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
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: