Mine are 1 mm stainless spring steel. I've used them for 23 years now and they are still like new. Easy to clean and have never kinked or creased after hundreds of bends. I had bought two sets but gave one away after it became apparent that one set will last forever. Prior to those, I had thinner ones which seemed O.K. but needed replacing after a few years. I got these from a sheet metal supplier who suggested the heavier ones when I told him what I was using them for. They absolutely do provide better support. I sometimes bend some very difficult wood and I cannot remember when I last cracked anything. I think they may also help to reduce the rippling and/or cupping one sometimes gets across the sides with some materials.