Klaf

Since the innovative ruling of Rabbeinu Tam (12th century Tosafist) it is primarily used to refer to parchment or vellum.

During Talmudic times, salt water and barley (or flours) were sprinkled on the skins which were then soaked in the juice of afatzim (gall nuts, or oak apples, and other tannins).

Some parchment (usually poor quality) is smeared with log, a chalky substance, to make it whiter.

Rabbi Abraham ben Isaac of Narbonne, in his day (early 12th-century), wrote the following account in his Questions & Responsa: Our Torah scroll, even though it is not made in accordance with the halakha, since it is written upon klaf called [in Arabic] req (= parchment), which is neither treated with [barley]-flour nor tanned with tannins, they (i.e. the Jewish community at large) have already relied upon its use owing to the extenuating circumstances, for 'it is a time for God to act, [so as to avoid] their cancelling of Your Divine Law.'

They are [obligated to] stand-up before it, and observe concerning it the sanctity prescribed for the Book of the Law (Torah), and it is forbidden to hold-on to it without the intermediate handkerchiefs.

Today there is a large amount of klaf processed under rabbinical supervision, and the variety, quality, and quantity are increasing.