I have been making these crystals since before there was an internet and recently began seeing a number of people having trouble with the process. Crystals were either not being produced or the color was very yellow among other concerns. I recognized the difficulty and I am posting this discussion to clear up some of the issues
The reactants necessary to synthesize Rochelle salt, (potassium sodium tartrate), which is a double salt, involves the use of potassium bitartrate, also sold as “cream of tartar” in markets in the spice aisle, and sodium carbonate, also sold in the market in the detergent aisle as Borax washing soda. Baking soda, (sodium bicarbonate), can be used in place of the washing soda but involves heating to drive off water and CO2. It is much easier to just use the borax washing soda.
The problem as I have seen it that the cream of tartar has a variable composition and may contain other chemicals to stop clumping, or other modifiers to make the product less expensive. This leads to at least two problems: The first is that the reaction is changed to produce unwanted byproducts, and the second is that the final crystals have inclusions which lead to poor crystal quality. The easy solution to the problem is to simply recrystallize the crude cream of tartar and then harvest the clean product for the synthesis of the Rochelle salt.
Fortunately, cream of tartar is easy to recrystallize as it is much more soluble in hot water than cold, which is the perfect condition for clean crystals. See the lead photos. At 100 degrees centigrade, 100 ml of water will hold about 6.2 grams of cream of tartar. At 10 degrees, it will hold only about 0.6 grams. So here is the process to do the recrystallization:
Place about 6 grams of the cream of tartar into a suitable container such as a Pyrex measuring cup. In a saucepan, bring a water bath up to just boiling and place the cream of tartar container in and immediately add about 100 ml's of distilled water. When the solution reaches just about boiling, stir to dissolve. Let as much dissolve as seems reasonable and filter through a coffee filter or filter paper. Let the clean solution cool to room temperature. The crystals will be forming at this point. Place in a refrigerator and let cool completely. Then, filter and collect the crystals and dry. That is really all that is necessary to now proceed to the synthesis of the Rochelle salt. Losses will vary but you can use the following formula to calculate recovery if desired:
% Recovery = amount recovered divided by initial quantity X 100. Good luck!