Chemical leavening, such as the use of baking soda to create bubbles of CO2 in baked goods, is a surprisingly recent phenomenon. The two main forms of leavening used were therefore "biological" and "mechanical", with different forms being used for different things.

Biological leavening is the use of yeast to make things rise. Bakers throughout history have often been close to breweries, in order to get their yeast supply from the brewer. This came in the form of the liquid yeast, "barm", that came from the foam which rose to the top of a vat of beer during the brewing process. Alternatively, spent yeast which fell to the bottom of the vat could sometimes be collected and reactivated by the baker, though this was rarer. The kind of dried, instant yeast we have today was a more recent invention, and "yeast" at the time pretty generally referred to this barm collected from brewing. As such, bakers' wives across Europe during the middle ages often were principal brewers in their communities, as the trades went hand in hand.

Sourdough was also sometimes used, which didn't need an ongoing supply of yeast, just a bit to get it started. A small piece of dough would then be broken off before baking and kept back to add to the next batch of dough before the process was repeated, allowing a yeast culture to persist and grow which was unique to each yeast population. This could even come in the form of new dough being made in a bread bowl which hadn't been cleaned of the traces of the last batch of dough.

Yeast was definitely the main method for leavening, however, in recipes where it wasn't desired, mechanical leavening was often used. This generally came in the form of eggs being aerated, and the forceful introduction of air bubbles to a mixture which can then be cooked before it sets to maintain the structure of air bubbles.

One of the first sights of possible usage of chemical leavening is in an eighteenth-century controversy in 1757 London, surrounding the adulteration of flour with chalk, possibly to make flour look more white as white bread was more desired, though also to let the miller turn a better profit by padding out his product. The public caught on with the publication of Peter Markham's "Ten Ingredients used in the Adulteration of Bread Flour" the same year. This started a back and forth with bakers or millers defending the quality of their flour which they well knew was adulterated. While this may have not been intentional as a leavening agent, chalk contains calcium carbonate which would have thermally decomposed much like modern baking soda to release CO2, creating bubbles within the dough and causing it to rise.

The first example of chemical leavening being used deliberately in American cooking comes in Amelia Simmons' "American Cookery", a 1796 cookbook which is one of the first examples of a properly American cookbook, as opposed to a British or other European cookbook. She uses pearl ash, a purified form of potash which contained potassium carbonate, and as such when heated would thermally decompose just like chalk or baking soda and release CO2 to create those bubbles and make cakes rise. By the 1830s, chemical leavening was common knowledge in British households.

Britnell touches on women's employment in the brewing trade in medieval England in 'England: Towns, Trade and Industry' from Rigby, "A Companion to Britain in the Later Middle Ages", as does Mate in 'Work and leisure' from Horrox and Ormrod, "A social history of England 1200-1500".

Peter Markham's "Ten Ingredients", sometimes called "Syhoroc" can be found in print on Amazon or other booksellers, as can Amelia Simmons' "American Cookery".