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Information for Preparation

One method of preparing fossils that can be easily overlooked is transfer preparation. It is a technique that must be used judiciously, as it is permanent and irreversible, but sometimes necessary. It is a common component of acid prep, specifically when working with specimens encased in nodules. Essentially, transfer prep involves embedding the exposed portion of a specimen in a resin block, enabling prep from the other, hopefully, more useful side. This is a technique that should be considered for its non-chemical applications. Though permanent, it can sometimes be the only way to view important anatomical features of a specimen.

Considerations

A specimen should be embedded if the bone is not strong or stable enough to be three dimensionally prepared, or if it is important to preserve elements in relation to each other. The decision shouldn't be made lightly, and definitely with the knowledge and collaboration of research and collections staff. One common use of the technique is in the case of Santana Formation fishes, in this instance, the fish is in a nodule, typically split along the midline of the skeleton. Important data is typically obscured, and the only way to view it is by removing the outer surface of the nodule, using a combination of mechanical and chemical techniques.

Now, we'll examine an opportunity to use the method in a strictly mechanical capacity. In this case, several gars are preserved in a block of Green River Formation sandstone, not the most common type of Green River preservation. The exposed surfaces were heavily weathered before collection, destroying the data apparent from the surface. After photographing, the decision was made to embed this side of the specimen, and prepare down from the "downside" of the block.

Again, under direction of the researcher, and with the knowledge of the collections staff, the block was cut to size so that the data was optimized, exposing the two skulls contained in the block. The surface to be embedded was cleaned with a solvent, and then a retaining wall was built to form a dam that resin could be poured into. In this case, we used Castolite AP Clear Casting Resin, that we liked in the lab for its resistance to acids, unlike epoxies. Castolite, being exothermic, will warp if poured too thick. As long as the Castolite is poured in thin sequential layers, no more than a 1/4" thick, no warpage will take place.

Sidebar

The primary reason to be worried about the resin warping is the occurance of "Silvering". Silvering is the effect of the resin pulling away from the back of the specimen that it is supposed to be adhered to. The air pocket caused by this effect looks similar to an air bubble in water trapped against glass, a silver reflection. Vibration from pnuematic tools can increase the size of these pockets, negating the purpose for embedding in the first place, which is to create a stable background for delicate elements. This can go wrong in two ways. Mechanically, further vibration can cause the bones to fall out, and chemically, acid can become trapped in the pocket, and eat away the bone from the backside.

At this point, the specimens are visible in dorsal view, as they were in the field, and immediately after collection. This view looks down through resin varying in depth from roughly 2 cm to 4 cm. Much important data exists in the braincase and palatal region of these fish, which could only be viewed by working from the other side.

See Page Two for more.