Additively colored single crystals heated in the presence of water vapor developed a colorless region around the surface of the crystal. This bleaching effect was explained by a model in which water diffused into the crystal and reacted with excess K to form a colorless photochemically-sensitive reaction product. Using this model, the diffusion coefficient of water in colored KCl was:

D(cm2/h) = 7.2 x 10-6 exp[-0.56(eV)/kT]

J.C.Penley, R.S.Witte: The Journal of Chemical Physics, 1964, 40[6], 1550-3