What Was Written Under That Prayer Book? Somebody Scraped Off Archimedes

Parchment is made from animal skin and a single substantial book consumed a herd, which made it far more expensive than the words on it in most circumstances. When a text became unwanted, obsolete or unreadable, the practical response was to scrape or wash the surface and reuse the sheet rather than to prepare a new one. The practice was routine in monastic libraries from roughly the seventh century onwards, and it accelerated whenever demand for books outran the supply of skins or when the older text was in a language nobody local could read.

Erasure was never complete and several traces remain:

• •Iron gall ink bites into the surface and cannot be fully removed
• •Scraping leaves faint depressions where the letters were
• •Chemical residues remain in the skin even where colour is gone
• •The old writing frequently runs at right angles to the new
• •Sheets were cut and refolded, so lines run across the new page
• •Ageing can darken the old ink back into faint visibility

Recovering the lower text used to involve chemical reagents that damaged the manuscript permanently, and modern methods are entirely non-destructive. Photographing the page under many separate wavelengths from ultraviolet to infrared exploits the fact that the two inks respond differently outside the visible range, and processing the resulting stack of images by computer separates one layer from the other. X-ray fluorescence maps the iron in the older ink directly. Raking light reveals the depressions. Combining these, text invisible to the eye for a millennium can be read reliably, and the work is now routine in major collections.

The word travelled out of manuscript studies and is now applied to anything showing traces of what it replaced, which is a useful metaphor rather than a loose one. Geologists use it for landscapes where an older drainage pattern or field system is still legible under a later one. Archaeologists use it for city plans in which a Roman street grid survives in the property boundaries of a modern town. Writers use it for texts that visibly carry an earlier work beneath them. In each case the point is the same, that erasure is rarely complete and the earlier layer constrains what was written over it.

The most famous case is a thirteenth century prayer book in which the underlying text turned out to be tenth century copies of several works of Archimedes, including the only surviving copy of a treatise showing that he used methods close to integral calculus and considered actual infinities. It was identified in 1906, disappeared, resurfaced at auction in 1998 and was imaged over more than a decade. Other recoveries include lost portions of Cicero, early biblical and Quranic texts, and works in languages with almost no other surviving witnesses, which is why the technique matters far beyond any single manuscript.

Parchment cost more than the words on it, so unwanted texts were scraped away and the sheets reused, usually incompletely because iron ink bites into the skin. Imaging across many wavelengths and mapping the iron directly now recovers the lower layer without damage. A prayer book treated this way preserved the only copy of an Archimedes treatise approaching integral calculus.