(i)Drying by chemical reaction between paint and air: Oxygen and water vapor in particular are reactive chemical ingredients of the air. The air is used as a chemical reactant and is kept apart from the reactive ingredients in the paint by the tightly fitting lid on the can. If it is not, it may cause the formation of a skin, which seals off the skin below from further reaction with the air. Alternatively, the skin may be permeable to the air and the reaction may spread right through the paint, turning it into a cross-linked polymer swollen with solvent. This is called an irreversible gel and of course at this point the paint becomes useless. However, assuming that the air is completely excluded, no reaction begins until the paint film is applied, presenting a large surface exposed to the air. As the solvents evaporate, cross-linking begins and the soft, sticky, low molecular weight, linear or branched polymers in the paint are converted to a hard, tough, cross-linked film, which will no longer dissolve in the solvents used in the original paint.