For the last few years I've been exploring iron glazes that produce spots without being layered. I believe that the key to the creation of the spots is my firing schedule, which is given below, with temperatures in Celcius:
60 minutes from 942 to 1022
65 minutes from 1022 to 1076
65 minutes from 1076 to 1112
80 minutes from 1112 to 1132
This amounts to Orton cone 4, or slightly higher. It's generally believed that traditional oilspot glazes depend on the thermal decomposition of hematite (Fe2O3) at around 1230C. The oxygen released forms bubbles, which create spots with high concentrations of Fe3O4. While the spots in my glazes are definitely formed by bubbles, the fact that the maximum temperature I fire to is only 1132C suggests that they may be produced through some other process. Alternatively, it may be that the thermal decomposition of Fe2O3 in a molten glaze with the correct composition takes place at a lower temperature, but at a slower rate, and that it is the long time spent near the top temperature at allows enough decomposition to take place to produce the spots.
I've had spots appear in four different families of glazes. All of these families contain boron, and all except Mirror Black II Surprise contain magnesium. When I get the time, I'll discuss each family in more detail.
The first family of spotted glazes I stumbled upon, and the one most similar to traditional oilspots in appearance. Initially formulated with a significant amount of clay (Overberg Earthenware), but since reformulated with less clay in an attempt to control crawling.
Examples on Glazy:
A modification of a glaze test in Mimi Obstler's book 'Out of the Earth, into the Fire'. High silica.
Example on Glazy:
Based on a John Sankey iron red glaze. Low alumina, high boron. Tends to blister.