Tyrian Purple

The fast, non-fading dye was an item of luxury trade, prized by Romans, who used it to colour ceremonial robes. It is believed that the intensity of the purple hue improved, rather than faded, as the dyed cloth aged. Pliny the Elder described the dyeing process of two purples in his Natural History[7]:
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... the Tyrian hue ... is considered of the best quality when it has exactly the colour of clotted blood, and is of a blackish hue to the sight, but of a shining appearance when held up to the light; hence it is that we find Homer speaking of "purple blood."
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Archaeological data from Tyre indicate that the snails were collected in large vats and left to decompose. This produced a hideous stench that was actually mentioned by ancient authors. Not much is known about the subsequent steps, and the actual ancient method for mass-producing the two murex dyes has not yet been successfully reconstructed; this special "blackish clotted blood" colour, which was prized above all others, is believed to be achieved by double-dipping the cloth, once in the indigo dye of H. trunculus and once in the purple-red dye of M. brandaris.
-wikipedia

In my opinion wikipedia should be double checked at every turn but, this is very similar to the way the process was explained to me in school and is eaisly checkout on via other sources on the web and in books.
What we went through for purple...amazing.