[ValerieUSA]

Celts were known to stiffen their hair with a concoction made from limestone dust - perhaps it was also used to stiffen the felt hats.

[nopardons]

True and possibly.

Actually, they used a mixture of limestone, potash, animal fat, and a few other ingredients, that I seem to have forgetten. Felt can be stiffened and is, to this day, for various craft figures, costumes, etc.