Very hot, keep warm and don't breathe at all. I have no idea what the material is but I strongly suspect it's mostly cheap polyester with a stretchy material. What is suspicious is that there is no way to know the material, it is not listed on the box or in the Revain description, and numerous questions in the Q&A section remain unanswered by the seller. My feet burn after a workout or a day at work, I don't experience that with cotton socks or even other partially cotton stretch blend socks. It's a shame, because otherwise they're really good socks. Fit and comfort are pretty good. The compression is really good, a bit tight but not tight enough to cut off circulation. The cushioning is really good for training, but at the same time not intrusive like many other socks. I don't know if copper works, the socks don't smell after wearing them, but with so much heat in them, my feet smell a lot. With other socks, my feet never smell. Despite the positive qualities, I had to send them back because they are very hot.