Wood is the main source of pulp and paper. Preliminary production steps are debarking and chipping. Pulping processes are of three principal types: mechanical, or grinding; chemical, or cooking with added chemicals; and semichemical, or a combination of heat or chemical pretreatment with subsequent mechanical reduction to fibres. The yield of pulp ranges from about 40 percent by chemical methods to 95 percent by mechanical ones. Chemical processes are based on either acids (i.e., sulfite pulping) or alkalies (alkaline pulping, including soda and sulfate [kraft] processes).