BS”D
The question is whether there is any NON kosher cream of tartar, and therefore whether making your own play-dough is even necessary.
Unless someone has another source, it seems that the information from R’ Berel Levy A”H is correct and that cream of tartar is kosher.
Again, there is NO WAY the small kosher winemaking industry could produce enough cream of tartar even to fill all those McCormick, Gefen and Frontier Herbs boxes.
To us, Kedem seems like a huge company but in the great scheme of things, their output is not very large. The amount of cream of tartar they, the Israeli firms, and the French, Australian and South African runs could produce would not be enough to account for all the cream of tartar out there that is kosher. The rest comes from non kosher wine production and because it is so heavily processed it does not have the din of grape products from what I understand of the information I linked to.