One of the jobs of the UCL Institute of Archaeology team is to preserve the bones as nearly as possible in the state in which we found them. Sometimes, they are extremely delicate and will not survive lifting from the soil intact. This is because, whilst they were in the ground, they were steadily becoming incorporated into the soil. Bones inside living people are composites of a fibrous protein called collagen and a mineral which comes from the family of minerals known as apatites. In the ground, the collagen component breaks down and the mineral component is gradually lost as groundwater washes through the soil. Also, the weight of soil can crack the bones. This doesn't happen to the same extent in all bones, even within one skeleton, but in some cases they are so delicate that the soil is all that is holding them together. In such cases, we support them with a consolidant. It is important to use a "conservation grade" material that has long term keeping properties and can relatively easily removed. We have done this with the bones in the picture on the right, which is why they are darker than the others. We've only consolidated those that really need it, because our guiding principle is to avoid any interference with the state of the bones wherever possible.
Some of the most delicate bones that we find are the thin sheet-like bones that form the brain case of the skull. When we excavate them, they may appear intact, but they have many tiny cracks and if we try to lift them in this state they would fall into fragments. In addition, under the weight of soil, the skull often collapses so that these bones lie on top of one another in a complex sandwich which is sometimes four bone sheets thick. For these very delicate bones, we need to provide really strong support. This is done by the technique archaeological conservators call "facing up". We use consolidant to fix a sheet of very fine tissue paper to the surface of the bone. When the "faced up" bones are lifted, they can be cleaned as normal and the picture on the left shows how delicate and fragmentary they are.