Freeze-drying is a technique where materials are dried by freezing them and subsequently evaporating the water under vacuum, without the water comming in the fluid phase.
A little bit theory:
Water has three phases, solid (ice), liquid (water) and gas (water vapor).
When an item dries in the air, there is a risk that this item collapses (shrinks) because of internal forces in the water. By ensuring that water is not ‘water’ when things dry, but forcing it directly from the solid phase to the gas phase, you will prevent this collapse.
The technique is, in the museum world, most commonly used for drying waterlogged wood (such as the viking ships in Roskilde fjord), but is also used for drying books (after water damage) and old skins (from bogs). Those items often first demand an impregnating treatment with PEG (polyethyleneglycol).
In addition, the technique is used in commercial purposes in the production of insulin and for the preparation of meals for astronauts and hikers, as well as, for example, instant coffee and tea.
I use freeze-drying for the preservation of small birds, reptiles and arthropods (insects, spiders etc.) and for drying plants
In my workshop I have a primitive freeze-dryer, without vacuum. The results are just as good but the process time is remarkably longer.
Below some examples of freeze-dried animals.