“Put it on the plaster” – people have been able to do that for at least 12,000 years. In any case, finds on the east coast of the Mediterranean, in the Levant, indicate that walls were already plastered with a mixture of quicklime and water. But the mix didn’t last more than a few decades there or here.
The Maya in Central America, on the other hand, added an ingredient to their mortar more than 3000 years ago that made the plaster particularly resistant and which has now been identified by a research team led by Carlos Rodriguez-Navarro from the University of Granada, Spain. It made sure that the plaster “withstood the aggressive, hot and humid tropical weather for more than 1,200 years,” the researchers write in the journal “Science Advances“.
Plaster that releases fewer ions
The fact that the ingredient could be part of trees goes back to reports from the Bishop of Yucatán. Diego de Landa wrote in the 16th century that the lime plaster on the roofs of the Maya was made more malleable and resistant with a liquid made from tree bark. In fact, Mayan stucco contains residues of biomolecules found in the sap of the American Balsam and Chukum trees.
Carlos Rodriguez-Navarro’s team has now examined up to 1400-year-old pink stucco from a temple and from the interior walls of a building in the Mayan city of Copan in present-day Honduras. Although it consisted of 95 percent of the typical crystals of lime plaster, there were also embedded organic particles – a structure similar to the hard shells of mussels or the spines of sea urchins.
“Following advice from local people descended from ancient Maya civilizations, we squeezed sap from the bark of Chukum and American Balsam trees that grow in the area around what remains of the Maya city of Copan,” reports the group led by Rodriguez- Navarre. When these polysaccharide-rich extracts were mixed into the quicklime during slaking, a plaster was formed with a structure similar to that from the Copan Temple. Without this additive, on the other hand, a lime plaster of low durability was created.
An experiment showed that the biomolecules apparently slow down the weathering of the material: The electrical conductivity of water in which the researchers placed samples of the Mayan plaster rose much more slowly than in water containing conventional plaster. This means that Mayan plaster releases fewer ions, so it weathers less quickly because of the bark extracts.
“Because some of the samples from the old Maya buildings were not mixed with any plant extracts, the bark juices were probably only used for plaster intended for particularly important or heavily used areas such as murals, stucco work or floors,” suspects Carlos Rodriguez’s team -Navarro. With the help of the rediscovered recipe, the Mayan relics could now be better protected from decay. Quite a reason to literally “smash it”: to celebrate exuberantly.
