was used to hold in the shells, but perhaps it was some other sort of fixative.

Ken

4/9/2016 04:28:29 am

JenA,
I suspect that POP over a paint primed surface will have very poor adhesion but if not disturbed will probably stay attached. The water would migrate into to the wood and the crystal structure would be so hard that it would fail. I would think that a mechanical approach might be better. If you staple either Fiberglas or metal window screen to the top and then used POP it would have a surface to hold on to. Another possibility would be 1/8 inch or even 1/4 in hardware cloth (zinc coated metal). Another possibility would be to use tile cement (Thin-set) instead of POP. This would give you longer working time as POP might get hard before you placed all of the shells. Hope that this helps, Ken.

JenA

4/9/2016 05:11:22 am

Hi Ken, this really does help, thanks so much. My late dad was a chemist and I appreciate you taking the time to share your knowledge, as I cannot ask him anymore. Have a great day!

Yvonne

4/12/2016 08:15:03 am

A question and a comment

Question: I plaster equally strong whether it's mixed thin or thick? I am using it to glue marbles & glass pebbles. I'm starting with pots just to get my bearings and moving on to cement stair risers. It works pretty well thickly, but I would like to increase the setting times, so I am wondering whether the result will be just as strong with a thinner mix.

Comment:For coloring, I used Universal Pigments--they're the same kind of pigments they use in paint stores for coloring the paints. I own a full set and it gives me a great feeling of freedom. If I order a paint that's not exactly to my liking, I can always adjust it. They really are universal ! And they mix in with ease.

Ken

4/12/2016 10:41:57 pm

Hi Yvonne,
You can add a little more water safely but the risk is that too much water changes the crystal structure and makes POP weaker. The fact is that although stiff is a little more difficult it makes a better product. The happy medium is a cream like consistency. However, if you use really cold water, refrigerated, it will slow the set time. POP is also available with retarders added. Thanks for the coloring tip, Ken.

Yvonne

4/13/2016 02:05:00 am

Just tried another suggestion. Joint compound. Now that I know the chemicals of POP from you, it seems it is a mixture of POP or something similar, and silicon, which was the other suggested thing to use. So far, much easier than mixing up little portions of POP and seems to be working. I'll see when it's hardened.

Ken

4/13/2016 04:05:07 am

Hey Yvonne,
I would question the use of joint compound as it become dry and powder-like. It is easy to sand when feathering and is typically protected by a coat of paint. So, I am not certain that it would be a good adhesive. One of the suggestions I had for a previous question from JenA was to use tile adhesive (Thin-set) as it is an adhesive, provides a good working time, and is inexpensive. Although silicone might be OK but stays flexible and does not get hard. Just a thought. Ken.

Yvonne

4/13/2016 04:44:52 am

P.S. Any way I can send you a picture when done?

Yvonne

4/13/2016 04:43:33 am

First wanted to tell you that your tip worked like magic !!! I used the joint compound because on another site, someone said they used it to affix pebbles to a fireplace & four years later they were still up. I used the joint compound on most of it and washed off the excess a few hours later. Only a few of the glass pebbles fell off, which is the same success I had with plaster. Then, for the heavier marbles which the joint compound wouldn't hold, I mixed up a super-thick plaster using refrigerated water, as you suggested. I couldn't believe how long I could work before the plaster set. I'm not sure whether the last few pieces will hold, as the plaster may have been setting, but with such a thick mixture, it's hard to tell. No heat.

After I finish, I intend to made a tinted plaster "grout" so that should serve the same purpose as painting the joint compound (I think) Anyhow, as I said, when I washed the surface, 99% if the glass pieces held. Wish me luck !

Ken

4/13/2016 05:23:46 am

Yvonne,
Well, I am always ready to be amazed and surprised and it sounds as if you have a system well in hand. Let's hope that it is permanent and provides good information for others. You can always send a photo to my email at: ken.quast@gmail.com. Thanks for the update, Ken.

Rai

4/14/2016 04:47:59 pm

I just wanna make a new born baby sculpture or figurine by using POP with natural skin tone color. Don't wanna paint after casting the figurine. Of course the figurine has last for long time with out any stress or airline cracks on it during the cold season as well. Could you write something that can be great to me.

Cam

4/16/2016 09:34:58 am

Hey, I have some quickset plaster and some of the normal version from a hardware store, do you know if the strength properties are different and if the normal plaster, although takes longer to set, is stronger than the quickset in the long run?

