I have james castner's book "Shrunken Heads" probably the best pictoral tsantsa guide on the (niche)market,as far back as the middle-ages heads of state,royalty,prominent figures had death masks taken,made from wax or plaster as mementos or for later portrait use,this was before the advent of photography.
i just googled it, i have to add that book to my collection, it certainly is intrigueing stuff