All About the Blue

Reviewed Book: Baruch Sterman, The Rarest Blue: The Remarkable Story of an Ancient Color Lost to History and Rediscovered (Guilford, CT: Lyons Press, 2012)


In the past twenty years, a new candidate has emerged for the biblical tekhelet, a central component of the mitsvah of tsitsit as commanded in the Torah.[i] The sky-blue dye that can be extracted from the murex trunculus shellfish matches many of the traditional descriptions provided by Hazal and the other bearers of Jewish tradition. Thousands of people have been convinced that this dye is exactly what God commanded them to place on the fringes of their garments, and they have purchased dyed wool to fulfill this commandment. Thousands more wait for more conclusive evidence, more widespread adoption, or more prodding. Each side of the debate is defended by vocal and prominent leaders, and a wealth of literature has been produced on the issue.

Dr. Baruch Sterman’s new book, The Rarest Blue, is not about this debate. It is not about Halakhah. It is not even written for a Jewish audience. In fact, The Rarest Blue defies categorization of any sort. It should not be labeled a story (as its subtitle suggests) for it contains too much science, though it cannot be called a book of science either since it focuses too heavily on archaeology. Nonetheless, while The Rarest Blue refuses to be measured by traditional yardsticks, it excels in the unconventional category it carves out for itself. With terrific prose and an inviting tone, the book appeals in both content and presentation to readers of all backgrounds and interests.

The Rarest Blue begins with a lengthy historical overview of dyeing in the ancient world. Evidence of shellfish dyeing dates back to eighteenth-century BCE Greece, and remnants of dyed fabric from as early as thefifteenth century BCE have been found in Syrian archaeological digs. Cuneiform tablets fromfourteenth-century Egypt contain the words takhilti and argamannu, referring, respectively, to the blue and purple dyed wool known in Hebrew as tekhelet and argaman.[ii] These precious dyes came to symbolize aristocracy and were highly demanded commodities. Trade and conquest spread these fabrics throughout the world, and they have been found as far away as St. Petersburg.[iii]

The shellfish-dyeing industry has ancient roots in Israel as well. Excavations in Dora, an ancient coastal city between Jaffa and Haifa, revealed a pair of pits, one full of shells of the murex snail, and the other containing coagulated dye. Although it is not fully understood what function each of these pits served, it is clear that a murex-dyeing factory was centered there.[iv] Digs on Mount Zion in Jerusalem have uncovered shells of the murex trunculus.[v] Tufts of purple and blue wool found in Masada from the time of the Bar Kokhba revolt have been chemically analyzed and found to have been dyed using the murex snail as well.[vi]

The prestige of these colors led to decrees on who could and could not wear them. In Rome, the color of one’s toga denoted his status and stature, and Julius Caesar and Augustus established limits on which royal advisors could wear purple robes. In the fifth century CE, the emperor Theodosius II forbade commoners not only to wear purple-dyed clothes, but even to own them. Other sources imply that the same restrictions applied to blue clothing. For Jews living under the Roman Empire, this made wearing tekhelet illegal and dangerous. Hazal, aware of these circumstances, comfort those who cannot fully observe the mitsvah of tsitsit.[vii] As is apparent from late Talmudic sources, the rabbis no longer wore tekhelet, but still tried to perpetuate the knowledge of how to produce it. In the seventh century CE, the last vestiges of the dye industry in Jerusalem were destroyed, and by 1453, murex dyes and fabrics had disappeared from the world entirely.[viii]

Although the loss of tekhelet was tragic for observant Jews, the global dyeing industry had a ready substitute: plant-based indigo dye. The chemical equivalent of murex dye, indigo is cheaper and easier to obtain and use, and has colored everything from King Tut’s kerchief to Levi Strauss’s denim jeans. It is also disqualified for use as tekhelet, which must come from the hilazon creature, rather than from a plant. The virtual equivalence of tekhelet and kala ilan (the Talmud’s name for indigo) is the focus of many strong statements in Hazal about the ethical imperative of dye sellers to market their goods truthfully.[ix], [x]

The modern quest to rediscover the hilazon began with the Radziner Rebbe, Gershon Henokh Leiner, in the nineteenth century. A Torah prodigy who independently studied secular subjects as well, Leiner traveled to the aquarium in Naples, Italy, hoping to find the sea animal that produces the biblical tekhelet. From Hazal’s many descriptions of the hilazon, he identified the ten most reliable characteristics and concluded that the cuttlefish was the right creature. Unfortunately, the cuttlefish creates a brown dye, not the blue that he had wanted. Leiner consulted a chemist, who devised a chemical process by which this brown dye could be made blue. In 1891, the Radziner Rebbe opened a tekhelet factory and began to sell dyed strings.[xi]

