As we saw on yesterday’s daf, if sacrificial blood was spilled on a garment, the Torah requires that it be cleaned and removed. We learned, however, that this only applies to a finished garment, but something that was not considered a keli – a usable object – would not be included in this category. Thus, unprocessed animal skin would not need to be cleaned of sacrificial blood, but leather, which has undergone processing, would need to be cleaned.
The Gemara on today’s daf asks how the law requiring cleansing would apply to leather. Does leather absorb blood in an ordinary fashion? Can it be cleaned of the blood in the same way that a garment is cleaned? The basis for the Gemara’s question are the laws of Shabbat that prohibit washing fabrics, where we find a Mishna that teaches that if clothing became soiled with lashleshet (Arukh: chicken excrement; Rashi: something disgusting like saliva or excrement) it can be removed by means of a dry cloth, but if the garment was made of leather then water can be poured on it until it becomes clean.
In response, Rava points out that it is clear that the halakha recognizes that leather can be washed, inasmuch as it is a clear passage in the Torah regarding a nega tzara’at – biblical leprosy (see Sefer Vayikra 13:58) – that washing leather is part of the purification process. Rava’s conclusion is that we must distinguish between cloth, where simply placing it in water would already be considered “washing” that is forbidden on Shabbat, and leather, where “washing” only takes place if there is additional agitation that enhances the cleansing property of the water, what he calls kiskus.
Rashi explains that kiskus is the activity that is normally done by launderers who rub the material of the clothing against itself (in modern washing machines this is done mechanically). Rabbeinu Ḥananel (in Massekhet Shabbat daf 140a) suggests similarly that it refers to folding and stretching the cloth that is being cleaned. Regarding this action in the context of leather, the She’iltot explains that even if this type of agitation cannot be done to all types of leather, the point is that an additional action is necessary beyond simply placing water on the garment.