The LORD said to Moses as follows:
Speak to the Israelite people and instruct them to make for themselves fringes on the corners of their garments throughout the ages; let them attach a cord of blue to the fringe at each corner.
That shall be your fringe; look at it and recall all the commandments of the LORD and observe them, so that you do not follow your heart and eyes in your lustful urge.
Thus you shall be reminded to observe all My commandments and to be holy to your God.
I the LORD am your God, who brought you out of the land of Egypt to be your God: I, the LORD your God.
The Talmud (b. Menachot 43b) engages in a discussion of the mitzvah of tzizit and tekhelet, in which multiple homiletical statements are found, including this:
ותניא אידך וראיתם אותו וזכרתם ועשיתם ראיה מביאה לידי זכירה זכירה מביאה לידי עשיה ורשב”י אומר כל הזריז במצוה זו זוכה ומקבל פני שכינה
And it is taught in another baraita: The verse states: “That you may look upon it and remember all the commandments of the Lord and do them.” This teaches that looking at the ritual fringes leads to remembering the mitzvot, and remembering them leads to doing them. And Rabbi Shimon bar Yoḥai says: Anyone who is diligent in this mitzvah of ritual fringes merits receiving the Divine Presence.
Subsequently Rabbi Meir describes the uniqueness of the color (or dye) tekhelet:
תניא היה ר’ מאיר אומר מה נשתנה תכלת מכל מיני צבעונין מפני שהתכלת דומה לים וים דומה לרקיע ורקיע לכסא הכבוד שנאמר (שמות כד, י) ותחת רגליו כמעשה לבנת הספיר וכעצם השמים לטהר וכתיב (יחזקאל א, כו) כמראה אבן ספיר דמות כסא
It is taught in a baraita that Rabbi Meir would say: What is different about tekhelet from all other types of colors? It is because tekhelet is similar in its color to the sea, and the sea is similar to the sky, and the sky is similar to the Throne of Glory, as it is stated: “[And they saw the God of Israel] and there was under His feet the like of a paved work of sapphire stone, and the like of the very heavens for clearness” (Exodus 24:10), and it is written: “The likeness of a throne, as the appearance of a sapphire stone” (Ezekiel 1:26).
It is commonly understood based on Rabbi Meir’s statement and the earlier quoted baraita that the seeing of the color itself is intrinsically related to the remembering of God’s commandments by means of this associative chain of thought:
Tekhelet-> reminds of -> Sea;
Sea -> reminds of -> Sky;
Sky -> reminds of -> Throne of Glory
While this color-themed mnemonic is ancient and stands on its own, modern science and archaeology can enhance our understanding of it.
The knowledge of the process for making tekhelet was lost in late antiquity, so until the 20th century, tzitzit were made without a tekhelet thread. Even before that, in general, in the greco-roman world, tekhelet dyed clothing was reserved for nobility and the super-rich. This is because the dye was so expensive to manufacture.
In recent decades thanks to archaeology and modern science, the ancient process has been rediscovered and tekhelet is being manufactured again (see https://www.tekhelet.com/) and many jews today have tekhelet on their tzitzit once again.
Various different ways of winding and knotting tzitzit with tekhelet. from https://www.tekhelet.com/
Modern research has produced overwhelming evidence that both the blue or indigo colored tekhelet dye and the crimson/purple argaman dye (see e.g. Exodus 25:4, 28:5-6) are made from a secretion of a snail called murex trunculus which is found in the Meditaranean Sea. This is in close agreement with the Talmudic description (b. Menachot 42:2):
אמר ליה אביי לרב שמואל בר רב יהודה הא תכילתא היכי צבעיתו לה אמר ליה מייתינן דם חלזון וסמנין ורמינן להו ביורה [ומרתחינן ליה] ושקלינא פורתא בביעתא וטעמינן להו באודרא
Abaye said to Rav Shmuel bar Rav Yehuda: How do you dye this sky-blue wool [to be used for ritual fringes]? He [Rav Shmuel bar Rav Yehuda] said to him [Abaye]: We bring blood of a ḥilazon and various herbs and put them in a pot and boil them. And then we take a bit of the resulting dye in an egg shell and test it by using [it to dye a wad of] wool [to see if it has attained the desired hue.]
The Roman Pliny the Elder also describes a similar dying process, en masse, to produce the royal purple dyes and clothes desired by Romans. While this Talmudic passage was known all those centuries, the identity of the species called chilazon was not. Begining in the late of the 19th century, various rabbis and scientists began researching this topic and by the end of the 20th century, a modern version of the ancient dying process was reproduced.
The dying process begins (after the catching of the snails, of course) by breaking open or puncturing the snail to remove a glandular secretion. This does not appear blue (or purple) immediately, the secretion is initially yellowish-greenish, and then if dye is made from it, it is usually purplish in color, not pure blue. (For more details, see https://www.tekhelet.com/tekhelet/introduction-to-tekhelet/, especially the “Chemistry of Tekhelet” section.)
