Who knew! I sure didn't. This blurb is from The NY Times

The secret to making Edna Lewis's recipe for ethereal, flannel-soft biscuits is to use a homemade baking soda -- two parts cream of tartar sifted with one part baking soda -- so there's no chance of a metallic aftertaste in the dough. She calls for lard, too, which is excellent if you can find the creamy white variety usually labeled "leaf lard," but if you have only butter, so be it. They'll still be among the best biscuits you've ever had.