R. Isaac Herzog was the second modern Jewish leader to investigate tekhelet. Educated in the University of London and later Chief Rabbi of Israel, Herzog was also obsessed with the study of tekhelet. He investigated the chemical protocol employed by the Radziner Hasidim and was shocked to discover that their process was the same one that was used to produce a synthetic dye known as Prussian blue, and the cuttlefish ingredient played no real role. Herzog’s own investigations led him to the murex trunculus, which was known to have been central to the dyeing industry in the ancient world and appeared to match many of Hazal’s descriptions. However, this snail produced only a violet dye. Though the scientific community widely accepted that this violet color must have been the biblical tekhelet, Herzog stayed committed to the halakhic sources that tekhelet is sky-blue. He died in 1959, uncertain whether he had discovered the ancient hilazon. In the 1980s, chemist Otto Elsner found that by merely exposing the ink of the murex trunculus to sunlight, its color changes from violet to sky-blue; the murex trunculus can make blue dye after all. Scientific acceptance of the murex as the hilazon and Herzog’s insistence on the dye’s color finally converged.[xii]

At this point in The Rarest Blue, the historical narrative ends, and the book contains chapters on the physiology of the murex and the chemistry of the dyeing process. The descriptions are tailored to non-scientists and explain many of the challenges involved in murex-dyeing. The subtleties and complexities of the procedure not only stimulate appreciation for the ancient dyers’ craft, but also clarify many challenging technical descriptions found in the Talmud and non-religious sources. For example, it is now understood why the Talmud says that one must remove some dye from the heated mixture in order to test its color, rather than simply looking in the pot.[xiii] The hot dye is in a chemically reduced state that allows it to bind to wool, but which changes its appearance; only when it is removed and exposed to oxygen does it take on the blue color and remain fixed to wool.[xiv]

The book then devotes a pair of chapters to the color blue. A physicist by profession, Dr. Sterman explains why nature’s palette is full of reds and greens, but mostly devoid of blues. Blue can be physically produced by one of five physical phenomena, ranging from Rayleigh scattering (which is responsible for the blue sky) to quantum absorption of radiation (the blue sea) to crystallography (blue gemstones). None of these, however, can lead to an organic dye. The exception is indigo, whose unique molecular structure has a high degree of symmetry, allowing it to absorb and radiate light of blue wavelength.[xv] Based on the physics, it is unlikely that an as-of-yet undiscovered organic pigment will be able to produce a blue dye.[xvi] Sterman then switches from science to society with a discussion of various cultures’ perspectives on color in general, and blue in particular.[xvii] He finishes the book with a brief summary of the birth of the modern tekhelet industry, which gets its dye from the murex trunculus.[xviii]

The book ends without a call to action and without an insistence that the proposed tekhelet is the biblical one. Sterman’s foundation, the Ptil Tekhelet Association, is mentioned only in passing, and its web address is not even provided.[xix] Nevertheless, the religiously disinterested format of the book is very effective in convincing the reader that the murex trunculus is the hilazon and that the blue dye it produces is the right tekhelet. By steering clear of the usual debate over admissibility of archaeology in the halakhic court or the degree to which this tekhelet has been adopted (or not) by rabbinic authorities, Sterman frees himself to make it undeniably clear that tekhelet was universally known throughout the ancient world as the sky-blue dye extracted from a murex trunculus. It existed with this name long before Jews were commanded to put it on their garments, and it persisted well into the period of Hazal. It was produced and worn in biblical Israel and the world over. Everyone knew what tekhelet was, and now we do as well. While it is not clear whether the author intended for this result, The Rarest Blue presents a powerful proof for the authenticity of the current tekhelet.

