Successful cooling requires special handling. We have arrived at the conclusion that in virtually all cases of ice formation on cooling to cryogenic temperatures the process starts in a layer of liquid on the surface of the crystal. It is therefore necessary to remove or modify this layer. In the simplest cases one can begin by covering the crystal and some of the mother liquor with oil. One can then blot off any visible liquid, or else strip it off by moving the crystal through the oil.
More difficult cases require that the surface layer be modified with a cryo-protectant.
Cryoprotectants are compounds that prevent ice formation on cooling when added to aqueous solutions. Because the action is at the surface of the crystal, it is normally not necessary to soak the crystal for any length of time. There may be crystals that require more time to disrupt the water layer on the surface. Some crystals do not tolerate the modified mother liquor well, so they should be cooled as soon as possible after rinsing. An effective way of cooling is to plunge the crystal on its mounting pin into liquid nitrogen.