To a great extent, the ultra heavy flannel (UHF) shirts define Iron Heart: heavy, rugged, but amazingly wearable. The 12oz fabric is woven with Aspero cotton from the foothills of the Andes mountains. We double brush it on the inside and single brush it on the outside, resulting in a very soft yet substantial, thick, warm and windproof flannel. This shirt is a wardrobe staple - a work cut in blue tartan check.