A chemical process in which a substance dissolved in a solvent becomes insoluble (either by chemical reaction or by saturation) and separates from (or "precipitates out of") the solution. In ink terminology, precipitation is a form of drying utilized by moisture-set ink, or ink containing a glycol vehicle, in which a water-insoluble resin (containing the pigment) is dissolved in glycol, which is water soluble. When water comes into contact with the glycol, the glycol is dissolved in the water, but the resin is not, and the resin and pigment "precipitate out" onto the substrate. Variations of this process can also occur in the absence of water. (See Glycol Vehicle.)
The term precipitation also describes a problem encountered in flexographic inks that utilize hard-to-dissolve resins that require many additional solvents to keep them in solution. Frequently, a resin will precipitate out of the solution. The use of a more powerful solvent will help alleviate the problem. (See also Souring.)