HappySqurriel on 19 November 2009
I don't know about porcelain in particular, but lots of ceramic and glass items do not respond well to being unevenly heated; and the microwave doesn't provide even heating. This doesn't mean that these items will break the first time (or in the first 1000 times) you heat them this way, it may require the uneven heating to be just-right to put pressure in a flaw that has developed (or possibly always been there) to crack/break the item.
