According to most views of Jewish Law, a sefer Torah (scroll) should be written on gevil as was done by Moses for the original Torah scroll he transcribed. Further, a reading of the earliest extant manuscripts of the Mishneh Torah indicate that gevil was halakha derived from Moses and thus required for Torah scrolls.
Maimonides wrote that it is the law transmitted to Moses on Mount Sinai that a Torah scroll must be written on gevil or alternatively on klaf and that scrolls written on an alternative material are invalid for use, however it is preferable that they be written on gevil (Maimonides, Hilkhoth Tefillin 1:14).
Definition of gevil
Gevil is a form of skin made from the whole hide, after the hair is removed. The precise requirements for processing gevil are laid by the Talmud, Geonim and Rishonim. They were reconfirmed as "the law according to Moses" by Maimonides, in his Mishneh Torah. According to law, the preparation of gevil hide must include salt, flour and mey afatzim (wasp residue/gall-nut water). Gall nuts—rich in tannic acid—are a tree's reaction to an invasive parasitic wasp's egg; the pure black tint of the ink used on Torahs results from the reaction between the tannic acid and iron sulfate (a powder used to make the ink).
There are three forms of skin known to Jewish law. The other two qualities result from splitting the hide into two layers; however, there is some confusion about their identification. Others deviate from this process, and use modern chemical processes. However, some believe that this invalidates the parchment for scribal use.
According to the Halakhot Gedolot, klaf is the inner layer, adjacent to the flesh, while dukhsustos is the outer layer, on which the hair grows. The same view is expressed in the oldest extant manuscripts and critical editions of Maimonides' Mishneh Torah and the Babylonian Talmud. This is also the same definition which appears in the minor Talmudic tractate called Sofrim. However, more recent authorities reverse the two descriptions, and many printed editions of the Mishneh Torah are "adjusted" to reflect this. The reason for this original adjustment away from the original definition is a mystery. Some suspect that copying errors are to blame. As a result, many have become confused, in terms of which part of the skin should be used for writing. Using the full hide known as gevil for Sifrei Torah does avoid this issue, unfortunately this solution won't work for tefilin which must be written on Klaf and are not kosher if written on gevil.
Uses of gevil
According to the Talmud (Tractate Bava Batra 14b and Gittin 54b), gevil existed during the time of Moses (approximately 1280 BCE); Moses is described as using gevil for the Torah scroll he placed into the Holy Ark of the Covenant. Elsewhere in the Talmud (Tractate Gittin 54b), there is testimony that Torah scrolls were written on gevil.
Today, a handful of Jewish scribes and artisans continue to make scroll material in this way. However, the majority of Torah scrolls are written on klaf, in their belief that the Talmud recommends (as opposed to requires) gevil and relates to the optimal beautification of the scrolls rather than an essential halachic requirement. Given the uncertainty about which layer of the hide is in fact the klaf, there is a growing movement for insisting on a return to gevil in Torah scrolls - in order to avoid all doubts. Clearly, the antiquity of gevil (as the original practice) is not being debated by anyone.
- The Gevil Institute: Machon Gevil The only online organization dedicated to the preservation of gevil.