Dye extraction
The snails were processed in the laboratory under normal room lighting conditions. The shells were carefully broken with a hammer blow such that the gland was punctured deliberately. The entire broken shell with the animal was then immediately placed in a glass jar (total volume 200 mL). Seconds after this puncture, as the snail was expiring, a white mucus-like fluid was observed oozing out of the gland and, within a few minutes, a violet ink-like fluid began forming in the mixture. A jar containing three snails was then sealed loosely, covered from all light sources, and left for three days in order to insure that all the dark violet pigment would be formed.
Dye vat
The dye vat was then prepared by adding aqueous sodium carbonate (Na2CO3) solution (pH 9.00 at 25 °C) … This was done in order to minimize the amount of air entering the solution so as to allow for the reduction of the pigment and to prevent the premature oxidation of the dissolved pigment. … Twice each day, the mucky mixture was stirred very gently, in order to avoid introducing an excessive amount of atmospheric oxygen.
After four days of fermentation in the alkaline solution, and a total of seven days since the glands were punctured, the pH of the mixture had not decreased, indicating the cessation of fermentation. The color of the dark, muddy mixture was green, as shown in Plate 15.3, clearly indicating that reduction of a brominated or unbrominated indigoid to its soluble alkaline leuco-form had occurred.
Dyeing
The first dyeing was performed with a woolen fleece (1.0 g) for four hours at 50°C.
Immediately upon the removal of the wool, its hue was green, but, after about 20 seconds in the air, oxidation of the leuco-indigoid components in the wool commenced and the purple color began to develop…
Plate 15.5 The three exhaust dyeings from the fermentative dye vat From Koren Z.C. 2005. ‘The First Optimal All-Murex All-Natural Purple Dyeing in the Eastern Mediterranean in a Millennium and a Half ’, Dyes in History and Archaeology 20, p. 142
Note that he (primarily) produced purple, not pure blue as well. This is due to the fact that he purposely avoided exposure to sunlight or other ultraviolet radiation sources. However, if the dye were exposed to sunlight, then it becomes de-brominated – i.e. it’s bromine atoms separate – and becomes indigo dye! (See http://tekhelet.com/pdf/Chem_Eng.html.)
Molecular diagrams, from R. C. Hoffman, R. Zilber and R. E. Hoffman, NMR spectroscopic study of the Murex trunculus dyeing process, in Magnetic Resonance Chemistry 2010, 48, 892–895
Also notice that he prevented “pre-mature oxidation” by not exposing the dye to air before the end of the process. Only when dying the wool itself was the dye, already absorbed in the wool, oxidized.
So we have now learned that two of the elements necessary to transform the snail secretion into a tekhelet blue dye are sunlight (or other UV light) and oxidation (after previous reduction and avoiding premature oxidation.)
Coincidentally, roughly speaking, these two things are also exactly related to the other end of the associative thoughts that Rabbi Meir describes begining with tekhelet – the blue color of sky!
I will explain, begining with a personal anecdote: When I was a college student there was a science course for non-science majors called “Why is the sky blue?” I, however, was a physics major and never took that class. But finally, after slogging through about four years of physics classes, I made it to an advanced senior-level physics course, Elctromagnetic Waves and Physical Optics, in which we finally learned why the sky is blue (using many equations, which I will spare you.)
The answer in super-brief terms is: sunlight and oxygen!
To explain a bit more: Earth’s atmosphere is about 21% molecular oxygen, O2. Recall that every atom or molecule absorbs and emits light at specific wavelengths. Molecular oxygen absorbs and emits in the blue portion of the visible spectrum. The emission is isotropic – i.e. equal in all directions – thus having the net effect of scattering that color of incoming light in all directions. Sunlight is a broad spectrum spanning the visible spectrum, and far beyond its edges. Atmospheric oxygen scatters incoming blue light from Sun “bouncing” it all around equally in all directions in the sky, thus making it appear to come from different directions. In contrast, the other colors of sunlight, e.g. yellow through red, are hardly scattered at all and travel in fairly straight rays from the Sun to the surface of Earth, so they appear to come from one direction – the Sun’s position – which they actually do!
Diagram of preferential scattering of blue vs. red light, at noon and sunset
So the sky appears blue because of the sunlight and oxygen, the same things that make tekhelet blue!
Science has provided another way to think of the connection between tekhelet and the sky – their “shared” color is caused in each by the same two proximate causes (albeit for different reasons.)
Here’s another interesting thought about sunlight and oxygen: they are necessary for (animal) life on Earth! Obviously, we need oxygen to breathe. Sunlight is the source of energy that indirectly, via photosynthesis and the food chain, provides us (and everything on Earth) with our nourishment and energy. (It also keeps us warm, in a temperature range in which we can survive and in many other crucial ways makes our lives possible.) Obviously, another indispensible requirement for life is water – which is also mentioned in Rabbi Meir’s chain of association – the sea, which is also the source of the snails themselves from which tekhelet is produced.
Thus the “tzitzit tekhelet mnemonic” includes an implicit reminder that God gives us life and provides for us – via the same things that make both tekhelet and the sky blue!
AstroRav: Torah & Astronomy 