Can I ask why you're asking this? I'm a part time tattooist and have seen some horrendous things by people trying homemade tattoos. If you want a tattoo please don't do it on the cheap, it really is not worth it..
There are many reasons to question woad as a good body paint or tattoo ink. Frankly it can not be tattooed and I caution anyone thinking of doing so that it is a foolish and dangerous exercise. I know I've been so stupid as to try it. It is caustic, will cause the wound to not heal properly and it will not heal in. You might get a vivid scar, but it will not be at all blue. I had a less traumatic time than others apparently, as can be seen by an account given by Pat Fish at the bottom of Woad and it's mis-association with Pictish Body Art. Kids, do not try this at home!
It also makes a terrible body paint. You must mix it with something and anything any one has come up with either dries and flakes or smears. I have been told repeatedly that it stains the skin, but neither I nor anyone I know who has used it has had this happen (with the sole exception of someone who worked some into cloth with her fingers, but anyone who has handled things like that knows that anything will get into the frayed skin of ones fingertips for a bit including things that don't stain...dirt normally doesn't stain skin, I have had plenty spend a few days staining my fingers when gardening). Those who have reported staining have mostly been telling tales of things they heard, nothing more than hearsay.
The earliest referencing I have found to woad as the translation for "vitro" or "Glastum" is to the 1695 edition of William Camden's Britannia (Laing and Laing The Picts and the Scots pg .2 ...it is unclear if this is just the edition they used or if it is not in the earlier ones) and this appears to coincide with the start of the "Indigo Wars" (when woad growers and processors were fighting the importation of Indigo, which is the same pigment but cheaper and easier to get out of the particular plant). From what little I have been able to find, it appears that this was first translating "vitro" and "glastum" to mean "woad." Chances are this is actually nothing more than propaganda to help create a sense of nationalistic pride in woad to support the woad growers and processors. However, this is something I have not yet done a great deal of research on.
Vitreum is latin for glass and vitrium is latin for woad both mean blue/green because glass was a blue/green so it's easy to see the confusion.Well, the other possible translations I've seen propose that Ceasar said something like "All the British colour themselves with glass, which produces a blue colour" (from the same link again) - and I know there's archaeological evidence for blue glass ornamentation, although it's not clear that that's what he meant. (How's your Latin? The original is "Omnes vero se Britanni vitro inficiunt, quod caeruleum efficit colorem." I haven't a clue.)
The "Picts", on the other hand, are described by Claudius Claudianus as marking their faces with iron - although whether this refers to scarification with iron implements or tattooing with iron-based pigments is unclear.
There is also a claim that Pliny the Elder referred to the use of woad in women's funerary rites, but he uses the word "glastum" and describes it as "plantain-like", whereas in another book he refers explicitly to woad as "isatis".
As for the Greeks... Don't you run into the the whole "who were the keltoi" problem? I don't think you can argue that the practices of the people the Greeks called keltoi necessarily applied to the Britons. (Although I'm not sure if we're talking about the Britons specifically here - I think I am, but I can't speak for anybody else.)
All this debate over the translations of classical languages is beyond me, I'm just repeating stuff I've read elsewhere... What we really need are more bog-bodies.
I doubt that the iron age Brits covered themselves in glass as we didn't have it before the romans came here.
Vitreum is latin for glass and vitrium is latin for woad both mean blue/green because glass was a blue/green so it's easy to see the confusion.
I doubt that the iron age Brits covered themselves in glass as we didn't have it before the romans came here.
Keltoi doesn't refere to the Brits only a certain Gaulic tribe in europe. We were called the Pretanni which Briton get it's name from.