Sotah, chapter two, mishnah 4. In this mishnah we will discuss the types of material on which the scroll may be written, and the type of ink that may be used to write it.
The materials on which the scroll may not be written:
"lo al haluach" - not on a tablet, which is made of wood.
"v'lo al haniyar" - not on paper, the paper they were accustomed to producing from grass, and even paper produced from rags.
"v'lo al hadiftera" - not on diftera, an animal hide that was not properly processed, that is, a hide whose processing into parchment was never completed.
"ela al hamegillah" - only on parchment, as it says "basefer" - "in a scroll," and in the language of the Torah this means writing on parchment.
The types of ink that are invalid:
"lo b'kumus" - not with kumus, a kind of sticky resin, and it may be said that this is gum arabic.
"v'lo b'kankantom" - not with kankantom, the substance known as vitriol, which is in fact copper sulfate.
"v'lo b'chol davar sherosheim" - not with anything that leaves a permanent mark, meaning any permanent ink, that is, ink that is absorbed into the parchment and cannot be erased.
The valid ink:
"ela b'deyo" - only with deyo, ink made from the ash or the charcoal of vines, of grapevines, which can be blotted out.
The reason for this is stated explicitly in the verse: "u'machah" - "and he shall blot out," meaning it must be possible to blot the writing into the water, writing that can be erased. Therefore one may use only that substance called deyo.
In summary: in this mishnah we learned that the scroll is not written on a tablet, on paper, or on diftera, but only on parchment, as it says "basefer." So too it is not written with kumus, with kankantom, or with anything that leaves a mark and cannot be erased, but only with ink made from the ash of vines - all because of the requirement of "u'machah," that the writing must be capable of being blotted out into the water.