Royal Baking Powder was a single-acting product made years ago. Here's a recipe for single-acting baking powder you could use in old recipes calling for Royal brand.

• 2 tablespoons cream of tartar
• 1 tablespoon baking soda
• 1 tablespoon cornstarch

Combine ingredients and store tightly covered. For double-acting baking powder, use 1/4 teaspoon of baking soda and 1/2 teaspoon of cream of tartar for each teaspoon needed.

 

Here is some interesting info i found on store bought baking powder:

“Aluminum in baking soda?

In striving for clarity, baking SODA is (by definition) sodium bicarbonate, a chemical that is extracted from the ground.  It does not contain aluminum nor is aluminum added to it in processing either by intention or through aluminum equipment. 

Baking POWDER is manufactured with baking soda as an ingredient and typically includes a compound containing aluminum.  Marketing baking soda as aluminum-free is simply a marketing strategy to play to consumers' confusion while charging more for it.  As has been noted, no baking soda has aluminum in it. 

Aluminum in baking powder has been known to contribute to a bitter taste. So switching to aluminum-free baking powder should eliminate that problem. “

http://www.welltellme.com/discuss/index.php?topic=14397.0

~ I’m not saying this information is correct, i haven’t searched it out.