If a physical comparison needs to be performed but the examiner may not be able to retrieve the physical bullet itself, a cast of the toolmarks may be made and sent to the examiner for comparison. This may occur if separate jurisdictions are working together, if chain of custody is a concern, or if there are policies against shipping physical evidence. The cast received should be compared with another cast and not a bullet, because casts are the inversion of the bullets surface. Common casting materials include Mikrosil, Coe-Flew, and Dip-Pak. The components of the casting material are mixed, applied to the surface of a fired bullet, and removed once hardened.