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Kala Namak (Bengali Biit lobon (বিট লবণ); Newari Be Chi; Nepali Birae Nun (बिरे नुन) ; Hindi काला नमक ; Urdu کالا نمک Marathi काळं मीठ ;kālā namak; Gujarati સંચળ Sanchal; Tamil இந்துப்பு; Malayalam ഇന്തുപ്പ്) also known as sulemani namak, black salt, kala loon or black Indian salt, is a type of rock salt, salty and pungent-smelling condiment used in South Asia. The condiment is composed largely of sodium chloride with several impurities lending the salt its colour and smell. The smell is mainly due to its sulfur content. Due to the presence of Greigite (Fe3S4, Iron(II,III) sulfide) in the mineral, it forms brownish pink to dark violet translucent crystals when whole, and, when ground into a powder, it is light purple to pink in color.
The raw material for producing kala namak was originally obtained from natural halite from mines in Bangladesh, India, Nepal and Pakistan in certain locations of the Himalayas salt ranges, or from salt harvested from the North Indian salt lakes of Sambhar or Didwana and Mustang of Nepal
Traditionally, the salt was transformed from its raw natural forms into commercially sold kala namak through a reductive chemical process that transforms some of the sodium sulphate in the raw salt into pungent hydrogen sulfide and sodium sulfide. This involves firing the raw salts in a furnace for 24 hours while sealed in a ceramic jar with charcoal along with small quantities of harad seeds, aamla, bahera, babul bark, or Sajji. The fired salt is then cooled, stored, and aged prior to sale. Kala namak is prepared in this manner in Northern India with production concentrated in the Hisar district, Haryana. The salt crystals appear black in colour, and are usually ground to a fine powder which is pink in colour.
Although the kala namak can be produced from natural salts with the required impurities, it is common to now manufacture it synthetically. This is done through combining ordinary sodium chloride admixed with smaller quantities of sodium sulphate, sodium bisulphate and ferric sulphate, which is then chemically reduced with charcoal in a furnace. Reportedly, it is also possible to create similar products through reductive heat treatment of sodium chloride, 5-10 percent of sodium carbonate, sodium sulphate, and some sugar.
Sodium chloride provides kala namak with its salty taste, iron sulphide provides its dark violet hue, and all the sulphur compounds give kala namak its slight savory taste as well as a highly distinctive smell, with hydrogen sulphide being the most prominent contributor to the smell. The acidic bisulfates/bisulfites contribute a mildly sour taste. Although hydrogen sulphide is toxic in high concentrations, the amount present in kala namak used in food is small and thus its effects on health are negligible: Hydrogen sulphide is also one of the components of the odor of rotten eggs and boiled milk
Kala Namak is used extensively in South Asian cuisines of Bangladesh, India and Pakistan as a condiment or added to chaats, chutneys, salads, all kinds of fruits, raitas and many other savory Indian snacks. Chaat masala, an Indian spice blend, is dependent upon black salt for its characteristic sulfurous hard-boiled egg aroma. Those who are not accustomed to black salt often describe the smell as similar to rotten eggs. Kala Namak is appreciated by some vegans in dishes that mimic the taste of eggs. It is used, for example, to spice tofu to mimic an egg salad.
Kala Namak is considered a cooling spice in ayurvedic medicine and is used as a laxative and digestive aid. It is also believed to relieve intestinal gas and heartburn. It is used in Jammu to cure goiters. This salt is also used to treat hysteria, and for making toothpastes by combining it with other mineral and plant ingredients.