Ken

4/16/2016 09:56:24 pm

Cam,
Under the same conditions both will crystallize in the same way and have the same expansion and strength and the hardness remains a 2 on the Mohs scale. Both are interchangeable. Hope this helps. Ken.

sanjeev link

4/20/2016 04:05:01 pm

does mixing baking soda with plaster of paris make it stronger and waterproof for outdoor use. I mixed a little baking soda with plaster of paris and applied it outside walls of my house to repair cement. It is still strong and the rains have not softened or damaged or dislodged it at all

Ken

4/20/2016 09:45:26 pm

Sanjeev,
There is no evidence that baking soda improves POP in either strength or water resistance. POP is already insoluble in water when cured. Too much baking soda might change the crystal structure and cause weakness eventually. Ken.

Dhawal

4/22/2016 07:29:28 pm

Dear Ken
Wonderful article.
I am a builder in india and I have built brick walls in residential project. To finish them from inside, I do not want to go for sand plaster.
I checked with local manufacturers here , I have alternative of putting up putty ( 4 coats - 2 coat course putty and 2 course fine putty ). However, because of unevenness of bricks , huge putty is required. SO instead I went for POP - 1 coat. I have 2 questions in this scenario -
1. Can I add any waterproofing solution in POP so that my walls will not show leakage in future?
2. I have added crushed sand in the POP. Will that have any adverse effect?

Thanks in advance.

Ken

4/23/2016 04:11:29 am

Hi Dhawal,
Too much sand will cause the POP to crystallize and cure insufficiently so try to limit the sand content. My recommendation for waterproofing would be to coat the cured POP with a paint-like material to fill the pores. POP is already water insoluble so leakage should not be a problem. Good luck, Ken.

Prashanth

5/7/2016 07:52:44 pm

Hello Ken.
I am doing a project on a helmet. I am using a part of a helmet shell as the mold and want a same shaped part in pop.I would like to know which material is better to increase the strength of the pop mixture in this case - gauze bandage or coconut fibers?
What would be the ideal ratio of powder and water?
What can I do to make the mixture stick to the helmet shell because I doubt that when I apply the paste,It will slide down the helmet shell due to the curvature.

Ken

5/8/2016 04:05:19 am

Hi Prashanth,
I would use gauze bandage in the standard 2:1 ratio and as I did in the video, saturate the gauze and lay the strips over the helmet until it was covered. Then, a straight mix could be added for thickness without slipping. The other possibility would be to use a disposable plastic container big enough to hold the helmet and fill part way with the POP. Put the helmet in upside down and you will get a negative mold. Good luck, Ken.

Prashanth

5/8/2016 09:32:34 am

I am coating the pop on the outside of the shell. I want a good finish on inside of the pop part and want to use this pop part as a mold to make a helmet shell from composites.
What releasing agent can i use?And how to obtain a fairly good finish?

Ken

5/9/2016 10:49:26 pm

Prashanth,
You may not need a release agent but if you do, a silicone spray would be best. Vegetable oil would work as well. Try some experiments to test. Ken.

Kenneth

5/11/2016 11:52:37 pm

Hi there! I recently poured a number of POP molds & accidentally used a ratio of about 1:1 POP to water. There was a little water on top when poured, & was slow to harden up, but did cure & come out of the mold with nice detail. After about a day, they are still a bit damp, & a bit soft when I press on them hard. Are they doomed do you think? Or is there a way to evaporate the extra water, & possibly reinforce them? The molds are gone now, so we would have to cast new molds from these pieces & then repour plaster in the new molds... a pain, to say the least! Any ideas?

Kenneth

5/12/2016 12:01:19 am

I should add, the 1:1 ratio was by VOLUME, not weight, so my hope is that it wasn't as far off as I've feared? & it's only been a day, so is it possible that some of the softness is just due to them not being fully cured?

Ken

5/12/2016 04:56:43 am

Hi Kenneth,
Even at 1:1 volume it has an excess of water and will cure and be very soft and crumbly. But heating will make it worse as it will revert some of the calcium sulfate to the hemihydrate making it even softer. My best guess would be to let it cure at room temperature and see how soft it really is. If it too soft then saturate it with dilute polyvinyl acetate, (Elmer's glue) to fill the pores and strengthen and make new molds. At least you have molds to work with. Best, Ken.

Bob

5/13/2016 09:27:12 pm

How about its use to fix cracks in a plaster ceiling. Will it work over a long period of time? Also, I've got a bag that has been sitting for three years. Is the product still usuable? Thanks. Bob

Ken

5/14/2016 04:55:47 am

Hello Bob,
My preference for the ceiling cracks would be joint compound. Although it contains gypsum, it has longer set time, better adhesion, and is easier to use and can be sanded and feathered for painting. Same hardness as POP at 2 on the mohs scale. The three year old bag has possibly picked up water as POP is deliquescent. If it is soft and unchanged it should be fine. But, it may be hard and already reacted. It will store over time but only if it is really sealed up from the environment. Check and test a small sample. Ken.

