Sir,

The Karnataka State Food Safety Commissioner has banned, inter alia, the sale of “limestone paste” which is used in the preparation of paan beeda in packets and tubes (SOM dated May 16). I am afraid,  there has been some mix-up in the nomenclature.

The mineral, calcium carbonate, occurs in several forms: Limestone, Chalk, Calcite, Marble, etc. Limestone is almost insoluble in water.  A  “limestone paste” is, therefore,  impossible.

Limestone, on heating, yields quicklime (calcium oxide). This quicklime further reacts violently with water and yields slaked lime (calcium hydroxide) — a white amorphous powder. However, slaked lime is very sparingly soluble in water, resulting in a white paste. It is this ‘lime-paste’ which is used in the preparation of paan beeda.

PS: A suspension of slaked lime in water is known as milk of lime.  It is used as a coating for the brick and mortar walls of buildings. The coating serves as a weather-coat. However, on prolonged exposure to atmosphere, the slaked lime (calcium hydroxide) reacts with carbon dioxide and water present  in the air to form calcium carbonate. The carbonate gets peeled off as flakes.

– K.N. Krishna Prasad on e-mail, Lakshmipuram, 17.5.2017