By Katharine Sorrows
I was able to go in twice to work with Alton Dooley and Raymond Vodden who are working on making a replica of the giant beaver skeleton. I spent a few hours this week in the shop with them. It was a really great break from reading. I helped to make two piece silicon molds. First, a clay base is made around the bone. I built the clay up so that it was sealed against the bone and was smooth. I then added a pour spout and the snake around the edge. The snake had to be closer to the bone than to the edge and sealed to the rest of the clay before putting on the walls. The most important part in making a mold is to spray it so that the silicon will not stick to the bone and clay. The silicon is then poured into it making the first half of the mold. It was really relaxing to sit and mold the clay and work until it was as good as it could be.
Pouring the silicon into the mold was messy but lots of fun. The silicon is made up of two liquids, one of which is so thick that it was a real challenge to mix.
The most stressful part for me was pouring the casts. The liquids were a syrupy consistency and had to be mixed really well and then poured quickly. I had just two minutes to pour all of the silicon I had mixed into casts. The tooth molds are the little molds on the side and those are always kept ready for overflow. Each cast has several little holes, and I was pouring out of a larger container, which made it hard to be accurate. In addition, it had to be poured at a slow enough rate that it did not get air bubbles and to ensure that the entire cast was filled. The molds in the picture are teeth, the bigger rounder ones are vertebrae, and the flat skinny ones are ribs.
The last part I did was to help make a mother mold. A mother mold is made for a silicon mold when it has to be made really thin. It is made to provide structure for the silicon mold. The skull was made from a very thin silicon mold that needed to be supported. In order for it to work properly we put plaster around the mold. The plaster had to be added to the water and mixed until it had a syrupy consistency with some lumps in it. We then dipped fiberglass into the plaster and overlapped it over the mold. The mold pictured below has the plaster part on the bottom, the silicon mold is on the top. The same process would be used in the field to bring bones out from a site, protecting them using burlap strips in plaster.