Ancient grave goods: Rare silk from the peacock moth

Manufacturing silk has a long tradition in China: As archaeological finds attest, the art of processing the webs of butterfly caterpillars into textiles dates back to the Neolithic period.

Chinese silk producers later succeeded

in breeding a domesticated version, the silk

.

Although silk remained a luxury good, it could become an export hit.

Some of the trade route with caravans led along the legendary Silk Road through deserts and mountains, while some Chinese silk was mainly sent west by sea.

This route passed through Sri Lanka or along the coast of the Indian subcontinent, where the peacock

is native.

A team of chemists and archaeologists from the University of Oxford, who examined fragments of silk fabrics from Palmyra in Syria, discovered that its silk was also exported to the Middle East.

Founded in the third millennium BC and declared a UNESCO World Heritage Site in 1980, the former oasis city of Palmyra made sad headlines in May 2015: members of the terrorist organization "Islamic State" looted the art treasures and destroyed a unique ensemble of ancient architecture.

However, researchers can continue to study the rich inventory unearthed by archaeologists between 1930 and 1990 from the up to five-story burial towers of Palmyra.

The archaeological highlights that came to light there include more than two thousand remains of various textiles.

Based on the grave inscriptions, they could be classified between the first century BC and the second century AD.

Examination under a microscope revealed that some of these colorful textile fragments are made of

silk , imported from China and then processed in Syria.

A number of suspected silk fabrics, on the other hand, could not be unequivocally assigned based on their microstructure.

Great tensile strength and stability

Another method had to be developed in order to be able to identify the poorly preserved fiber samples with certainty.

Scientists working with Boyoung Lee and Elisabete Pires therefore took samples from three silky-appearing textile fragments from Palmyra in order to examine them biochemically.

In addition to the domesticated silk moth

and its wild form, five moths from the peacock moth family that supply or have supplied so-called wild silk in the past were also included in this study.

It is common for many butterfly species that the caterpillars spin a protective cocoon in which they pupate and transform into a moth.

The sequence of the amino acids in the fiber proteins is known from the few whose cocoons can be used to produce silk.

While these long protein molecules, called fibroin, leave the spinning glands in the caterpillar's head, they align themselves in parallel and form quasi-crystalline areas in their three-dimensional structure.

This gives them great stability and tensile strength.

Silk can absorb up to a third of its weight in moisture without looking wet.

However, it is neither soluble in water nor in common solvents such as ethanol.

The fibrous proteins of silk moths, which do not belong to the genus

, could only be dissolved at temperatures of over 100 degrees with the help of special salts.

Using an enzyme cocktail of trypsin and chymotrypsin, the researchers then succeeded in comminuting the fibroin in such a way that the various silk moths each provided characteristic fragments.

As Boyoung Lee and his colleagues report in the "Scientific Reports", they then used nanoflow liquid chromatography in combination with tandem mass spectrometry to analyze silk from the cocoons of different silk moth species.

When the archaeological samples from Palmyra were subjected to the same procedure, all three textile fragments were confirmed to be made of silk.

Not made from silk of Chinese origin, however, but from wild silk from the peacock

, also known as the Indian tussah silk moth.

This majestic moth, whose wings bear magnificent eyespots, is mainly found in the east of the Indian subcontinent.

In some places its cocoons are still used today for the production of wild silk.