This tub of plasticine (see photo below) has been sitting in a box at the back of my study. I don鈥檛 think it has been subjected to any particularly extreme environmental events, yet it looks like it is covered in white crystals. Any ideas how this has happened?
鈥� Plasticine is a mixture of calcium salts 鈥� mostly calcium carbonate 鈥� petroleum jelly and aliphatic acids 鈥� mainly stearic acid. , meaning that it is a hydrocarbon chain with a carboxyl group at one end. Rather than having the consistency of a fat, it forms crystals. Most fatty acids crystallise at room temperature.
The stearic acid crystals in plasticine have a solubility in water of 3 grams per litre at 20 掳C. I suspect that moisture in the air has caused stearic acid to leach out of the plasticine in a solution and recrystallise on the exterior when the water has evaporated. Cycling humidity or temperature would promote this process.
Advertisement
Eric Kvaalen, Les Essarts-le-Roi, France
