Well I certainly learned a lesson today.

I had a cabernet all finished sitting in a demijohn, just waiting a
bit to age and bottle. From last years harvest from Lanza Ranch. I
had put it in a room with some Sauvignon Blanc that is just finishing
fermentation, so I was keeping the room cool, about 50 degrees. The
air lock on the cabernet kept draining into the wine as it changed
volume due to changing temperature. So I put a bung in it. Good and
tight.

Today my wife calls me at work to say that there is cabernet all over
the floor! The demijohn cracked, apparently due to pressure due to
temperature changes. 54 liters of fine wine all over the carpet. The
carpet was crap and due to be tossed anyway.

Should a demijohn be able to take the small stresses from temperature
changes when it is bunged tight? Seems weak to me. Anybody else
experience this?

I'm bummed.

Dan