Baker's Borax—An Experiment
Or so I would have said a year ago. It turns out that al-Warraq's 10th century cookbook contains references to something he calls "Baker's Borax" which pretty clearly is not borax and apparently was a chemical leavening. I have been trying for some time to figure out what it was.
1. In a recipe for a leavened fritter, al-Warraq writes:
"If there was not enough yeast in the batter, wait until it ferments well. If the yeast was bad, add some more borax (būraq) to the batter."Which seems to imply that it functions as a chemical leavening.
2. Baker's borax was used by bakers to make bread shiny.
3. Another form of "borax" was natrum. Natrum is still used under that name for various purposes. It's a naturally occurring mix of sodium carbonates found in dry lake beds in Egypt.
Point 3 suggested that baker's borax might be, or contain, Sodium Bicarbonate--baking soda. That works in the al-Warraq recipes I've tried it in as a leavening. When I tried brushing the top of a loaf of bread with a baking soda solution before putting it in the oven, however, the result was brown, not shiny.
Various things, including comments on the SCA Cooks email list, suggested an alternative possibility, Potassium Carbonate, one of several things called "Potash." That also seemed to work, at least in the recipe I tried it in, but also did not make a loaf of bread shiny.
Today, for other reasons, I was planning to make some al-Warraq flatbreads. It occurred to me that although loaves of bread baked in an oven existed in al-Warraq's time, a lot of the bread consisted of flatbreads cooked much more rapidly by sticking them to the inside wall of a tannur, an effect I try to get by using a baking stone in a hot oven. There is no particular reason why the effect of baker's borax, whatever it was, would be the same for both kinds of bread.
So when I made my flat breads, I brushed part of some of them with a solution of Sodium Bicarbonate, part of some with a solution of Potassium Carbonate, before putting them on the baking stone. The result was pretty clear. Sodium Bicarbonate produced a dull surface, Potassium Carbonate a shiny surface. I took some pictures, and here is one. The loaf on the right has had all of it brushed with Potassium Carbonate. The loaf on the left has had the lower half brushed with Sodium Bicarbonate, the top half with nothing.
Hence my current best guess for baker's borax is Potassium Carbonate.