Not to beat a dead horse, but the Manoppello Holy Face doesn’t involve any paint. It’s an ancient piece of “sea silk” or “sea byssus,” a material which is made from a certain rare kind of shellfish.
Anyway, very few people even know how to extract and weave this material from the shellfish. Most of them live in Italy, although it was originally an Ancient Greek and Byzantine Greek craft. (Which is why it shows up in parts of Italy that used to be Byzantine.) Women were usually the only ones who made byssus, even more than other woven goods: because it demands great care, skilled hands, great color vision, and sharp eyes. You had to learn early.
It was always an extremely expensive and valued material, because it looks golden in certain lights and is dark in others, and basically has a whole range of subtle tones at all times. (UPDATE: It also appears transparent in direct light. Thus all the dark pictures of the Holy Face.) Unfortunately, this quality of sea byssus is difficult to reproduce with photography or videography, and I don’t think I’ve seen any good digital video of it, either.
The Holy Face apparently takes advantage of this tonal quality of sea byssus, because folks say the Face looks startlingly realistic in person. Unfortunately, this means that the Face looks pretty horrible in photographs.
There is no known way to paint on this kind of byssus. (UPDATE: There are ways to dye byssus thread, although the techniques are kept secret by the women who know them. You can also vary the tones of the thread, by altering the acidity of the stuff used in its initial production.)
It is possible that there was such a technique in ancient times, but there is no record of sea byssus weavings ever having pictures on them (although decoratively embroidering on other kinds of cloth with byssus threads is a thing). The ancient Greeks and Byzantines did weave other kinds of cloth into patterns or pictures; Egyptian weavers in Roman times specialized in making curtains with people depicted on them.
The Holy Face has a fair amount of provenance from pretty far back, as books about it will tell you. No scientific analysis has found paint or dye on it.
So either it’s the only example in the world of an ancient art technique of weaving a pattern into byssus from indetectably dyed thread with extremely subtle color differences, invented and used with mastery by one great woman artist who destroyed all other pieces using the technique, and never reproduced by any other byssus weaver – a piece of delicate fabric which has survived from the days before Constantine; or it’s a miraculous image that appeared on St. Veronica’s incredibly valuable byssus hankie.*
Either way, it’s something amazing.
- It also might have been an incredibly valuable hankie used as a funeral wrapping, to cover Jesus’ face before starting in with wrapping the whole head, putting on the burial shroud, etc. A lot of people think this explains the extreme resemblance to the Shroud of Turin. But the traditional association is with the women of Jerusalem, and specifically “Veronica,” trying to comfort Jesus on his way to the Cross.