Methods of removing the ancient texts
Physical erasure was one of the common methods for textual correction. It is commonly assumed that ancient scripts were similarly erased from the surface of the parchment with a pumice stone. This is an incorrect assumption however; instead, more often than not, a gentler method was used to wash off the unwanted text from the parchment and make it again ready for new use. We can imagine the actual process from a recipe which remained in a codex that was copied in Tegernsee (Germany) in the tenth century (Munich, Bayerische Staatsbibliothek, Clm 18628, f. 105v).
Munich, Bayerische Staatsbibliothek, Clm 18628, f. 105v
Quicunque in semel scripto pergameno necessitate cogente iterato scribere velit, accipiat lac inponatque pergamenum per unius noctis spacium. Quod postquam inde sustulerit, farre aspersum, ne ubi siccare incipit, in rugas contrahatur, sub pressura castiget quoad exsiccetur. Quod ubi fecerit, pumice cretaque expolitum priorem albedinis suae nitorem recipiet [cited by Wattenbach, Das Schriftwesen, p. 303].
Whoever might need, for whatever reason, to write on a parchment sheet which had already been written, should take some milk and should put the parchment in it for one night’s time. As soon as it is taken out, it should be strewn with flour in order that it not be wrinkled after it begins to dry, and so as to be kept under pressure until it dries out. After it is done, the parchment will regain its former quality, shining and lucid, by means of pumice stone [which was often not used] and chalk.
Thanks to the non-intrusive erasure, the underlying texts can be recovered and deciphered. When the text was physically erased by means of a more invasive method by thinning the parchment itself, the lower text cannot be recovered at all or only to a limited extent. Recycled documents were often erased in this way. Invasive erasure is more frequent in later medieval Latin manuscripts in the West than in the East. Older texts and palimpsests copied in the East (Christian Palestinian Aramaic, Armenian, Coptic, Ethiopic, Greek, Church Slavonic, Syriac, etc.) tend to be rewashed by means of gentler methods.
It seems, judging from the orientations of the upper and lower texts, that the purpose of erasure was not to remove the former writing completely but just to clear off the surface adequately enough for the new script. For this reason, scribes often were careful enough to turn the pages at a 90 or 180-degree angle, or write the upper text between the lines of the lower text or use parchment with faded graphic elements that differ from the new script, just to avoid overlapping scripts which make reading difficult. This tendency helps to decipher the lower texts. Unfortunately, often the two scripts completely overlap each other and this is the major obstacle which is very difficult to surpass even with the latest technical methods.
Urb. gr. 154, f. 128v (and f. 128v rotated 90° ac with ultraviolet illumination)
Pal. lat. 24, f. 46r (and f. 46r rotated 180° with ultraviolet illumination)