Cobalt Chloride: Cobalt Oxide Dissolved in Hydrochloric Acid

Cobalt chloride is the starting salt for cobalt blue and cobalt violet. Thorpe (1912): dissolve cobalt oxide in hydrochloric acid and evaporate. Crystallised cool it holds six molecules of water; hotter crystallisation gives four or two. A strong solution turns blue. The picture: cobalt chloride crystals (Wikimedia Commons).