Oily inks form a layer on the surface which is often called the skin . The skin protects the ink below it from being oxydized but, in the long run, the ink below becomes thicker and thicker until it is unusable. Synthetic inks also dry up and form a skin which does not protect the rest of the ink from thickening. Cellulose and ethyl cellulose inks do not dry up if they are kept in a hermetically closed container. Glycerophtalic inks form a skin on the surface which may protect the ink below it for quite a long time.
Varnish reacts more or less in the same way as ink.
Alcohol base varnish dries by evaporation. The drying of preparations is of great importance in lithography. Depending on the work, drying must be done immediately after the applications or then might be done after a varying amount of time. The stones used in lithography must be perfectly dry before being worked on. If they are a bit damp they can be dried out in a stove or in front of a heater.