a. In plural. An alkaline substance consisting largely of impure potassium carbonate and originally obtained by leaching the ashes of burnt plant material and evaporating the solution in large iron pots. Now rare (chiefly historical).Formerly not always distinguished from impure sodium carbonate produced in the same way: cf. barilla n.

b. singular. Originally: = sense 1a. In later use: purified potassium carbonate, esp. as obtained from potashes (sense 1a) and regarded as a simple, indecomposable substance rather than a compound. Now usually: potassium carbonate as used in the manufacture of glass; (also) potassium oxide as supplied by potassium carbonate during the process of manufacturing glass.Sometimes not distinguishable from sense 3c.

a. Chemistry. In names of chemical compounds: = potassium n. Cf. also potash alum n., potash feldspar n. at Compounds 2.Such compounds were formerly thought of as compounds of potassium oxide or of potash (potassium hydroxide) when regarded as a simple substance (cf. sense 3b); for example, carbonate of potash (potassium carbonate, K₂CO₃) was regarded as K₂O·CO₂, so while potash is here effectively synonymous with potassium, sense 3b is usually intended.

b. More fully caustic potash. Potassium hydroxide, KOH, a white, caustic, deliquescent solid which gives a strongly alkaline solution in water (originally regarded as a simple, indecomposable substance rather than a compound). Also: potassium oxide, K₂O, a yellowish white solid (now rare except as in sense 3c; cf. also senses 1b and 3a).

c. Any of various (soluble) salts of potassium, esp. as occurring naturally and extracted for use e.g. in fertilizers as a source of potassium (frequently quantified as the equivalent amount of potassium oxide); such salts collectively; potassium as a plant nutrient or as occurring naturally in the form of salts. Cf. also sense 1b.