Monday, July 19, 2010

Wax On. Wax Off?

In April, Lori and I had the privilege of attending the Third Annual Fossil Preparation and Collections Symposium at the Field Museum in Chicago. One workshop we attended introduced us to the concept of using water-soluble wax to support delicate areas of specimens while working on them. Debbie Wagner showed us how PEG (polyethylene glycol a.k.a. Carbowax) can be used with and without cheesecloth reinforcement. We were also introduced to cyclododecane, a wax that sublimates in air that's suited for specimens that can't be washed in water. Carbowax is much easier to find and much cheaper, so as soon as we got back from Chicago I ordered some wax from Museum Service Corporation. They carry Carbowax 4000 for $22 per kg. available online.

Lori has been working on some Ghost Ranch material with tiny bones and scutes, and I've been working on an ornithomimid which has long, delicate spines on the caudal vertebrae. Months after the Chicago trip, we finally decided to try using the PEG 4000 wax on a couple of delicate bones.

We had to figure out how to deal with molten wax in the lab. I tested the melting point of the wax and found it didn't need an expensive lab-grade hot plate to keep it liquid. We found a very inexpensive solution while at (apologies) Wal-Mart. We were looking for a small crock pot or hot plate with temperature control, but we settled instead on the candle warmer in the photo above. The warmer is designed for use with jar candles, so we picked up a candle and a warmer (under $15 total). At home, we melted the candle to get an empty jar, and tried melting the PEG 4000. The warmer will melt the PEG very slowly, but it's perfectly suited for maintaining the wax as liquid once it's already melted. The warmer has no temperature control, and it doesn't get hot enough to be scary in a busy lab. Since we don't have a lot of time in the lab to wait for a very slow melt, we melt the wax in the oven at home and then use the candle warmer to keep the wax liquid while we're at the lab. We use low temperature oven at home, 200° F or less — well below the flash point where the wax can catch fire. We transport the wax in a small, soft-sided lunch box cooler and the wax stays liquid for the half hour before we get set up in the lab.

The above photo shows wax and a small piece of cheesecloth applied to a delicate caudal vert. At the Field Museum, one application tool they use is a pair of tweezers with the ends held closely together. Lori's holding a ruling pen above.

The tweezer applicator technique reminded me of the ruling pens I used to use for drafting and design work, and I think a ruling pen is ideal for controlled wax application. It's easy to adjust and hold and will maintain the gap you choose indefinitely. I leave the pen in the molten wax so that it warms up. The warm pen keeps the wax molten while I work.

This image gives you an idea of how delicate the spine is. The matrix is hard and I had to use an air scribe to remove it from the exposed bone surface. With the wax holding the structure together, I was able to much more easily do this delicate work. I've consolidated the cracks visible in the spine, but haven't removed the wax yet. I want to show this to Mike Getty and Eric Lund before I put water to the wax.

Lori's using a thin bead of wax to stabilize a very thin bone in her Ghost Ranch cast. She's also stabilized a fragmented scute so she can remove surrounding matrix.

Thanks to the preparators at the Field Museum for teaching us this technique. I hope that the cheap candle warmer and the ruling pen applicator are useful contributions.


1 comment:

  1. Thanks Guys!
    I've just been asked by a curator here in Australia to use PEG4000 to reinforce a skull before I do some moulds of the dentition. Your experience has been invaluable!
    David Hocking