Sal ammoniac is a rare mineral composed of ammonium chloride, NH4Cl. It forms colorless to white to yellow brown crystals in the isometric - hexoctahedral class. It has very poor cleavage and a brittle to conchoidal fracture. It is quite soft, with a Mohs hardness of 1.5 to 2 and has a low specific gravity of 1.5. It is water soluble.

It typically forms as encrustations formed by sublimation around volcanic vents. It is found around volcanic fumaroles, guano deposits and burning coal seams. Associated minerals include: sodium alum, native sulfur and other fumarole minerals. Notable occurrences include: Tadzhikistan; Mt. Vesuvius, Italy and Parícutin, Michoacan, Mexico.

It is also the archaic name for the chemical compound ammonium chloride; from Greek, ??? ?µµ???a??? hals ammoniakos, salt of Ammon, because of its early manufacture in Egypt.

It is commonly used in stained and leaded glass manufacture as a flux and tinning agent for irons used in the soldering process. In both jewelery making and the refining of precious metals, potassium carbonate (cream of tartar) is added to gold and silver in a borax-coated crucible to purify iron or steel filings that may have contaminated the scrap. It is then air cooled and remelted with a one-to-one mixture of powdered charcoal and sal ammoniac to yield a sturdy ingot of the respective metal or alloy in the case of sterling silver (.75% copper) or karated gold. Anything other than 24-karat gold has silver and copper added. Usually the addition of silica, zinc, and deoxidants in very small amounts relative to the pennyweight (dwt.) of gold are processed into gold from as low as 8kt to as high as 23.5 karat gold. This is added to prevent porosity and/or cracking while milling the ingot further into wire, sheet, or tubing. Without those additives an otherwise poor-quality ingot will result in open crucible melting with a hand torch or blowpipe and flame, as was done before electric melting furnaces were invented for use in the precious metals industry. These practices are still used by metalsmiths and jewelers today.

Sal ammoniac has also been used in the past in bakery products to give cookies a very crisp texture, although that application is rapidly dying due to the general disuse of it as an ingredient. The term fell out of general use largely in the 20th century.

On February 26, 2007 late night talk show host David Letterman mentioned sal amoniac when talking of his memories of metal shop class. He did not know the word he just remembered it, only learning the meaning after looking up the term in a dictionary.