The problem was transport, not discovery. The fossil fields of the American West opened in the 1870s, in the middle of the rivalry between Othniel Marsh and Edward Cope, and the bottleneck was immediate: specimens that survived eighty million years in rock disintegrated on the wagon ride east. The plaster-and-burlap jacket is generally credited to Charles H. Sternberg, working in the late 1870s, though as with many field techniques the first use is not cleanly documented and rice-paste, flour-paste and rag-and-glue wrappings were all tried in the same period. What is certain is that the plaster jacket won, and that it is still standard practice on every professional dig today.

Why the rock is kept. The instinct is to free the beautiful bone in the field. It is the wrong instinct. Matrix supports the specimen mechanically, and it also carries evidence — associated microfossils, pollen, sediment structures, the exact three-dimensional arrangement of scattered elements. Preparation is destructive and irreversible, so it belongs in a laboratory where it can be done slowly and recorded, not on a hillside in the wind.

What the plaster is doing chemically. Plaster of Paris is calcium sulfate hemihydrate. Adding water rehydrates it to gypsum, which crystallises as an interlocking mesh — that is why it warms as it sets and why the reaction cannot be reversed by drying. It is a different chemistry from lime plaster, which sets slowly by absorbing carbon dioxide from the air; gypsum's speed is exactly what a field team needs before the light goes.

The mistake that ruins specimens. Omitting or skimping the separator layer. Wet plaster keys into every pore of exposed bone, and in the laboratory the only way to remove it is mechanically, taking bone surface with it. Preparators regularly open century-old jackets to find the specimen fused to its own shell. The damp paper towel is the cheapest and most important material in this blueprint.

Paleontology

Blueprints (1)

Plaster Field Jacket