Urethane is an excellent medium for use with the press as it does not compress under load. Like a waterbed, if you push down in one spot, it raises elsewhere, causing metal to flow into the contours of the punch or die. Because the force is spread over the entire surface of the metal, damage is minimized and fragile "married metals" and "mokume gane" can be formed without de-laminating. Upon release of pressure, the Urethane returns to it's original shape with no lasting impression of what you have just put it through. Some forming pads and blocks have been in use for over ten years and are still being used every day.

Regardless of what you do to it through pressure (short of cutting it), Urethane will resume its original shape in a very short time with no significant impressions remaining.

It is available in various hardnesses (durometer). 95 durometer is the hardest. If used with a matrix die, the raised form will be flatter across the open area with a sharper fall off at the edge. 80 durometer is softer and if used in the same matrix die, the raised area will be more rounded with a more gentle fall off at the edge. 60 durometer pads must be fully contained and are used in the form boxes. Urethane is also available in the form of Pads or Contained Blocks.

Further information is available on page 21 of Susan Kingsley's book: "Hydraulic Die Forming for Jewelers and Metalsmiths".