מגופת החבית

The stopper of a barrel

that is loose

but does not fall off,

Rabbi Yehuda says

it protects,

and the Sages say

it does not protect.

If it's finger-hold (this was an indentation in the top of the stopper)

was sunken into it (i.e. the indentation was so deep that it's bottom was, while still inside the stopper, below the top of the barrel and therefore inside its airspace),

if a sheretz is inside it

the barrel is tamei,

if the sheretz is in the barrel

food that is in it (the indentation) is tamei. (Note that the stopper is not considered a utensil, as otherwise this rule would be different; see 8:2.  )

פרק י משנה ד

A ball (which was made of pieces of cloth)

or a coil of reed-grass

that were placed over the mouth of a barrel (to serve as a stopper),

if one plastered from the sides (where the ball or coil come into contact with the opening of the barrel, which would be sufficient to seal an ordinary stopper in place)

does not protect

until he plasters them

from above and below (i.e. from all sides; some say this should be read "from above or below");

and similarly by a patch of textile.

If [the cover of the barrel] was of papyrus

or of leather

and one tied it with a cord,

if one plastered from the sides

it protects (as these materials were not porous).