Outside of salted food storage, the ancient Egyptians are famous for their perfection of the art of mummification. A key ingredient in the process was natrun, which is a natural mixture of halite, trona and sodium sulphate (Edwards et al., 2007). The ancients knew its preservative properties as it readily absorbs water, making it an excellent desiccant/preservative of organic material.

Natrun is found in large quantities in the beds of several Egyptian playa lakes (e.g. Wadi Natrun and El Kab, as well as Behiera in the nearby Libyan desert and in Lake Natron in the African Rift valley). It has been mined and traded from such localities for thousands of years. Writings as old as the reign of Rameses III (1198-1166 B.C.) refer to natrun deposits. Its preservative qualities must have been immediately apparent to the ancient Egyptians from its effects on any wildlife that had died in these lakes. 

Egyptian mummy on display at the British Museum

There is some evidence that the ancient Egyptians artificially precipitated natrun by isolating shallow basins of salt lake waters for faster evaporation, as is still done in parts of the Faiyum depression today. For purification and preservation, natrun was preferred over pure halite as it chemically attacks and destroys grease and fat, and so is a superior drying agent (as is sodium borate). Natrun residues are found not only in tombs and in pits, along with other discarded embalming materials, but also forms nodules and residues in the mummies themselves.

There is some popular debate over the method in which the natrun was used for mummification by the ancient Egyptians. Some argue it was used as a desiccant after the removal of organs, in a way similar to the contemporary method for "salting" fish. Dry natrun would be sprinkled over the body, perhaps with sawdust as an absorbent. After drying the body was wrapped in linen.

Others with a more starry-eyed bent, believe that pre-wrapping the body was immersed in vats containing a natrun solution and coated with bitumen. This now-debunked theory comes from studies such as the1821 work on the "Granville mummy," held in the British Museum. This study documents a famous unwrapping and dissection of a mummy by Augustus Granville, during the peak of mummy mania in 1821. Granville had deduced that 'the body must have been plunged into a vessel containing a liquefied mixture of wax and bitumen and kept there for some hours or days, over a gentle fire'. Such a wet method would have been odiferous and accelerated putrefaction, thus counterproductive to the preservation of the body, although it makes for good Hollywood images.

Later, 20th Century studies using GC-MS found no evidence for beeswax or bitumen but instead found degradation products from conifer resin acids. We now know that post-desiccation a combination of oils and linens were used or wrap and preserve. The basic recipe involved a combination of 1) plant oil - possibly sesame oil; 2) "balsam-type" plant or root extract that may have come from bullrushes; 3) plant-based gum - a natural sugar that may have been extracted from acacia; crucially, a conifer tree resin, which was probably pine resin. When mixed into the oil, that resin would have given it antibacterial properties, protecting the body from decay. A dry body (post-natrun) was more readily bandaged, as well as being more amenable to the attachment of amulets and other jewellery. Although mummification has supernatural trappings in popular culture and ancient religions, its basis is rooted in simple chemistry and processes as mundane as salting fish.

Ancient Egyptians believed in an afterlife where if your body rotted or was damaged after death your spirit would suffer, hence the practise of funereal mummification

The mummies of some Buddhist monks (Sokushinbutsu sect) in Japan resulted from the practice of nyūjō, which ultimately aimed to cause their own death and mummification by encasement in salt. This ritual took years to complete and involved starvation and dehydration. During the first three years, an ascetic monk significantly decreased his body fat by eating only nuts, seeds, and berries, while he increased his physical activity. Towards the end of the ritual, the monk reduced his food intake even further by only consuming bark, roots, and sometimes stones. Post-mortem preservation was further aided by the consumption of toxic herbs and tea that eliminated bodily fluids and killed bacteria that aid in decomposition. Japanese Sokushinbutsu monks were known to drink a tea made from the urushi tree, also known as the Chinese lacquer tree because it’s sap is used to lacquer tableware, instruments, and jewellery.

After years of starvation and dehydration, when the monk felt like he was close to death, his fellow monks arranged his body in the lotus position inside a coffin or a tomb. Then they surrounded the dying man with salt, wood, paper, or lime to pull more moisture away from the body and further prevent post-mortem decay. Only a small opening for air was allowed when the tomb was closed. The monk then chanted, meditated and occasionally rang a bell until he died.

A Sokushinbutsu sect monk on public display in a glass case and well along the salty “highway to heaven.” The sunglasses were added well after the mummified body was disinterred from its salty surround. Its hollow eye sockets were scaring children visiting the temple.

Mummification can occur naturally if a body is encased in halite, and natrun is not necessary, although it improves preservation. In 1593 AD, and again in 1616 AD, several tombs encased in salt were exposed by natural salt weathering and collapse in the Hazel Mountains. When the coffins were opened by the local people of Hallein and Hallstatt, there was astonishment that the bodies inside had very well preserved soft tissues. It was the result of the hyperarid encasement in a Neolithic salt mine, but frightened religious locals, encouraged by the local clergy, insisted on prompt reburial, along with additional religious efforts to lessen ambient sin levels (in part in the form of alms to mother church) and hence more prayer to create more effective seals. There was a similar popular response in 1734 AD when the salt-preserved body of a man wearing mountain clothing (likely a salt miner) was discovered. Fearful locals, once again encouraged by the local clergy, insisted on immediate reburial with no further scientific study or observations on the remains (Aufderheide, 2011).

In Iran, first in the winter of 1993 and later in 2004, in the modern Chehr Abad Salt Mine, near Hamzehloo, Zanjan Province, a total of five salt-preserved male bodies were found in a collapsed tunnel of a former salt mine, which was active around 400 BC.

The first discovery in the winter of 1993 was a salt-encased bearded head and some artefacts, the later discovery, beginning in November 2004, was of the remaining bodies. All five men likely died in an earthquake-induced collapse in the salt mine (Pollard et al., 2008). Encasement in the hyperarid atmosphere of the collapsed salt mine tunnel led to natural mummification of the bodies.

Isotopic analysis on the five salt-preserved bodies from the salt mine date from the 4th Century, BC through to the 4th Century AD (Ramaroli et al., 2010). Faunal remains associated with the remains show the  ecosystem is enriched in 15N, due to the semi-arid nature of the region. It seems that two of the "mummies" came from the Tehran/Qazvin Plain region (i.e., relatively local to the salt mine), and a further two appear on isotopic grounds to have come from the northeast of Iran or the Turkmenistan steppes. The fifth, the best preserved, appears to have come from further afield. Collectively, these mummies indicate social mobility and technological innovation associated with the salt industry in Iran during the Achaemenid period. 

Salt-preserved miners in the Douzlakh Salt Mine NW Iran. A) Salt man No 4. as he was discovered in 2004 in trench 4 (back view). B) Frontal view of salt man No.4. C) Reconstruction of a salt miner based on all 4 mummified bodies found at the site (see Öhrström et al, 2015 for radiological study of damage to miner No. 4 created by the roof collapse).