Many years ago while attending a state fair in the mid-west one vendor would give away a one inch by three inch piece of paper that had the thickness like a manila file folder. This was impregnated with a chemical on a spot about the size of a dime. It had a pink color to it and when the humidity increased it would change to blue. Or maybe it was the other way around. When the humidity decreased, it would revert back to the original color. What chemical would that have been?

Mix of cobalt chloride along with other salts.

The cobalt chloride changes color depending on the moisture level. I don't know, but I imagine this is from waters coordinating with the salt... other salts will coordinate waters as well, like epsom salt (MgSO4) but epsom salt looks white when coordinated with water or dehydrated. Cobalt chloride shows a color shift.

Added salts like NaCl or CaCl2 have nothing to do with the color other than they absorb moisture.

Here's a link. You may have problems getting cobalt chloride unless you buy from a chemical supplier or know someone who works in a chemical lab.

http://www.kcsd.k12.pa.us/piper/weat...erlesson9.html

Or you can buy paper already manufactured at :

http://www.sciencekit.com/category.asp_Q_c_E_435196

Haven't used either the recipe or the product, but I'd do some more searches for cobalt chloride and see what you get.

Chemists use a brand called drierite to dry air supplies to chemical reactions and they sometimes use this indicator to show the chemist when it is "spent".. and some moisture sensitive instruments utilize the same kind of discs with indicator... shows when the dessicants are spent and need replaced.