Red-Figure Vase Firing Stages

“The brilliant effect of figured Attic vases with their characteristic lustrous black gloss and reserved ares of orange-red was produced by a three-step firing process consisting of a single cycle of oxidizing, reducing, and reoxidizing atmospheres in the kiln.  The heat and oxygen were carefully adjusted to assure that the required physical and chemical reactions would occur during each step of the cycle.

Oxidizing phase – The significant content of ferric oxide (Fe₂O₃) gives Attic clay a reddish color.  The black gloss, made from the same clay, contains ferric oxide as well. When fired in an oxidizing atmosphere, the reserved and the glossed areas will both turn red. This occurs when the temperature of the kiln reaches about 800⁰ C and air, admitted through a vent, brings oxygen into the firing chamber. Thus, during the oxidizing phase the reserved clay areas fire to a light red, while the gloss turns a brownish red.

Reducing phase – In the middle of the firing process the temperature of the kiln was increased to 950⁰ C, the air vent was closed, and moisture was added (possibly on the form of green wood and leaves or damp sawdust introduced into the kiln), causing incomplete combustion. As a result, instead of carbon dioxide (CO₂), which is present if the combustion is complete, carbon monoxide (CO) was produced in the kiln. Since oxygen was prevented from entering the kiln, the carbon monoxide combined with the oxygen molecules of the red ferric oxide in the clay, chemically changing it to a black ferrous oxide (FeO) or to the even blacker magnetic oxide of iron (Fe₃O₄). The chemical reaction that took place during the reduction cycle turned the reserve areas of the vase to a matte dark gray color while the gloss, which was made of finer clay particles, sintered to a deep, shiny metallic black.

Reoxidizing phase – During the last phase of the firing, the kiln was gradually cooled to about 900⁰ C and some oxygen was allowed to enter the firing chamber through the vent. This changed the atmosphere in the kiln from reducing back to oxidizing. The oxygen combined with the more porous reserved clay, reverting its matte gray to an orange-red color. The black gloss, however, retained its black color, for its sintered surface could no longer absorb oxygen, so no further chemical reaction occurred.”

“Greek Vases” in Touchables Manual, Getty Villa, n.d.