An alkaline chemical pulping process in which wood chips are digested in a caustic soda containing lye (NaOH), produced by adding sodium carbonate (Na2CO3) to lime (CaO). The soda process is useful for digesting softwood or hardwood. The first commercial production of soda pulp was in 1851, and is used infrequently in the present-day due to the superiority of newer processes. In 1879, sodium sulfate (Na2SO4) was added to the soda process, and a much stronger pulp resulted. This process, dubbed the sulfate process, replaced the traditional soda process, although it was found later that the ingredient primarily responsible for the increased strength wasn't sodium sulfate, but sodium sul'fide (Na2S). The sulfate process is also called the kraft (after the German and Swedish word for "strength") process and is the most prevalent chemical pulpng system currently in use.