A marble head belonging to a centaur statue from the ancient Greek Parthenon has a substance on it that continues to stump scientists.
The head, which has been housed in Denmark since 1688, is partly coated with a thin brown film that matches similar coatings found on parts of the Parthenon.
It’s unlikely to have a biological origin, but researchers still don’t know what might have caused it. The study is published in Heritage Science.
“There have been many attempts to explain the peculiar brown film,” says Kaare Lund Rasmussen, an emeritus professor specialising in archaeometry and analytical chemistry at the University of Southern Denmark.
“In 1851, German chemist, Justus von Liebig, performed the first actual scientific investigation and determined that the brown film contained oxalates – salts of oxalic acid.
“This has been confirmed by later analyses, but the origin of the oxalates has remained a mystery.”
Rasmussen and his colleagues have used several microscopic and chemical analysis techniques to learn more about the film.
“We especially wanted to examine whether the brown film could have been formed by some biological organism, such as lichen, bacteria, algae, or fungi,” he says.
“This theory had been suggested before, but no specific organism had been identified. The same goes for the theory that it could be remnants of applied paint – perhaps to protect or tone the marble surface.”
The researchers examined 5 samples taken from the centaur head in 1999, each less than a square millimetre in size. As well as optical and electron microscopy, they used chemical techniques such as chromatography, X-ray diffraction and mass spectrometry to identify the elements and molecules present.
“We found no traces of biological matter in the brown layers – only from our own fingerprints and perhaps a bird egg that broke on the marble in ancient times,” says Rasmussen.
“This doesn’t prove that there never was a biological substance, but it significantly reduces the probability, making the theory of a biological organism less probable now.”
It also makes it less likely the stain is an ancient paint as these were typically made from things like eggs, milk, and bones – no evidence of these substances were found in the stain.
The researchers did find the film was made of 2 separate layers, each about 50 micrometres (or 0.05mm) thick. Both layers are made of the oxalate-based minerals whewellite and weddellite, but they differ in trace element composition.
This makes it unlikely that the film came from materials leaching out of the statue, or from air pollution.
“As there are two different brown layers with different chemical compositions, it is likely that they have different origins. This could suggest that someone applied paint or a conservation treatment, but since we haven’t found traces of such substances, the brown colour remains a mystery,” says Rasmussen.