This past week, I've made 3 batches of Merken's white choc candy clay that have crumbled upon kneading. I do 1lb of chocolate per 1/3 cup of corn syrup. This does not happen with the Merkens milk chocolate batch. What is going? Is there some other white  choc brand I can use for this recipe? I need some SOS right now because I have a birthday cake due saturday and the kid's mom asked for edible chocolate animal toppers?

Sounds like you microwaved it when making it and overcooked it.  I no longer make it in the micro because it's very easy to do and you can't fix it.  Do it over a water bath and melt it slow, don't let the water boil so much it spatters the underside of the bowl.  Good luck!

I'll add the crappy Merckens white chocolate itself as a possibility. It probably was over heated, but when I melt it over a waterbath I can never get it to melt completely until I add crisco to it. It smooths out then, but if you don't do that it's hard to get completely smooth.

But the crumbling does sound like overheating.