Natural Coloration

4th century A.D.
Ht., 12.3 cm

Glass is not naturally colorless. Beach sand invariably contains black particles of iron oxides scattered through it. When fired in a usual, oxygen-rich environment (i.e., with the furnace in an oxidizing state), these impurities give the glass a natural aquablue to light green tinge. The typical amount of iron in beach sand is 0.3%: the intensity of the color increases as the amount of iron increases.