I just shut off the kiln for the night and I thought I'd share some of the reading I've been doing while the firing's been going on. A while ago I posted a little something about Achaemenid glazes. I've kept thinking about them since then, curious about figuring out a formulate for myself. I found a very interesting essay here (http://www.gustav-we...s-of-Glazes.pdf) on the origin of glazes. I highly recommend giving the essay a read, it's very insightful about just how glazes came about.
"Chemical analyses (Hedges 1982) from the Achaemenid, Parthian and Sassanid periods (from 700 B.C. revealed glazes with the following boundary val- ues: 6-8 % Na2O, 2.5-4 % K2O, 4-8 % CaO, 2-4 % MgO, 2-3 Fe2O3, 4-8 % Al2O3 und 65-75 % SiO2. After a phase diagram by Morey (1930), this ratio of alkali to earth alkalis and silicic acid needs a firing range of 900°-1100°C. These glazes were ap- plied to a body with 16-17 % CaO, 5-6 % MgO und 50 % SiO2, which corresponds to the lime-rich clays of the Middle East."
10% soda ash, 10% potash feldspar, 10% dolomite, 10% kaolin, and 60% silica puts you right in the ball park of that chemistry. Historically, this would likely have been soda/natron and local sand, just like Egyptian glass.
Edit: It occurred to me that I should share how I arrived at that recipe. The essay talks about plant ash and sand as the origin of this type of glaze, but I'm not sure how realistic this is when trying to replicate the result. I tried every known plant ash, and I couldn't make it work. There are plants in the mediterranean and elsewhere that take up soda and potash in tremendous quantities, e.g saltwort (salsola soda), but I don't have those, and it was later found in ancient glass making that substances like natron form a better glass than ash. In fact, it wasn't until the breakdown of trade routes that ash became important in glassmaking again.
Thought I'd share