Re: What is the substance that changes from blue to pink depending on weather?

Dear Laurie,

The substance is cobalt chloride. When it is dry it has the formula CoCl₂ and is a bright blue color.

When it is wet, it changes to cobalt chloride hexahydrate, meaning that it picks up six molecules of water for each cobalt atom. It has the formula CoCl₂.6H₂O, but it is more correct to write [Co(H₂O)₆]²⁺Cl₂^2- because the water molecules are coordinated, or associated, with the cobalt itself. So, when the humidity is high, the color changes to pink when the maximum amount of water molecules have been absorbed.

When the humidity is in-between, each cobalt has just two water molecules. The "dihydrate" is a sort of purple color.

Because the cobalt chloride must be in contact with the air, it cannot be incorporated into a paint, but rather it is used to impregnate a porous medium. In the case of the figurine, the pottery is unglazed, and in the case of the postcard part of the picture is coated with "flock" (normally very finely chopped wool) which absorb a solution of cobalt chloride (and is allowed to dry).

Thanks for your question!