From The book called "Exactly" by Simon Winchester, 2018, ISBN 9780008241773, in a section discussing standardising pistol parts in France in 1785 so that spare parts were readily available and interchangable, initially done to be of benefit to French army.

page 90 ........... Blanc made all the necessary tools to help in his efforts - according to one source, hardening the metal pieces by burying them for weeks in the copious leavings of manure from the castle stables.

I've not seen that method of hardening described before, google doesn't yield may hits, wouldn't it take an awful long time for the carbon to be absorbed by the metal parts by that method ?

I've always wanted to learn the details regarding how accurate screw threads were first made but the book has failed to deliver on that subject.

Bill


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