Ferrous sulfate - FeSO4·7H2O
Frequently, ferrous sulfate has a green color with brownish hue, especially the cheap material from a pottery supplier usually looks like that. Pure ferrous sulfate has a beautiful mint green color as shown in the picture. The brownish hue is due to aerial oxidation. Ferrous sulfate is air-sensitive and in contact with air it quickly is covered with a brown layer. For this reason, the material must be stored in a tightly closed container.