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Klaf or Qelaf (Hebrew: קְלָף) is the designation given a particular piece of skin. The Talmudic definition includes both the form of the skin and the way it is processed, in particular, that it must be tanned. Since the innovative ruling of Rabbeinu Tam (12th century Tosafist) it is primarily used to refer to parchment or vellum. It is one of the materials upon which a sofer writes certain Jewish liturgical and ritual documents.
Klaf is a specially prepared, tanned, split skin of a kosher animal—goat, cattle, or deer. Rabbinic literature addresses three forms of tanned skin: gevil, consisting of the full, unsplit hide; and klaf and duchsustus which are the two halves of the full hide. The rabbinic scholars are divided upon which is the inner and which is the outer of the two halves. Maimonides is of the opinion that klaf was the inner layer and that duchsustus was the outer layer  The Shulchan Aruch rules in the reverse that klaf was the outer layer and that duchsustus was the inner layer 
The legally required method of cleaning and preparing klaf has been altered over the centuries. During Talmudic times, salt water and barley (or flours) were sprinkled on the skins which were then soaked in the juice of afatsim (gall nuts, or oak apples). Nowadays, most processors dip the skins in clear water for two days and then soak them in limewater for nine days to remove the hair. When it is a hairless surface, the scribe stretches it on a wooden drying frame and scrapes it until it is dry. Creases are ironed out with presses. Then it is sanded until it becomes a flat, smooth sheet fit for writing. The reasons for the change in this process are lengthy and controversial. Today, a few Jewish scribes still prepare klaf in precise accordance with the Jewish Law.
Some parchment (usually poor quality) is smeared with log, a chalky substance, to make it whiter. Occasionally this is only done on the reverse. Some scribes object to the use of log as it forms a barrier between the ink and the parchment.
In Talmudic times klaf was primarily used for tefillin and at times mezuzot; since the 9th or 10th centuries it has become more widespread to write Sifrei Torah on klaf; however, even today, there are still groups who continue to adhere to the ancient prescription described in the Talmud, and continue to write on gevil.
The parchment must be prepared "for the sake of use for the Divine act) and the processor must declare what he is preparing it for, as one cannot use klaf destined for a lesser holiness (kedusha kallah) - e.g. a mezuzah to write tefillin or a Sefer Torah, which are of weightier holiness (kedusha chamurah). If necessary, the scribe should state that he is preparing for the sake of a Sefer Torah but that he may change his mind if he wishes. Some Rabbinic scholars hold that , a non-Jew may prepare it. However, a Jew must stand over him, directing him in his work and stating verbally that the preparation is for the sake of heaven.
Today there is a large amount of klaf processed under rabbinical supervision, and the variety, quality, and quantity are increasing.
- "Mishneh Torah, Tefillin, Mezuzah and the Torah Scroll 1:7". www.sefaria.org. Retrieved 20 December 2022.
- "Shulchan Arukh, Orach Chayim 32:7". www.sefaria.org. Retrieved 20 December 2022.