Plaster Field Jacket

Vertebrate fossils are legally protected in most countries and on most public land, and collecting without a permit is an offence. Check the law for your jurisdiction and landowner before a tool touches the ground. An illegally collected specimen is scientifically worthless as well as illegal — no museum will accept it.

Photograph the exposure, record coordinates, the rock unit and the bed the specimen came from. A fossil without stratigraphic context has lost most of its information — the age and environment come from the rock, not the bone.

Work the overburden down with hand tools until the upper surface of the specimen is defined and you know its outline. Do not chase the bone downward and do not clean it — dirt is protection at this stage.

Brush a thin reversible consolidant, such as dilute B-72 in acetone, into visible cracks and let it wick in. Use it thin and only where needed — a thick surface skin traps moisture and causes worse problems later.

Dig a channel right around the block, at least a hand's width clear of the fossil on every side, and deeper than the fossil goes. You are carving a mushroom of rock with the specimen inside the cap.

Cut inward at the bottom of the trench to narrow the stem. Undercut enough that the block will snap free cleanly when the time comes — but not so much that it topples while you are plastering.

Cover the exposed rock and fossil completely with damp paper towel, tissue or aluminium foil. This is the step that cannot be skipped. Plaster keyed directly onto bone cannot be removed in the laboratory without destroying the specimen.

Tear or cut hessian into strips roughly 80-150 mm wide and long enough to cross the block and lap onto the pedestal. Prepare them all before mixing plaster — you will not have time afterwards.

Sift plaster of Paris into water — not water into plaster — until islands stop sinking, then stir smooth. Mix only what you can use in a few minutes; it begins to set as soon as it is wet.

Draw each strip through the plaster, strip off the excess between two fingers, and lay it over the block so it beds down onto the separator. Overlap every strip by half its width.

Run each successive layer across the one below. The strength comes from the burlap fibre, not the plaster — plaster alone cracks under its own weight on a large block.

For a block over about 300 mm across, bed timber battens into the wet plaster and bandage over them. A heavy jacket flexes in transit, and a flexing jacket grinds the specimen inside it.

Wait until the plaster is warm and rigid — the set is exothermic, so heat means it is working. Then lever or chisel the undercut stem until the block parts from the bedrock.

Turn the block carefully, trim any loose matrix, apply separator to the new face, and plaster the bottom the same way. The specimen is now fully enclosed.

Write the field number, locality, date, collector and a large arrow marked TOP directly onto the plaster. Jackets sit in stores for decades and an unlabelled one is a mystery block nobody will open.

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.