Transfer technique

The transfer technique is a technique to stabilise and prepare fossils preserved in oil shale and other substrates that will deteriorate under atmospheric conditions. It was developed (and mostly used) on fossils from the Messel Pit in Germany.

Deteriorating fossils
The Messel fossils, noted for their exquisite preservation, including soft tissue, body outline and even color sheen on beetle wings, are notoriously difficult to preserve. The fossils themselves are flat, sometimes film-like on the surface of the rock layers. The oil shale contains 40% water. When a slab is broken free of surrounding rock, it will soon dry out and crack. A slab with a perfect fossil will turn to a heap of rubble in a few hours, destroying the fossil with it. This was the fate of numerous Messel fossils until the transfer technique was developed and perfected in the 1970’ies.

In order to preserve the fossils once their slab is taken out of the rock, the fossil need to be transferred from the rock surface on to a durable, artificial surface. The water in the fossil itself also needs to be replaced.

Lacquers and epoxy
As soon as the slab bearing the fossil is worked free from the rock, it is submerged in water to stop it from cracking. This commonly involves packing it in wet newspaper. While in the wet state, it is cleaned up and all preparation needed for the transfer conducted.

Once ready for transfer, the fossil (but not the surrounding rock!) is dried off with a blow-dryer. As soon as the fossil start to lighten, a sign of drying, water soluble lacquer is applied. The lacquer will penetrate the bone and other organic remains, but not the shale itself, as shale is impenetrable to watery solutions.

When the lacquer has set, a frame of modelling clay is built on the rock face around the fossil. A two-component epoxy is pored onto the frame, forming the new artificial surface for the fossil. The composition of the resin is important, as it will have to soak into the fossil to further strengthen it and to bind it to the new surface.

Back-to-front preparing
When the epoxy has set, the slab is turned over, and preparation begins from the shale at the back. Layer by layer of oil-shale is removed with brush and scalpel. When the preparator hits the fossil, more lacquer and glue is applied to further stabilize the fragile fossil. When the work is done, all traces of oil-shale have been removed, only the fossil itself remain on the epoxy slab.

The contrasting physical property of the rock and fossil are essential for this technique to succeed. The organic remains of the fossil are porous and hygroscopic, while the oil-containing rock is not. Thus, the lacquer can penetrate fossils, and not rock, enabling the preparator to "glue" the fossil to the artificial slab, without at the same time gluing it to the shale.