Greg

5/30/2016 11:04:32 am

Hi Ken,
I am making little statues out of POP. I have a silicone mould that I pour and set them in. However painting them after is always a struggle. I tend to find the acryllic paints do not stick to the POP. will often bubble and peel off. What can I do to not have this happen?

Ken

5/31/2016 05:40:53 am

Hi Greg,
The first thing that I think may be that the POP is not fully cured and the mold is still releasing water. I am assuming that you are not using a mold release as none is needed with RTV. If you are using your own with caulk then it could be acetic acid. POP is very porous and should accept acrylic paint with no problem. Let me know if either of these conditions apply. Thanks, Ken.

Stephen Coleman

6/16/2016 10:29:41 am

I'm working with POP for the first time making a mould with about 3 sq.ft. surface. The POP sets far too fast and when I mix a second batch into the same bucket it sets within 3 minutes. The POP comes from China and is the only POP I can get in this area. This forces me to make the mould into 4 layers, I can't make it in one attempt. Cold water is out of the question as I am in the humid tropics and refrigeration is unavailable. The temperature from my well water is 84 degrees F.

Kari

8/2/2016 03:37:20 pm

Hi, I have a plaster of Paris mould which has had washing soda spilt on it. The plaster mould is now slowly disintegrating. Any ideas on how I can stop the process?
Thanks

Ken

8/3/2016 12:06:30 am

Hi Kari,
The easiest way to stop the damage would be to submerge the mold in water to remove the sodium carbonate since it is water soluble and the cured plaster is not soluble. Then let the mold dry (dehydrate) and the damage should be stopped. Alternatively, a dilute solution of vinegar would react with the sodium carbonate diluting its damage to the mold. Hope that this is helpful, Ken.

Debbie

8/5/2016 02:46:02 am

Hi Ken, some friends and I want to get together to make a picture we've seen. It looks like you take a canvas, pour plaster of Paris on, then write a quote in it, draw a simple picture then paint this with

Ken

8/5/2016 11:39:25 pm

Debbie,
Please finish your question and I will try to provide an answer, Ken.

Sam

8/6/2016 07:45:06 am

G'day mate,
My son is involved in a school challenge with limited materials, one of which is Plaster of Paris. The challenge is to make a machine and in his design a flywheel is required.
We have tested the design using a push bike wheel and 5 kg of extra weight added with great success. Now we need to build with the allowable materials.
My question is mainly one of chemical reinforced plaster.

Ken

8/6/2016 09:49:32 pm

Hey Sam,
If I understand correctly you want to use POP as the flywheel and need to make it stronger or more durable. What I am not clear about is what you mean by chemical re-enforcement. And, are modifications to the plaster within the challenge? Thanks, Ken.

Sam

8/9/2016 05:29:10 am

Ken, Chem strengthening by way of PVA or admix. We have an admiix for concrete products- Bondall "Bondcrete". I'm about to begin scaled tests and advice on quantities would help.
Also what is the weight ratio from initial mixture to cured product? If we use 1kg of POP powder do we have 1kg after curing? What factors effect this ratio? water to POP?
Cheers, SAM>>>

Ken

8/9/2016 10:26:05 pm

Sam,
Actually POP is fairly strong but the problem is in the softness (2 on the mohs scale) and a 5 % admix will help. What may be better would be to put the flywheel in a dilute solution of Elmer's glue (PVA) and let it fill any interstitial voids and coat the outside. Just a thought. With regard to the weight, most of the water enters into the reaction to make the hemi-hydrate and the left over water is statistically irrelevant. The caveat here is that this is true if mixed in the 2:1 ratio. Good luck, Ken.

ANIL

8/17/2016 02:18:02 pm

Hi
Very informative site. What is the best drying conditions of the plaster mold??We found some times long last, some time very short. what are the reasons

Ken

8/18/2016 04:41:57 am

Anil,
The two determining factors assuming that the mix was correct at 2:1, are the room temperature and the humidity. If it is very cool or very humid the water will evaporate more slowly. Warm and dry is the quickest condition for the cure as well as time. Ken.

Anil

8/18/2016 08:14:16 am

Hi Ken
Thanks for the prompt reply, how we know the mold is completely dried ?when you say 2:1 is plaster and water ratio ?

