Before paper was commonly used to write on, papyrus and parchment were the writing materials of choice. When texts fell out of favor or use, the base material was too precious to throw away. Instead, it was recycled. The recycling means that many texts that might have ended up in a landfill may still be accessible to us today.
Before the advent of industrially produced paper, papyrus and parchment were the chosen carriers for knowledge and information. The production of papyrus was dependent on the harvest of the plant of the same name growing in the Nile Delta in Egypt. Parchment could be produced from any kind of animal hide available. Parchment (Latin pergamentum) was named by the Romans after the Kingdom of Pergamon in modern Turkey, where the highest quality material of the time was produced.
While leather is produced in a tanning process, parchment is produced by liming. The result is a very even writing surface; depending on the quality of the production, the hair and the meat side of the parchment are almost identical. The Romans appreciated parchment made from calfskin above all others, calling it vellum. While continental European languages apply the term to calfskin exclusively, English broadened its use to mean any high quality parchment made from calf, kid, or lamb. High quality vellum is made from the thinner skins of young animals, while the cheaper quality parchment is made from the hides of older beasts which have been split to make them thin enough to be pliable.
The word palimpsest derives from Greek and is composed of the Greek words palin (again) and psaein (to scratch). The Romans called a reused parchment or papyrus document a codex rescriptus (rewritten book or text); Cicero on the other hand seems to refer to the reuse of wax tablets when using the word palimpsest. Wax tablets were the Romans notepads with wax applied to a wooden base; notes could be scratched into the wax and later be smoothed over easily for reuse.
The expression palimpsest was received into the European languages whereas codex rescriptus was not. While the word superficially might seem to refer to the scratching off of the old text from the parchment, it actually refers to the process of rewriting it as used by Cicero when re-scratching into wax. A palimpsest therefore refers only to cleaned and rewritten documents and not to merely cleaned parchment or papyrus that has not been reused.
There were various reasons for reusing older material; budget considerations were not the least of them. Reuse by reason of shortages and for cultural and religious reasons are also documented. Instead of just burning the offensive material, reuse was a more economical way of getting information deleted. A famous example for such a use is the Codex Ephraemi Rescriptus; in the 12th century, the treatises of Ephraem of Syria were written over the text of an Egyptian Bible of the fifth century that was not in keeping with what the Church wanted to tell the faithful.
There were several ways of getting rid of unwanted texts on these documents. One way was obviously to carefully scratch off the ink. This method was mostly used on papyrus. With parchment, there was a way to wash ink off; and quite early a chemical solution in the form of citric acid came into general use. All these methods left traces on the writing material which modern science is able to decipher. By the 14th century, it was common practice to use powdered pumice to abrade the parchment surface; the treatment effectively deletes any traces of prior content.
19th century researchers used gall and later ammonium bisulphate to make erased writing visible again, often with destructive results. Modern techniques include ultraviolet exposure and multispectral imaging processes. Multispectral imaging means the exposure of documents to various light spectra and the superimposition of the resulting pictures into a single image. One of the latest techniques is X-ray fluorescence imaging which reveals the iron contained in ink.
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