In May 1898 Italian photographer Secondo Pia was allowed to photograph the shroud. Pia was startled by the visible image of the negative plate, implying that the shroud is effectively a negative of some kind.
A variety of methods have been proposed for the formation of the image, but the actual method used has not yet been conclusively identified.
Despite numerous investigations and tests, the status of the Shroud of Turin remains murky, and the nature of the image and how it was fixed on the cloth remain puzzling.
The origins of the shroud and its images are the subject of intense debate among theologians, historians and other researchers.
Diverse arguments have been made in scientific and popular publications claiming to prove that the cloth is the authentic burial shroud of Jesus, based on disciplines ranging from chemistry to biology and medical forensics to optical image analysis.
A burial cloth, which some historians maintain was the Shroud, was owned by the Byzantine emperors but disappeared during the Sack of Constantinople in 1204.
Although there are numerous reports of Jesus' burial shroud, or an image of his head, of unknown origin, being venerated in various locations before the 14th century, there is no historical evidence that these refer to the shroud currently at Turin Cathedral.
A drop of molten silver from the reliquary produced a symmetrically placed mark through the layers of the folded cloth.
Poor Clare Nuns attempted to repair this damage with patches.
Secondo Pia's 1898 negative of the image on the Shroud of Turin has an appearance suggesting a positive image.
It is used as part of the devotion to the Holy Face of Jesus. The shroud is rectangular, measuring approximately 4.4 by 1.1 metres (14 ft 5 in × 3 ft 7 in).
The cloth is woven in a three-to-one herringbone twill composed of flax fibrils.