Last year, Mehran Matin and his daughter Moujan Matin, working at the Shex Porcelain Company in Saveh, Iran, showed that these materials didn’t need to be in physical contact at all. All that would require is for a bit of quartz, a ubiquitous mineral in the Middle East, to have been lying around in a copper-smelting kiln.
Or does it? To get the rich turquoise blue, you also need other ingredients, such as salt. So Moujan Matin, now at the Department of Archaeology and Art History at the University of Oxford in England, has undertaken a series of experiments with different mixtures to see if she can reproduce the shiny blue appearance of the earliest blue-glazed stones. She used a modern kiln fired up to the kind of temperatures ancient kilns could generate – between 850 and 980C – in which lumps of quartz were placed on a pedestal above a glazing mixture made from copper scale and other ingredients.
Matin found that copper scale and rock salt alone covered the quartz surface with a rather pale, greenish, dull and rough coating: not at all like ancient blue glaze. An extra ingredient – calcium carbonate, or common chalk, which the Egyptians used as a white pigment among other things – made all the difference, producing a rich, shiny turquoise-blue glaze above 950C.
That looked good – but it forces one to assume that salt, chalk and quartz all somehow got into the kiln along with the copper scale. It’s not impossible, but as Matin points out, such accidents probably had to happen several times before anyone took much notice. However, there’s no need for the least likely of these ingredients, rock salt. Matin reasoned that dried cattle dung, which contains significant amounts of both alkalis and salt (chloride), was widely used as a fuel since the beginnings of animal domestication in the eighth millennium BC. So she tried another mixture: copper scale, calcium carbonate and the ash of burnt cattle dung. This too produced a nice, shiny (albeit slightly paler) blue glaze.
Of course, there’s nothing that proves this was the way glazing began. But it supplies a story that is entirely plausible, and narrows the options for what will and won’t do the job.