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Information for Preparation

The following is a pictorial demonstrating the use of air conditioner filter foam in creating a storage cradle. While the methods will be familiar to most, this foam is an outstanding alternative to burlap. The A/C foam soaks up the gypsum, in this case, Hydrocal, just like a sponge. Simply soak the foam in your mixture until it starts to thicken, and apply it to your specimen. On small to medium sized specimens, one layer of foam is more than sufficient to support the element, and you can use the same barrier between the jacket and bone as with burlap and plaster. In this case, a thin piece of Ethafoam was sandwiched between the two for padding. Thanks to Russ McCarty of the Florida Museum of Natural History for bringing this method to our attention.

see Notes at the bottom of the page for more info

The skull roof of a phytosaur buried in sand.

The skull roof is covered now in a thin sheet of Ethafoam, so that there is no gapping or ill fitting later in the process. The A/C foam(blue) is cut to size, and the Hydrocal is mixed.

The foam is left in the bucket of mixed Hydrocal until it becomes thick enough to stick to the surface of the A/C foam. The weight of the gypsum allows the user to form the foam tightly around features of the bone, forming a well fitting jacket. It is usually heavy enough to also hold the the 1/8th inch Ethafoam tightly to the bone, creating a snug but well padded cradle.

At this point, the cradle is effectively finished, the last steps could include trimming the edges, and adhering the ethafoam to the cradle. This specimen is destined to be shipped in the near future, so the process will be repeated on the other side, and the two halves strapped together.

The trimmed jacket in cross section. You can see the uniformity of the plaster soaked foam. This was very easy to cut with a pneumatic hack saw.

Materials:
Blue and White polyester AC filter media (1" x 40" x 90ft $84+shipping)
from Troy filters (1-866-817-TROY, www.troyfilters.com)
Hydrocal White from www.plaster.com

Notes:
Regarding strength, I tested a few different variations, large squares, narrow strips, and a few double layered thicknesses of foam. As with plaster and burlap, you can break single layers by hand. I was able to snap a 4x4 in. square of foam, but it took considerable effort. Keep in mind also that a one inch thickness of A/C foam compresses to about a 1/2 in. once saturated. I was not able to break a doubled 4x4 in. square by hand. I enjoy testing to failure, so, I clamped the piece in a vise, and swung a two pound hammer at it. A light blow did not break the section, however, a firm strike completely cracked the foam at the fulcrum created by the jaws of the vise. One thing to note about using this technique is that when cracked, the foam snaps entirely. That is to say, when it fails, the failure is complete, as opposed to burlap, which usually retains some form, even though it may not be very supportive. However, especially in the case of Hydrocal, the foam seems to require quite a bit more force to break it. Additionally, best results come when the foam is completely saturated. It is best to let the foam soak in plaster until the plaster is too pasty to migrate back out.