San Diego Natural History Museum--Your Nature Connection[BRCC San Diego Natural History Museum: Paleontology]
Master castmaker Fritz Clark tells all
(all about how to cast a T. rex skull)
What is so challenging about casting different skulls and bones is that each job presents different considerations. Skulls that are brittle and delicate require extra care and handling in order not to damage them. Creating a technique that provides a reproduction of excellent quality, at low cost, and without damage to the specimen is the fun part of it.

Fritz sitting at a bench, prepares a modern whale skull for mold making. Museum paleontology associate Fritz Clark likes to get his hands dirty. He also likes to make up the rules himself. He's been known to use an electric jackhammer in the pouring rain to dig a site, invent one-of-a kind casting techniques like using 110 pairs of pantyhose to help make molds of a T. rex skull, and tour movie stars like Laura Dern around his bone-filled workshop for her research on the film "Jurassic Park."

For those ambitious hobbyists out there, here is a step-by-step guide to casting a T. rex skull the "Fritz" way:

  • First, carefully examine the skull to determine where the seam line will be.

  • Cover one half of the skull in Plastaline clay with a flange extended out from the seam line. Cut a groove into the flange to form a lockline so both halves of the mold will fit together exactly.

  • Mix and apply RTV rubber (Room Temperature Vulcanizing, considered the best, but very expensive) to the skull to a thickness of 1/2 to 3/4 inches thick. To reinforce the mold, and prevent tearing, cut up and place 110 pairs of pantyhose on the outside layers of both halves.

  • Apply a fiberglass layer, called the mothermold. This enables the mold to retain its exact shape when it's removed from the T. rex skull. Mothermolds are usually made in sections that are held together with nuts and bolts.

  • Remove the clay from the underside and apply a separator to the seam lock so the two halves of the mold can later come apart. Install a plug at the back of the skull in order to provide a pour hole.

  • When the rubber and fiberglass are cured, remove the mothermold and peel the rubber mold from the T. rex skull and place it in the assembled mothermold. When the mold is bolted back together, a cavity will be created that exactly represents the skull. The skull cast is then produced when two parts of liquid foam are mixed and poured into the mold. Five minutes later, the foam sets up, and your cast of a T. rex skull is done.

Photo: Fritz Clark prepares a modern whale skull for mold making.
Photo Credit: Judy Gibson 1997
Article by Andrea Chatwood