http://tekhelet.com/tekhelet/introduction-to-tekhelet/Tekhelet
The vein is removed… and to this salt has to be added… three days is the proper time for it to be steeped, and it should be heated in a leaden pot with 50 lbs. of dye to every six gallons of water. (Pliny the Elder, Natural History 9.61.133, first century BCE)Rav Herzog knew of the work done by Lacaze-Duthiers and others, and realized that all the evidence pointed to Murex trunculus as the most likely candidate for the tekheletsource. Two problems, however, prevented Rav Herzog from positively identifying that snail with the Chilazon. First, the dye obtained from the trunculus was purplish-blue, not pure blue as tradition maintained. Second, this snail has an off-white shell with stripes of brown, hardly fitting the Talmudic description of the Chilazon as appearing similar to the sea.
Current research has supplied the solutions to these objections. The shell appears off-white with brown stripes when it is out of the water, cleaned and polished. In its natural element, however, trunculus is covered with a coat of sea fouling the color of the ocean. Everything in its vicinity is covered with the same fouling, making it almost impossible to distinguish the snail from the sea bed on which it is found. The Talmud’s description is of the Chilazon in its natural habitat!
The riddle of producing a pure blue color from the snail was serendipitously solved. While researching the methods used by the ancient dyers, Prof. Otto Elsner, of the Shenkar College of Fibers, noticed that on cloudy days, trunculusdye tended towards purple, but on sunny days it was a brilliant blue! He found that at a certain stage of the dyeing process, exposure to sunlight will alter the dye, changing its color from purple to blue. To the dye masters of old, working in the bright Mediterranean sunlight, this was certainly no secret.