Kala namak (Urdu / Hindi) or bire noon (Nepalese; literally "black salt") is a type of rock salt, a salty and pungent-smelling condiment used in South Asia. It is also known as "Himalayan black salt", Sulemani namak, bit lobon, kala noon, or pada loon. It is found mostly in the Himalayas.
The condiment is composed largely of sodium chloride with several other components lending the salt its colour and smell. The smell is mainly due to its sulfur content. Because of the presence of Greigite (Fe3S4, Iron(II,III) sulfide) in the mineral, it forms brownish pink to dark violet translucent crystals when whole. When ground into a powder, its color ranges from purple to pink.
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 Salt Lake or Didwana and the Mustang District of Nepal.
Traditionally, the salt was transformed from its relatively colourless raw natural forms into the dark coloured commercially sold kala namak through a reductive chemical process that transforms some of the naturally occurring sodium sulfate of 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, amla, bahera, babul bark, or natron. 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 Hisar district, Haryana. The salt crystals appear black and are usually ground to a fine powder that is pink.
Although the kala namak can be produced from natural salts with the required compounds, it is common to now manufacture it synthetically. This is done through combining ordinary sodium chloride admixed with smaller quantities of sodium sulfate, sodium bisulfate and ferric sulfate, 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% of sodium carbonate, sodium sulfate, and some sugar.
Sodium chloride provides kala namak with its salty taste, iron sulfide provides its dark violet hue, and all the sulfur compounds give kala namak its slight savory taste as well as a highly distinctive smell, with hydrogen sulfide being the most prominent contributor to the smell. The acidic bisulfates/bisulfites contribute a mildly sour taste. Although hydrogen sulfide 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 sulfide 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, Nepal, 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 season tofu or avocado to mimic an egg salad or deviled eggs.
Kala namak is considered a cooling spice in Ayurveda and is used as a laxative and digestive aid. It is also believed to relieve flatulence and heartburn. It is used in Jammu to cure goitres. This salt is also used to treat hysteria and for making toothpastes by combining it with other mineral and plant ingredients.