In textual studies, a palimpsest is a manuscript page, either from a scroll or a book, from which the text has been scraped or washed off so that the page can be reused for another document. Parchment was made of lamb, calf, or goat kid skin and was expensive and not readily available, so in the interest of economy a page was often re-used by scraping off the previous writing. In colloquial usage, the term palimpsest is also used in architecture, archaeology and geomorphology to denote an object made or worked upon for one purpose and later reused for another, for example a monumental brass the reverse blank side of which has been re-engraved.
The word "palimpsest" derives from the Latin palimpsestus, which derives from the Ancient Greek  (palímps?stos, from + = "again" + "scrape"), a compound word that describes the process: "The original writing was scraped and washed off, the surface resmoothed, and the new literary material written on the salvaged material." The Ancient Greeks used wax-coated tablets, like scratch-pads, to write on with a stylus, and to erase the writing by smoothing the wax surface and writing again. This practice was adopted by Ancient Romans, who wrote (literally scratched on letters) on wax-coated tablets, which were reusable; Cicero's use of the term "palimpsest" confirms such a practice.
Because parchment prepared from animal hides is far more durable than paper or papyrus, most palimpsests known to modern scholars are parchment, which rose in popularity in Western Europe after the 6th century. Where papyrus was in common use, reuse of writing media was less common because papyrus was cheaper and more expendable than costly parchment. Some papyrus palimpsests do survive, and Romans referred to this custom of washing papyrus.[note 1]
The writing was washed from parchment or vellum using milk and oat bran. With the passing of time, the faint remains of the former writing would reappear enough so that scholars can discern the text (called the scriptio inferior, the "underwriting") and decipher it. In the later Middle Ages the surface of the vellum was usually scraped away with powdered pumice, irretrievably losing the writing, hence the most valuable palimpsests are those that were overwritten in the early Middle Ages.
Medieval codices are constructed in "gathers" which are folded (compare "folio", "leaf, page" ablative case of Latin folium), then stacked together like a newspaper and sewn together at the fold. Prepared parchment sheets retained their original central fold, so each was ordinarily cut in half, making a quarto volume of the original folio, with the overwritten text running perpendicular to the effaced text.
Faint legible remains were read by eye before 20th-century techniques helped make lost texts readable. To read palimpsests, scholars of the 19th century used chemical means that were sometimes very destructive, using tincture of gall or, later, ammonium bisulfate. Modern methods of reading palimpsests using ultraviolet light and photography are less damaging.
Innovative digitized images aid scholars in deciphering unreadable palimpsests. Superexposed photographs exposed in various light spectra, a technique called "multispectral filming", can increase the contrast of faded ink on parchment that is too indistinct to be read by eye in normal light. For example, multispectral imaging undertaken by researchers at the Rochester Institute of Technology and Johns Hopkins University recovered much of the undertext (estimated to be more than 80%) from the Archimedes Palimpsest. At the Walters Art Museum where the palimpsest is now conserved, the project has focused on experimental techniques to retrieve the remaining text, some of which was obscured by overpainted icons. One of the most successful techniques for reading through the paint proved to be X-ray fluorescence imaging, through which the iron in the ink is revealed. A team of imaging scientists and scholars from the United States and Europe is currently using spectral imaging techniques developed for imaging the Archimedes Palimpsest to study more than one hundred palimpsests in the library of Saint Catherine's Monastery in the Sinai Peninsula in Egypt.
A number of ancient works have survived only as palimpsests.[note 2] Vellum manuscripts were over-written on purpose mostly due to the dearth or cost of the material. In the case of Greek manuscripts, the consumption of old codices for the sake of the material was so great that a synodal decree of the year 691 forbade the destruction of manuscripts of the Scriptures or the church fathers, except for imperfect or injured volumes. Such a decree put added pressure on retrieving the vellum on which secular manuscripts were written. The decline of the vellum trade with the introduction of paper exacerbated the scarcity, increasing pressure to reuse material.
Texts most susceptible to being overwritten included obsolete legal and liturgical ones, sometimes of intense interest to the historian. Early Latin translations of Scripture were rendered obsolete by Jerome's Vulgate. Texts might be in foreign languages or written in unfamiliar scripts that had become illegible over time. The codices themselves might be already damaged or incomplete. Heretical texts were dangerous to harbor--there were compelling political and religious reasons to destroy texts viewed as heresy, and to reuse the media was less wasteful than simply to burn the books.
Vast destruction of the broad quartos of the early centuries took place in the period which followed the fall of the Western Roman Empire, but palimpsests were also created as new texts were required during the Carolingian Renaissance. The most valuable Latin palimpsests are found in the codices which were remade from the early large folios in the 7th to the 9th centuries. It has been noticed that no entire work is generally found in any instance in the original text of a palimpsest, but that portions of many works have been taken to make up a single volume. An exception is the Archimedes Palimpsest (see below). On the whole, early medieval scribes were thus not indiscriminate in supplying themselves with material from any old volumes that happened to be at hand.
About sixty palimpsest manuscripts of the Greek New Testament have survived to the present day. Uncial codices include:
Porphyrianus, Vaticanus 2061 (double palimpsest), Uncial 064, 065, 066, 067, 068 (double palimpsest), 072, 078, 079, 086, 088, 093, 094, 096, 097, 098, 0103, 0104, 0116, 0120, 0130, 0132, 0133, 0135, 0208, 0209.