The color purple

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Imperial Purple

Purple was the color of royalty in the ancient world because purple was the most expensive dye in the world. The purple dye used at the time of the Roman Empire was extremely colorfast and nonfading. The various purple dyes were made from the secretions of Mediterranean Sea snails from the area about the Phoenician city of Tyre in what is now Lebanon. From the shells found in ruins around Tyre, it is believed that the primary snails used were Helix Ianthina, Murex Brandaris, Murex Trunculus, and Purpura Lapillus, also call Nucella Lapillus.
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Murex brandaris and murex trunculus
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Nucella (purpura) lapillus

Each different variety produced a different color of purple or blue and the colors behaved differently depending on how the dye and material were handled. The smaller varieties, like murex trunculus, of the snails were crushed and allowed to decompose in vats until the dye could be separated from the shells. This is known from the crushed shells found in the dye factory ruins and from comments from ancient writers about the smell in Tyre! The larger varieties had the dye producing gland removed or were forced to excrete the dye. Ancient dye recipes were written down but were not complete enough to allow accurate reproduction of the various purple dyes. It is believed that the best Imperial Purple was produced by dyeing the thread in the dye from one kind of snail and then overdyeing it with dye from a second kind of snail. Some writers have claimed that Imperial Purple produced this way was iridescent and that this accounted for it's popularity. Purple cloth and threads were very expensive. It took 12,000 murex trunculus to create just 1.5 grams of pure purple dye, enough to dye one toga.
Currently, dye made from a sea snail, purpura pansa, is still used for dyeing thread in Oaxaca. Purpura pansa is a large snail that can be forced to excrete the mucus which turns purple on when exposed to the sun. Purpura pansa is a protected species that lives in the tide margins. The dye process involves removing the snail from it's rock, using it to squirt a little of the dye liquid on a skein of cotton thread looped over the dyers arm and then replacing the snail in the same place on the rock. To see more of the process there is a video here.

Photo Credits
Murex brandaris and trunculus http://www.manandmollusc.net/advanced_uses/personal_adornment.html
Nucella Lapillus http://www.conchology.be/en/availableshells/searchspeciesgallery.php?family=MURICIDAE&page=3