As heavyweight flannels go, the Iron Heart Ultra Heavy Flannels are the champs. The UHFs are an annual, limited release of original crazy check colorways that are never reproduced. But what really makes this shirt really crazy is the use of Aspero cotton - a naturally occurring wild cotton from the foothills of North East Peru. It's actually a tree (yes, a tree!) so it's one of the purest, most organic forms of cotton in the world. It cannot be mass produced and has been prized by the Peruvians for centuries for its softness, fineness, volume and strength.