Most modern baking powders are double acting, that is, they contain two acid salts, one which reacts at room temperature, producing a rise as soon as the dough or batter is prepared, and another which reacts at a higher temperature, causing a further rise during baking. Common low-temperature acid salts include cream of tartar, calcium phosphate, and citrate. High-temperature acid salts are usually aluminium salts, such as calcium aluminium phosphate. Baking powders that contain only the low-temperature acid salts are called single acting.
While various baking powders were sold in the first half of the 19th century, our modern variants were discovered by Alfred Bird. August Oetker, a German pharmacist, made baking powder very popular when he began selling his mixture to housewives. The same recipe he created in 1891 is still sold as Backin in Germany. Oetker started the mass production of baking powder in 1898 and patented his technique in 1903.
Eben Norton Horsford, a student of Justus von Liebig, who began his studies on baking powder in 1856, eventually developed a variety he named in honor of Count Rumford. By the mid-1860s "Horsford's Yeast Powder" was on the market as an already mixed leavening agent, distinct from separate packages of calcium acid phosphate and sodium bicarbonate. This was packaged in bottles, but Horsford was interested in using metal cans for packing; this meant the mixture had to be more moisture resistant. This was accomplished by the addition of corn starch, and in 1869 Rumford began the manufacture of what can truly considered baking powder.
During World War II, Byron H. Smith, an inventor in Bangor, Maine, created a substitute product for American housewives, who were unable to obtain baking powder, cream of tartar or baking soda due to war food shortages. Named "Bakewell", a mixture of sodium pyrophosphate and corn starch, the product is still part of regional culinary history. When combined with baking soda, it is essentially the same as any single-acting baking powder, the only difference being that the acid is sodium pyrophosphate.
In 2006 the development of Rumford Baking Powder was designated an ACS National Historical Chemical Landmark in recognition of its significance for making baking easier, quicker, and more reliable.
Moisture and heat can cause baking powder to lose its effectiveness over time, and commercial varieties have a somewhat arbitrary expiration date printed on the container. Regardless of the expiration date, the effectiveness can be tested by placing a teaspoon of the powder into a small container of water. If it fizzes energetically, it's still active and usable.