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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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I think the ancients did produce purple dye via admixture.

Various challenges with that approach (and purple dyes in general) likely affected its historical ubiquity or our perception of that ubiquity.

Regarding your specific sub-questions:

  1. Though challenging, ancient humans have mixed dye components to create purple dye, however, more complex factors probably limited the ubiquity of this approach and, likely, our current perception of the ubiquity of that technique.

  2. The physical and chemical mixing and application of dye components have challenges unique to the principles of mixing and applying acrylic paint, notably in the mass-produced textile context you discuss.

Related details:

My basic art training hints that creating a color-fast, long lasting dye of a singular hue and applying in bulk to textiles is more challenging than applying opaque pigments mixed as a media and applied to a surface. Color combinations often muddy or dilute themselves, which is why a distinct natural source like chemical (animal, vegetable, mineral) derivations are superior as a pigment and probably more so in a dye context.

There are historical precedents for using mixtures to create purple dye (in Western culture and beyond) but it is likely that certain chemical or environmental factors limit our perception of the ubiquity of purple dyed textiles (admixture or otherwise) throughout antiquity.

Long-term instability of dye and pigment constituents for a purple hues can affect their discoverability. Defining the optical aspects of colors described in spoken or written history and matching to modern colors becomes more challenging with fewer corroborating artifacts. From Wikipedia:

A popular new dye which arrived in Europe from the New World during the Renaissance was made from the wood of the logwood tree (Haematoxylum campechianum), which grew in Spanish Mexico. Depending on the different minerals added to the dye, it produced a blue, red, black or, with the addition of alum, a purple color, It made a good color, but, like earlier dyes, it did not resist sunlight or washing.

Though the time period for the above excerpt might not be considered ancient, the techniques used could be applied further back in time.

Social constructs including popularity and marketability are also likely discriminating factors. Iconic desirability can actually hamper something becoming popular or ubiquitous when other forces influence availability.

In the middle ages, color-segregated guilds were forbidden to dye colors by other guilds. That sort of rigidity certainly wouldn't invite more inventive experiments - admixtures included - during that time period.

Though an edge case example, one possible reason ancient tekhelet (one of it's suggested color ranges being purple) dye was not replaced with an alternative replacement was the precious nature of the source that reinforces the sacred ritualistic nature of its use. From Wikipedia:

Yet, although this dye was much cheaper to obtain, the rabbis cursed those who substituted techelet with some low-priced equivalent and in fact preferred to annul the obligation altogether rather than to compromise its value

Perhaps it's possible that an absence in the historical record of a societal mass-market of a cheap or plentiful purple in ancient times is also related to both this and other intentional or unintentional limitations in the traditions and trade of dye production and human-made artifacts.

The intersection of ritual, including apprenticed traditions and closely held trade knowledge, combined with rigid societal structures (rare-colored goods as a symbol of power) and market forces further restricted by comparatively less free time or interest in novelty when compared to modern times could have simply choked out more egalitarian access to this royally symbolic hue by any number of subversive inventors or entrepreneurs.

I think the ingenuity required to source a stable purple hue - let alone devise a process through mixing - and establish a mass market definitely has had numerous challenges through the ages.

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