Hot weather is not a friend of hat blocks.
If you leave a hat block in the sun or worse, sitting on the floor of a car for example on even a warm day, you may well come back to a cracked block. You see, hat blocks are timber, and timber always moves. Heat and cold expand and contract all things especially wood.
The hat blocks that I make are made from kiln dried timber. That doesn't mean they are as dry as an old bone! They still retain about 12 to 15% moisture content and that can go up or down a bit depending on the weather. A stinking hot day will suck all the moisture out of the air and thus also a block of wood, causing it to shrink and pull, causing cracks and the breaking of glued sections.
Varnished blocks are subject to the same problem, especially once they have been used...pin holes will have broken the seal of the varnish allowing moisture in and out too.
So, keep those blocks stored well. Don't leave them in the sun in your studio all day and definatly not in your car!
If you can't control the temperature put the block into a plastic bag and seal it up. That way, the moisture content of the block will stay the same no matter what the weather does.
Darryll
