Histology tissue blocks can melt? 

Paraffin wax tissue blocks can be sectioned again and again (as long as there is still tissue in the block), so it is important that they be stored properly.

If tissue blocks have been stored improperly (such as near a window, or in a car during our AZ summers, or next to the exhaust fan of an instrument that tends to run hot) the blocks can melt. Once the wax begins to soften, the smooth surface of the block can become deformed. To recut the sample, the block may need to be re-embedded (melted down and re-oriented). If the piece of tissue in the block is small or thin, some of the valuable tissue may be lost while trying to section into the sample. Please store your blocks with care.

NOTE: The paraffin used in histology labs often has a melting point of 55-57°C (131-135°F).