A great deal of prestige is attached to the colour purple - the hue of bishops and emperors alike - and, if you've read a little about the ancient world, you'll probably have heard the story explaining why this is the case. The Greeks and Romans - the story goes - knew of only one way of obtaining purple dye: a painstaking and eye-wateringly expensive process involving the extraction of the precious substance from sea snails.

As a boy, I always imagined that the root of this purple business was an inability on the part of the Greeks and the Romans to produce blue dye. I know little about dyes, but the colour blue is notable for occurring in the natural world only very rarely. So it makes sense, at least at first glance, that the ancients would have struggled to produce dye of this colour. The colour purple was the closest that they could come to achieving to such a rarity, hence its prestige. In any case, if they could produce blue dye in quantity, they could just have mixed it with red dye - which has always been one of the cheaper pigments - and every Roman and his dog could have strolled across the forum in lurid purple vestments.

This theory took a bit of a knock a couple of years ago, when I read that the colour purple only became available to the non-fabuously wealthy in 1860 with the invention of magenta pigment (named for the Battle of Magenta of the previous year). Given that, for centuries before that date, the armies of Britain and France marched into battle wearing vivid red and blue uniforms respectively, it seems unlikely that lack of blue pigment was the main contributor to the rarity of purple.

Given that both red and blue dyes were plentiful pre-1860, what, then, was stopping the ancients (or anyone else for that matter) from mixing the two to create purple? Do clothing dyes not mix the way that acrylic paints do?

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    This might be a good question over on chemistry.stackexchange.com or on hsm.stackexchange.com – Ryan_L Jan 7 at 22:21
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    The jews had Tekhelet which was an ancient blue dye, it is produced from murex shells just the same as tyrian red. I seem to remember reading in an article if you exposed this dye to a catalyst it would change it into a more purple color. See the pic at the bottom of en.wikipedia.org/wiki/Tekhelet . Also i believe our current idea of purple is not the same as the ancient idea of purple, but that is just a personal opinion. – ed.hank Jan 8 at 13:39
  • Google images for bishops and you'll see they wear red. Because that was the purple colour back then: a bright, intense, pure red. Other reddish dyes were known, but they weren't as bright, and usually turned brownish with use. According to Vitruvius, the exact hue of the "purple" could vary from red scarlet to violet, but it was the high quality of the dye what they really appreciate, since most dyes washed away quickly. – Rekesoft Jan 8 at 14:26
  • @Tom Hosker Possibly the ancients did make purple dye by mixing red and blue. Remember that there is a difference between dye that produces a purple color in cloth and dye that is PURPLE, the super expensive sea snail dye which had several good properties. I once read that in medieval Ireland there were allegedly rules about which colors which classes could wear, kings wearing the most colors, and the extra color which kings wore was purple. I doubt that the Irish imported PURPLE so they probably mixed red and blue dyes to make ordinary purple. – MAGolding Jan 8 at 16:10

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