Ink setting and ink drying are two different phenomena. Ink that has set allows the sheet to be handled without easily offsetting or smearing. Many printers refer to ink setting as ink drying, since some chemical drying has occurred at this stage. Ink setting involves a rapid increase in the viscosity of the ink on the printed sheet. The causes of ink setting include absorption of solvents into the substrate, solvent evaporation, increases in the ink's viscosity due to the ink's thixotropic properties, and reduction in the temperature of the ink along with quick setting varnishes. Actual drying of the ink is a complicated chemical reaction involving oxidative polymerization using drying oils and other drying agents that react with oxygen in the air. A "hard dry" or completion of the chemical drying process can take as long as 24-48 hours.