Originally Posted by smoothseas
You can make your own baking powder by combing 1/2 teaspoon cream of tartar and 1/4 teaspoon baking soda, this equals I tsp of bp.
Unless I'm missing something, if you combine 1/4 teaspoon of a powdered ingredient with 1/2 teaspoon of a similar powdered ingredient, you end up with 3/4 teaspoon of the two ingredients. In order to create a full teaspoon, you will need to mix up more.
And, I agree with Joe. Buy fresh. It's too cheap to risk possible failure.