Though Sterman generally avoids matters of Halakhah, his occasional references create a deeper appreciation of traditional Jewish sources in light of the history and archaeology he discusses. For example, it is known from the archaeological record that blue and purple dyes were very difficult to manufacture and very costly, so their use was mostly limited to noblemen. The Torah reflects the same phenomenon: The priests wore garments dyed with tekhelet.[xx] The novel extension in Jewish law is that every male affixed a string of tekhelet to his clothing, illustrating “the epitome of the democratic thrust within Judaism which equalizes not by leveling, but by elevating: All of Israel is enjoined to become a nation of priests.”[xxi]

Halakhah can also explain perplexing archaeological finds. All ancient dyeing factories were located on the sea, both because it is the natural habitat of the murex snail and because the odor of the dye fermentation had to be kept away from cities. Why, then, were murex shells found in Jerusalem? Sterman suggests that their presence does not mean that dye was produced within the city. Rather, shopkeepers selling dyed wool for tsitsit proudly displayed these shells in their stores to show that their products were from authentic shellfish, not indigo.[xxii]

Sterman could have included more discussion on religious philosophy and Halakhah, albeit at the risk of limiting the book’s audience. Two particular subplots in the history of tekhelet stand out as deserving further attention. R. Herzog’s steadfastness to the tradition that tekhelet is blue, in opposition to everyone in the scientific community of his time, is an inspiration for other debates between science and tradition.[xxiii] R. Leiner and R. Herzog’s unenviable responsibility to decide which traditional descriptions of tekhelet were reliable and which others were hyperbolic or misinformed[xxiv] has implications for other modern questions in Halakhah.[xxv]

Needless to say, Sterman is not as ambivalent to the authenticity of his foundation’s tekhelet as his book suggests. He certainly has more to say, as evidenced by the number of articles he has posted on the Ptil Tekhelet website. In a recent lecture at Yeshiva University, he firmly but respectfully challenged the common counterarguments to the adoption of tekhelet for the mitsvah of tsitsit.[xxvi] But his book operates independently of this dispute. It delivers a historical and scientific account of the murex tekhelet that can be appreciated by any audience, a valuable contribution to the public understanding of this important mitsvah.


Gilad Barach is a third-year YC student majoring in Physics and Mathematics, and is a staff writer for Kol Hamevaser.


[i] Numbers 15:38. Tekhelet is also required in certain priestly garments and Tabernacle materials.

[ii] Baruch Sterman, The Rarest Blue: The Remarkable Story of an Ancient Color Lost to History and Rediscovered (Guilford, CT: Lyons Press, 2012), chapter 2.

[iii] Ibid. chapter 3.

[iv] Ibid. chapter 4.

[v] Ibid. chapter 5.

[vi] Ibid. chapter 6.

[vii] “Greater is the punishment for [those who do not wear] white, than for [those who do not wear] tekhelet” (Menahot 43b, cited by Sterman, p. 85).

[viii] Ibid, chapter 6.

[ix] Ibid. chapter 7.

[x] For example: “The Holy One, blessed be He, will exact vengeance from him who attaches to his garment threads dyed with kala ilan and maintains that they are genuine tekhelet’” (Bava Metsia 61b, cited by Sterman, p. 70).

[xi] Ibid. chapter 8.

[xii] Ibid. chapter 9.

[xiii] Menahot 42b.

[xiv] Sterman, The Rarest Blue, chapter 11.

[xv] Ibid. chapter 12.

[xvi] As Sterman recently said in a lecture in Yeshiva University, “At least as far as science knows today, there is not even the possibility for there to be another blue dye that is created in a natural process.” (Baruch Sterman, “Evidence for Techeiles,” YUTorah Online, 2 December, 2012, available at:

[xvii] Sterman, The Rarest Blue, chapter 13.

[xviii] Ibid. chapter 14.

[xix] The website,, is an impressive resource on the history, science, and halakhot of tekhelet. The site contains many articles and multimedia links exploring every aspect of the mitsvah of tekhelet and murex-dyeing. Tallitot and tsitsit with tekhelet, as well as sets of dyed string, are available for purchase.

[xx] Exodus 28.

[xxi] Jacob Milgrom, cited by Sterman, The Rarest Blue, p. 29.

[xxii] Ibid. 67-70.

[xxiii] Ibid. 129-132.

[xxiv] Ibid. 112, 129-130.

[xxv] Two possible areas where the methodologies of R. Leiner and R. Herzog might be applied are the identification of shibbolet shu’al (usually translated as oats), one of the five grains in Halakhah, and the pronunciation of certain letters and vowels in Hebrew. Interestingly, these matters are also heavily influenced by the relative weight one gives tradition versus other sources of knowledge, a question which arises regarding the adoption of the modern tekhelet as well.

[xxvi] Sterman, “Evidence for Techeiles.”