ALVIN

8/19/2016 02:29:02 am

HIE
i need to make dustless chalk using pop how do i go about it

Ken

8/19/2016 04:30:52 am

Alvin,
POP is not used as a chalk so it won't work. The primary ingredient is calcium carbonate. But all chalk has some dust. Google search for "make your own chalk at home" for some ideas. Ken.

Ken

8/19/2016 04:26:42 am

Anil,
The best way to know when the POP is fully cured is to weigh the mold periodically and when the weight no longer goes down it is fully cured. Watch the first minutes of the video and you will see that it is two parts POP (dry volume) to 1 part water, (wet volume). Ken.

Anil

9/17/2016 08:13:18 am

Thanks Ken
We found difficulty in take out the mold from the mother mold ( both are plaster of paris ). We tried varsaline, then it affect the surface of the mold ( not smooth), any alternative?? Also we found that if the setting time is more , more difficult to remove the mold. Please advise
Thanks in advance
Anil

Ken

9/17/2016 10:12:01 pm

Anil,
Plaster on plaster is not always easy to do but as below, I would suggest cooking oil or olive oil. I don't really know of other materials suitable for release. Ken.

Sam

8/19/2016 08:05:28 am

Hey Ken,
Results from testing the addition of PVA or Admix to the water in POP concluded that Admix created excessive bubbles even in low dilution ratios. PVA was used up to 50% with water in 2:1 POP mixes with 18% improvement during destructive impact tests. Best results were from 10% PVA for economy and added strength. The most noteworthy point is that PVA eradicated virtually all bubbles and produced a remarkably perfect, non chalky smooth exterior.

Ken

8/19/2016 11:08:07 pm

Hi Sam,
Thanks for the added information but some clarification would be helpful. Was the PVA (polyvinyl acetate) in the form of Elmer's glue or similar, or was it a solid? The reason that I ask is that Elmer's glue is typically about 40 % solids (PVA) and that would be what was added as your 10 %. Thanks again, Ken.
NOTE: I did a video on the determination of percent solids if anyone is interested in knowing how to find the amount of solids in a solution or emulsion here:
http://www.observationsblog.com/sciencetechnologyexperiments/percent-solids-determination-and-homemade-weighing-cups

ahmad

9/13/2016 11:51:29 pm

Hi ken. I've read all above and found it very helpfull. Please I want more clarification on the issue of pop mixed with Portland cement. I have been using it recently and it is wotking fine. What I am concerned about is how durable will it be in exterior applications. Being under climate changes and stuff. Thanx a lot for ur article it was very very helpfull

Ken

9/14/2016 09:55:12 pm

Ahmad,
The finished product is still relatively soft and I would recommend a coating of either clear or a paint of some kind to protect the surface. Thanks, Ken.

ahmad

9/17/2016 04:06:06 am

Thanks alot sir much appretiated

pramod link

9/16/2016 06:24:28 pm

Hi,
I would like to know best parting agent in available in India and can I make single mould for a wooden shape bottle which is with me .

Ken

9/17/2016 04:03:59 am

Pramod,
Vaseline, also called petroleum jelly will work and any vegetable or cooking oil like olive oil will also be a good release agent and they are available everywhere, Thanks, Ken.

Angel

9/20/2016 01:10:49 am

Hi!! I am using pop to make baby foot print. I am using clay to make the impression and pour pop into it. After some 12 hrs when I try to remove the mould it breaks into pieces. Please suggest

Ken

9/20/2016 05:24:15 am

Hello Angel,
I have some thoughts but first some questions: Did the POP release from the clay or did it seem to stick? Was the POP very thin? Did you reinforce the POP with something like gauze bandage to give it some strength? Was the POP mixed at the 2:1 ratio needed for best results? One thought that I had was that the POP had no way to release water against the clay and 12 hours may not have been long enough. Let me know what you have done and we can fix the problem. Thanks, Ken.

Angel

9/22/2016 12:12:58 am

Answering your questions first. Ya POP mould is perfectly released from the clay. Ya it is slightly thin. Small layer. Like a bread. I dint use any guaze. And I mixed it with the same proportion 2:1. Before using clay I tried dough. In which the POP is not getting dried soon. It's still wet. Not able to take the mould out. Please advice.

Ken

9/22/2016 04:49:44 am

Angel,
Something is terribly wrong if the the material is truly Plaster of Paris and is mixed with two parts plaster to one part water. It should set in thirty to 40 minutes and mostly cure overnight. The fact that it is still wet causes me to think that there is an excess of water. Try making a small batch and pouring it on a surface like waxed paper or parchment paper and see if the same thing happens. Ken.

Plaster of Paris Mixing Guide...

Leave a Reply.

Author

Archives

Categories