I would wager on it being the mash water. Much more thermal mass than the water used to preheat it. That is, of course, unless you fill 'er up with hot water to preheat it. (I just put a gallon or two of hot water in to preheat mine.)
TB
Ya, I do the same. O well. Guess we will see how it goes on the second use. Hopefully it doesn't just crack in half on me.