Fish scale the surface, interacting only with the riblet tips, not causing any high-veloctiy flow in the valleys. Since this high velocity flow now only interacts with the riblet-tip, which is a very small surface area, the momentum transfer which causes drag is now much lower than before, thereby effectively reducing drag. Also, this reduces the cross-stream velocity fluctuations, which aids in momentum transfer too. The rough, sandpaper-like texture of shark and ray skin, coupled with its toughness, has led it to be valued as a source of rawhide leather, called shagreen. One of the many historical applications of shark shagreen was
Tanning (leather) the process, cedar oil, alum, or tannin were applied to the skin as a tanning agent. As the skin was stretched, it would lose moisture and absorb the agent. Following the adoption in medicine of soaking sutures in a chromium (III) solution after 1840, it was discovered that this method could also be used with leather and thus was adopted by tanners. The tanning process begins with obtaining an animal skin. When an animal skin is to be tanned, the animal is killed and skinned before the body heat leaves the tissues. This can be done by the tanner, or