The Blue Colour of Tekhelet
1. In my initial research, I considered that tekhelet was coloured violet (= purplish blue = bluish purple), because violet was the colour reportedly obtained from the colourless glandular fluid taken from the Mediterranean seashell trunculus חילזון . Furthermore, a violet colour corresponds to the chemical composition reported for trunculus-dye, consisting of a mixture of two dyestuffs, namely dibromoindigotin (DBI) and indigotin. DBI is a purple dyestuff, which is also the main constituent of the Biblical dye argaman. Indigotin is the main constituent of indigo dye as obtained from the indigo and woad plants, and is generally identified as Talmudic kala ilan.
Now violet is the terminal colour in the visible spectrum and is translated in Hebrew as "segol". Therefore I wrote that tekhelet violet is "segol". However, the Hebrew word "segol" is also the translation of "purple", referring as it does to the whole range of shades that includes both violet and purple. But this gave rise to the erroneous contention that I hold that tekhelet is purple, which I do not credit; because purple is the colour of argaman, Tyrian purple, the dye obtained from all other species of dye-snails except for trunculus.
2. The literature and discussions published so far have been based on the old scientific reports that tekhelet from trunculus is essentially a mixture of two main constituents, namely DBI and indigotin.
However, these reports have since proven to be erroneous. We now know that the main constituent of tekhelet from trunculus is, in fact, a violet-coloured dyestuff named monobromoindigotin (MBI), which is accompanied by smaller amounts of DBI and indigotin.
We then discovered that MBI has the unique property of readily changing colour to blue when heated to above 60 degrees C. We have also proved that there is no change in chemical structure or composition during this colour transition, which is irreversible.
3. In order to obtain a blue from trunculus-violet/purple, Amutat Petil Tekhelet introduced subjecting the natural dye to a modern chemical transformation (called "debromination" that was discovered in 1944), which changes it essentially to indigotin. However, this manipulation is unnecessary, now that we know that blue is obtained by simply heating the natural dye.
Adoption of this heating treatment, instead of debromination, to produce blue tekhelet will have two noteworthy advantages: firstly, heating was, of course, readily available in antiquity, giving historical authenticity to the process; and, secondly, the original dyestuff constituents from the snail are conserved in the blue, giving chemical integrity to the product.
4. However, the fact is that different samples of trunculus snails may yield different shades of blue. This is a consequence of the dye being a mixture of several dyestuffs that are formed in varied proportions from the colourless precursors. These will all be genuine tekhelet notwithstanding their varied shades, because they all fulfil the actual halakhic requirement הביא מן החילזון כשרה as promulgated in the Tosephta.
In this connection it may be noted that the exact shade of tekhelet should not be defined from its similarity to kala ilan, since there is no proven identification of kala ilan. In any case, such a similarity is not a halakhic criterion (סימן). Furthermore, the comparison of tekhelet to the colours of the sea, the sky, trees, plants, etc.is not a halakhic criterion defining tekhelet: that comparison is merely a homiletic simile that Rabbi Meir uses to remind us of כסא הכבוד the Heavenly Throne:-
רש"י סוטה יז ע"א ד"ה שהתכלת דומה לים וכו'. דתכלת לא דמי לרקיע כל כך אלא דומה לדומה כמו תכלת דומה למראה הים ואנן קחזינן שים דומה למראה הרקיע
Such comparisons should not therefore distract us from the coloristic consequences of the truly halakhic requirement for a seashell source of tekhelet.
5. Both tekhelet and argaman were used together for the Bigdei Kehuna and the Mishkan artifacts and coverings. Because of this and other weighty arguments, we believe that there would have been only one blue tekhelet for all end-uses, both by Jews and gentiles.
6. As regards the propriety of using only white tassels at a time when tekhelet is available: although תכלת אינה מעכבת את הלבן, one has not then fulfilled the mitzvah of tekhelet! Rabbi Zvi Hershel Schachter's Ginat Egoz is very helpful on this issue; see the two marked paragraphs in the chapter in the folder תשובות מאת פוסקים below.