הלכות כלים

Whenever the external surface of a k'li that has a receptacle contracts impurity from liquids, its inner side does not contract impurity vis-à-vis terumah. Nor does the outer side of the k'li's neck, its handles, or the fingerhold on the rim of the k'li contract impurity. If the inner portion of the k'li contracts impurity, all of the above components also contract impurity.
If impure liquids fall on the bases of keilim, their rims, their projections, or handles of keilim with receptacles, one should dry them and the receptacle is pure. Even the entire outer surface does not become impure.

The following law applies to keilim made from goats hair, e.g., sacks and carrying bags, leather keilim, e.g., pillows and bolsters, and wooden keilim, even baskets and dispensers. If their external surface contracts impurity from liquids, their inner portion does not contract impurity.

Flat keilim that can be purified by immersion, that are not fit to sit or lie upon, e.g., a table or countertop that does not have a side are not susceptible to impurity according to Scriptural Law. Accordingly, if the external surface of such a k'li contracts impurity from liquids, its inner surface does not contract impurity. If, however, flat keilim that can be purified by immersion are fit to sit or lie upon, e.g., a bed, a hide on which one sleeps, a chair or the like, the distinction between an inner surface and an outer surface does not apply. Instead, whether impure liquids touch their inner surface or their outer surface, the entire k'li contracts impurity.
Similarly, the distinction between an inner surface and an outer surface does not apply with regard to measures for wine and oil, a two-sided cooking k'li, a filter for mustard, and a filter for wine. If impure liquids fell on a portion of the k'li, the entire k'li contracts impurity, as is the law with regard to garments.

A concept paralleling an external surface and an inner surface apply with regard to a plow.
What is implied? If impure liquids fall on the lance, only seven handbreadths of the beam adjacent to the lance become impure and the remainder of the beam is pure. Similarly, if impure liquids fall on the metal peg, only the four handbreadths of the beam that is adjacent to the metal peg become impure. If the liquids fell beyond the seven handbreadths and the four handbreadths, the situation resembles a receptacle whose outer surface was touched by impure liquids, in which instance, only the place where the liquids touched and the outer surface contract impurity.

When there is a pouch with an inner pocket and one of the two contracted impurity due to contact with impure liquids, the other does not contract impurity.
When does the above apply? When their rims are on the same level. If, however, the external one overlaps the inner one, and the inner one contracts impurity, the external one also contracts impurity. If the external one contracts impurity, the inner one does not contract impurity. If the impurity stems from the carcass of a crawling animal, in all instances, if one contracted impurity, the others also did.

When one carves out a measure of a revi'it and half a revi'it into one piece of wood and one of them contracted impurity from impure liquids, the other does not contract impurity even though they are on the same piece of wood.
What is implied? If impure liquids touched the inner surface of the revi'it measure, that measure and its outer side are impure, the half-revi'it measure and its outer side are pure. If the liquids touched the inner surface of the half-revi'it measure, that measure and its outer side are impure, the revi'it measure and its outer side are pure.
When one immerses this piece of wood in a mikveh, it should be immersed in its entirety all at one time. If the outer surface of the revi'it measure or of the half-revi'it measure contracted impurity from liquids, the entire outer surface of the piece of wood contracts impurity, because an outer surface is not divided.

When the outer surface of a kettle was impure and it was boiling, we do not suspect that maybe liquids flowed out from it, touched its outer surface, and then receded back inside it. Instead, the liquids that are inside it are pure with regard to terumah.