This section is from the book "Cassell's Cyclopaedia Of Mechanics", by Paul N. Hasluck. Also available from Amazon: Cassell's Cyclopaedia Of Mechanics.
For relaxing bird skins, line the inside of a wooden box with a 1 in. layer of plaster-of-Paris, well mixed. When dry, the box is ready for use. Pour water inside sufficient to saturate the plaster, and, after turning out the surplus water, place the skins inside. Cover them with a damp cloth and close the lid, which should fit well. Now place the box in a damp shady place (such as a cellar) until the skins are relaxed; this will be known by the feet, wings, and tail being soft enough to spread out with gentle handling. Another method is to half fill a box with silver sand and well damp it. Wrap each skin in a piece of rag and cover the whole with more damped sand. The rag will keep the feathers from actual contact with the sand, but will allow the moisture to penetrate. The average time for small birds up to the size of a thrush will be about twenty-four hours; for grouse size, about two days; for heron size, three days; for eagle size, four days. When the legs will bend a little, work them about till they bend easily.
 
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