Here’s what you’ll learn when you read this story:

• In previous archaeological excavations, Neolithic burials in Southwest Asia have been found with traces of symbolic red ochre and the remains of plaster casts, which are thought to have protected the bodies.
• Researchers recreated these burials, painting three donor bodies in red ochre, then covering one in lime plaster, one in gypsum plaster, and leaving one unplastered.
• As they expected, lime plaster resulted in the best preservation, but the ochre nearly vanished in all three.

Usually, when human remains surface after thousands of years, bones are all that remain. However, mummification—whether total or partial—can result in the preservation of much more of the human body

Around 10,000 years ago, Neolithic peoples in Southwest Asia attempted to mummify their dead by covering the bodies with red ochre, binding them, and coating them in lime or gypsum plaster before burial. That said, many of the finer details of these processes are still unknown to science. Now, with the help of several deceased who had donated their bodies to science, an international team of researchers recreated ancient plaster burials to find out which type of plaster was best at preservation. The results are published in the journal Archaeological and Anthropological Sciences.

“While it remains difficult to define a single type of plaster burial across prehistoric Southwest Asia and understand the intent behind this funerary practice,” the researchers wrote, “this experimental study offers an important opportunity to gain a more dynamic understanding of the early stages of decomposition in the presence of plaster, insights that are not otherwise obtainable from archaeological contexts.”

Plaster can refer to any material made by mixing water with a binder, but those made with gypsum and lime—which can be anything derived from limestone, such as calcium oxide or calcium hydroxide—are some of the most common. Lime becomes quicklime (or calcium oxide) when heated enough to release carbon dioxide, and absorbs moisture. When it reacts with water, quicklime produces a slurry that hardens through carbonation when exposed to CO₂, turning into calcium carbonate. Its durability after it dries is the reason it has been used to protect exterior walls of houses for millennia. Hydrated lime and quicklime are also thought to have antibacterial properties, which is why the dead were often covered in the substances during epidemics.

Gypsum plaster is better known as plaster of Paris (calcium sulfate hemihydrate) when heated. Compared to lime, gypsum plaster is more porous, and breaks down with moisture.

When peoples in Southwest Asia began to shift from hunting and gathering to agriculture, burial grounds began to appear in their settlements. Many Neolithic plastered skulls have been unearthed in the southern Levant (and show signs of postmortem facial recreation done with plaster), but burials in which the deceased was covered completely in unheated gypsum or lime plaster are less common. While the effectiveness of lime on delaying or preventing decomposition in pig cadavers had been tested before, it had never been experimented with on humans.

To model what a Neolithic South Asian burial would have probably looked like, the researchers bound the donor bodies, then painted red ochre onto the hair, scalp, and upper arms. They then covered one body in hydrated lime and another in gypsum casting plaster ( the third donor was used as a control). After five years, the bodies were exhumed from their burial pits.

The body that had gone unplastered had mostly skeletonized. Only crumbling pieces of tissue were still clinging to the bones, and the hair (on which ochre was still visible) had detached from the scalp. Hardly any ochre was visible on the rest of the skeleton or on the surrounding sediment.

Plaster made from gypsum had formed a hard cast around the second body, and ochre stains and impressions of facial and body features, hair, and binding rope were visible inside the cast. The body was found partly skeletonized, with the scalp and hair detached. While ochre was still visible in the hair, it could no longer be seen on the rest of the remains, and termites had nested in some of the bones.

The lime plaster was the most effective. Lime also formed a hard cast around the body, and the inside showed impressions much like those found in the gypsum cast. But despite having partly skeletonized, this donor still had patches of skin covering large areas, including the head, fingers, torso and abdomen. The scalp and hair were detached, but preserved, and there was still ochre detectable on the hair (but not on the skull or body).

This experiment showed the effectiveness of both lime and gypsum plasters in ancient burials. Both the lime and gypsum plasters resulted in limited interactions between the bodies and the soil, and were found to act as bacterial barriers. Despite lime and gypsum increasing soil acidity (which is supposed to invite bacteria) both slowed decomposition, and lime ultimately proved to be tougher and more resistant to water. Interestingly, ochre burials were difficult to detect, which directly contrasts with ancient burials in which red ochre—which is thought to have symbolized life, blood, and rebirth—has been somehow preserved over thousands of years.

“The presence of ochre in this study furnishes an important example of the effect of time,” said the researchers. They added that their findings “also raise questions about the quantities of ochre used in prehistoric burials, as they are discovered with substantial amounts still present over thousands of years later.”