Bechorot 25: Using a Stopper on o Barrel on Shabbat

Today's daf discusses a 'classic case': stoppering a barrel on Shabbat.  The rabbis argue about why this is forbidden.  

First they consider that the stopper is a piece of cloth.  Of course, at some point the cloth could absorb the wine and become ineffective, allowing liquid to squeeze from the cloth.  This would prohibit three of of the 39 possible prohibited categories on Shabbat;

• dash, threshing, or mefarek, removing desirable contents from an unwanted peel or cover
• melanin, whitening, as if squeezing out is part of a washing process, or
• tzove'ah, colouring, when squeezing out liquid changes the colour of the cloth

Meforek is ruled out by the Sho'el U'meishiv because the activity is part of food preparation, which only forbidden on Shabbat but permitted on Yom Tov.

The Aruch calls on an argument from the time fo the Ge'onim.  The suggestion is that the stopper is not made of cloth but is mesochraya denazyata, a wooden stopper used to cover to an opening in the side of a wine barrel.  The prohibition would not be squeezing but bones, building.  Even though the intention would be to keep the wine in the barrel, the action would fall under the category of building.