I was wondering about the three archaeological finds you cite as proof that trunculus was used in ancient times to make blue dye. How can a chemical test prove that they are really dyed from trunculus as opposed to from another – as of now unknown (nignaz, perhaps?) – chilazon?
The dye that comes from murex sea-snails contains three major components, indigo – which is blue – along with monobromoindigo and dibromoindigo – compounds based on the fundamental indigo molecule with one or two bromine atoms respectively. Those brominated indigo molecules have colors which are shades of purple. The mix of the three taken together generally gives a violet blue-purple to red-purple depending on the relative amounts of each – but never the pure blue that our tradition maintains is the color of Tekhelet. In order to achieve that, there are various ways to de-brominate the di- and mono- indigo molecules, one of which is to expose the dye solution (in a specific stage as it is prepared for dyeing wool) to sunlight. Once that is done, there is a much higher proportion of indigo and, to the naked eye, the color is that of pure indigo. This is, of course, similar to the color of the fraudulent Tekhelet known in the time of the Talmud as Kala Ilan, which was plant-based indigo. However, no matter how hard you try to debrominate the mix of molecules, there will always be *some* brominated indigos left. So if you find any trace of dibromindigo/monobromoindigo in a blue-dyed fabric, you can be sure, 100%, that it came from a sea-snail and not from a plant. And that is exactly what the chemical tests using a very sensitive detection apparatus (called an HPLC) determined about the blue-dyed fabrics from ancient Israel.
Therefore, the archeological finds prove beyond doubt that murex was used to dye blue in Israel during the time these fabrics were made – somewhere between the 1st century BCE and the 1st century CE (roughly, the times of the Tannaim). There is also no doubt that the blue of the murex is visually identical to kala ilan. It is also a very strong and fast dye (lasting 2000 years!). This is enough evidence to convince many (the Radzyner and Rav Herzog write this explicitly) that the murex should be acceptable to dye Tekhelet, since if not, then chazal would have had to warn against using murex – which was clearly available and used and identical to Tekhelet. In theory, there could have been another snail that was also acceptable for use as Tekhelet and had the same properties mentioned, but that does not negate the logical argument just presented that the murex also must have been acceptable for use.
I will add one more thing. Natural dyes are rare – very much so. Blue dyes are extremely, extremely rare to the point that the only colorfast sky-blue naturally occurring dye known to man is indigo – whether from a plant or from an animal. There are reasons for that, having to do with the physics of light and its interaction with matter. If we were to ever find another creature on this or any other planet that produced a sky-blue dye, it would almost certainly be indigo. So even if there were to be some other snail, or fish, or worm, or whatever – nignaz or not – that made the dye for Tekhelet, it would be the indigo molecule.
– Baruch Sterman