Back with the hot slates

 When it was really hot at the weekend, I went back to the pile of slates. The front slate was too hot to touch all the way down.

 I went deeper into the pile. The top parts were hot but lower they were much cooler.

 It seems that where the top of the slate was in the sunshine, it absorbed infra-red and heated up. The thermal energy then conducted down each individual slate. This broken slate deep into the pile had no part of it in the sun so was cold. It seems that the slight air gap between slates is enough to stop conduction from one slate to the next.