Hi, all.

Regarding being "purpled" by various molluscs, this may not be the
final word on whether a species could be used for the dye. Daniel V.
Thompson says, "It [the dye] was most laborious to produce..."

One thing all the sources make clear is that the "purple" of murex was
often very red, more crimson coloured than anything, though it could
vary from the red to violet, and even be quite bluish.

The murex dye, Tyrian Purple (aka Byzantium Purple, Imperial purple,
"porphyry", whelk red) comes from Murex trunculis or Murex brandaris,
according to Meyer.

Thompson, however, says:

"The most familiar of the names of these shellfish is murex, and we
often speak loosely of the 'murex purple', meaning the purples of
antiquity. Actually, a good many other molluscs besides the murex
yielded the porphyry dye."

Daniel V. Thompson, _The Materials and Techniques of Medieval
Painting_

He also adds that the lack of particularity about what shellfish used
was probably instrumental in the variability of the colour produced.

Meyer speaks of the synthetic dye:

"In 1908, Friedlander discovered that the coloring matter of the
ancient purple was identical with a purple coal tar color that had
been introduced in 1904."

Ralph Meyer, _The Artist's Handbook_

This may refer to "mauve", but I would need more evidence to pin this
down.

Hiler says Tyrian Purple is:

"Formerly made, like Peruvian yellow, from uric acid."

Under Peruvian yellow, he elaborates:

"A product due to the genius of the Depouilly Brothers, who obtained
murexide by treating the uric acid extracted from guano, or from the
droppings of Peruvian seabirds."

Hilaire Hiler, _Notes on the Technique of Painting_

This product ran into trouble when guano became harder to obtain, and
the manufacturers turned, first, to snake excrement, and finally to
pigeon droppings (which was only 10% uric acid, unlike the 80% of
seabirds). It was eventually discontinued as being too expensive.

Indigo, on the other hand, comes from the leaves of the indigo plant,
which contains indican, which forms indoxyl when hydrolyzed or
fermented, finally creating indigotin when oxidized. The blue colour
is made by the exposure to oxygen.

I don't believe there is any chemical connection between indigo and
murex dyes. Artificial indigo, thioindigo, is also different from the
synthetic purple, but most books don't get into formulas when dealing
with complex organic compounds.

Even when you do get colours from the same basic compound, variations
in some of the elements can often cause colour shifts.

As for the colour from blueberries, this is not a true dye, but only a
stain, and is quite fugitive.

I hope this helps somewhat.

Cheers,

Carin Perron

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