A palimpsest is a manuscript scraped clean and written on again, so that the earlier text still shows through where the second writer pressed hard, or where the erasure failed. Monks called it a gift: more text on a smaller page, and a way of keeping a secret in plain sight.
Herein the brother who holds the purse shall set down each sum at the hour it is counted, and beside it the name of the house from whence it came, so that the sums may be compared at the Michaelmas audit.
Item: a small casket of oak and silver, taken from the chapel of Saint Æthelwold, and not returned. The brother who knows of this is enjoined to say nothing, which is a heavier thing to carry than a casket.
The ink is very bad. Whatever is written here will not outlast the page, and the page is already older than anyone who has read it. Write large. Write often. Do not bother with a fair copy.
Scrub with the pointer. The layer underneath is not stored anywhere except in the pixels you uncover.
When a monastic scribe erased a page, the knife did not take the parchment with it. It flattened the fibres and left the ink sunk in the skin of the leaf, and under raking light or ultraviolet the earlier writing comes back — not perfectly, but enough.
What survives is the part the second writer pressed hardest against, and the part that happened to be in the shadow of the erasure. It is an accidental archive: the parts of the erased text that somebody tried hardest to destroy are the parts we can still read.