The first baking powders were made of cream of tartar and bicarbonate of soda, mixed with a starch, to prevent the slight chemical action which would cause the powder to lose strength; and these two substances are now used in the best baking powders. The resulting salt is the Rochelle salt of medicine.
An acid phosphate is sometimes used with soda, and this gives a harmless neutral substance..
Cheaper acids have sometimes been used, especially alum. It is best not to use an alum powder. Select a standard kind, avoiding those that offer prizes for a certain number of boxes purchased. Even if these latter do not contain alum, there is probably an excess of starch or flour.
The advantage of baking powder is in the accuracy of the proportions of the two substances by weight. Even though the measuring of the cream of tartar and soda separately is accurate, the proportions may not be correct. There is no great advantage in homemade baking powder. It costs almost as much as the manufactured, and is not as perfect a product.