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"Fuligio pictoria [soot] that the painters use is taken from the glassmakers, as that is the best." (Diocorides, De Materia Medica 5-182) http://www.cancerlynx.com/BOOKFIVEWINES.PDF

While glassmaking involves potash, it doesn't seem to include charcoal or soot. Potash is pale in colour. Why would anyone search the best soot at the glassmakers?

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It looks like this was literally soot taken from the furnace walls themselves, not the soda ash which was used as a flux in the Roman glassmaking process itself.

from A Dictionary of Greek and Roman Antiquities, Volume 1

Atraimntum tectorium or pictorium a black pigment used by painters. Pliny describes many varieties the best of which was made by collecting the soot arising from the combustion of the pitch pine on the marble walls of a specially constructed furnace mixing it with glue and then drying the mixture in the sun Pliny H.N. xxxv 41 Vitruv vii 10.

So apparently the Diocorides text considers the walls of the glassblowers furnace as a good place to gather this soot, though Pliny seemed to describe special furnaces which contained the smoke.

Pliny informs us how it was made. He says, "It was made of soot in various ways, with burnt resin or pitch: and for this purpose," he adds, "they have built furnaces, which do not allow the smoke to escape. The kind most commended is made in this way from pine-wood:

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