Crush washers are stamped rings of soft metal, such as aluminum or copper. They create a seal by crushing down along the sealing ring to take the shape of the surface to be sealed. The aluminum crush washers are most noted for sealing swivel seals on carburetors. The copper counterpart is most known for Banjo bolt and brake fitting lines. In both cases, upon removal it's easy to see the textures transferred from the sealing surfaces to the crush washers. Because the washers seal from distortion, you need to change them every time you break the seal.