|Dionysos in a ship, sailing among dolphins,
(iron oxide red and black glazes)
bowl discovered at Vulci, Viterbo.
Artist: Exekias (c. 530 BCE)
The three most common forms all occur as dark grey or black mineral, but each possesses distinctive properties. "Wüstite" (FeO) is often found in meteorites, "hematite" (Fe2O3) forms a red powder when scraped, appearing to bleed, hence the name and finally, "magnetite" (Fe3O4), which is magnetic. These oxides can occur chemically bound with water, as mixtures and in different crystal structures, forming a family of compounds. The iconic red and black pottery of the Greeks was colored with iron oxide; the red (or orange) was hematite and the black was magnetite. Chinese "celadon" glaze is a green-blue that resembles jade; it is derived from an iron oxide (FeO), which does occur in nature as wüstite, but in this case forms chemically in the potters kiln.
Iron, as a pigment in glass, can be responsible for several different colors, notably green and according to Antonio Neri also red. Today red from iron is common in pottery glazes, but not in glass. In his 1612 book, L’Arte Vetraria, Neri does not offer a specific recipe for iron red, but in chapter seventeen he teases us with this passage:
This second way to make crocus martis although very easy should not be disparaged, but rather highly regarded, since crocus produced this way causes the glass to appear a rather bright blood red.Crocus martis was the alchemical name for the red oxide of iron (Fe2O3). Neri’s iron red may depend on making a suspension of the finely ground powder in the glass. It is questionable how stable it would be in the furnace; eventually the powder would dissolve in the glass turning it transparent green. But as mentioned above, this oxide is successfully used in red pottery glazes. For glassmakers in the seventeenth century, not iron but copper was the most common pigment for producing red, yet here he presents us with an intriguing alternative. However basic crocus martis might be, Neri holds it in great esteem, using his alchemical reasoning to explain that it has the power to expose colors which are usually hidden in the glass. He continues:
The way to make it is like this: Obtain some iron filings, although steel is better if you are able to get it. Mix it well in a terracotta oven-pan with strong vinegar, which is sprinkled only until it is moist throughout. Now spread it out in the pans, and put it in the sun to bake. When the sun is clouded, leave it in the open air to dry. Now turn it into powder. If it leaves any hard lumps, sprinkle and moisten them with new vinegar, leave it to dry again and then pulverize, as above. This work should be repeated eight times, then grind and sift it through a fine sieve, which will make a very fine powder the color of brick dust. Store this in well sealed vessels for use in the coloring of glass.From this prescription, we can guess that he was producing a mixture of hematite and iron acetate. The fact that iron does not appear as a red pigment in other glassmaking references makes his statement something of a mystery, perhaps to be solved by engaging our glassmaker's own favorite activity: experimentation.