The Right Chemistry: Rediscovering a biblical blue dye

The Right Chemistry: Rediscovering a biblical blue dye

The Right Chemistry: Rediscovering a biblical blue dye 1000 750 rhecht

Baruch Sterman, an Israeli scientist, has taken an interest in producing the blue dye he believes is tekhelet from snails.

The ancient Phoenician city of Tyre on the Mediterranean coast was permeated by a putrid smell. It came from the processing of mounds of snails harvested from the ocean to yield the most famous dye of antiquity, Tyrian purple. As early as the 17th century BC, Phoenicians had discovered that extracts of three types of sea snails, Murex brandaris, Thais haemastoma and Murex trunculus were capable of yielding dyes ranging in shades from reddish to bluish purple.

Sea snails do not produce coloured compounds to satisfy human vanity. They produce them to ward off predators. The very compounds that are prized for their ability to dye fabrics with stunning colours have flavours so bitter that predators learn to leave the snails alone. The three most noteworthy compounds the snails produce are dibromoindigo, monobromoindigo and indigo, with the relative ratio of these determining the colour of the dye that can be produced.

Phoenician dyers mastered the art of using the three types of snails to produce a variety of purple hues. Since huge numbers of snails were required to produce a small amount of dye, Tyrian purple was more expensive than gold! Given that only the very rich could afford purple fabrics, the wearing of such apparel became a status symbol. This was particularly the case in ancient Rome with purple togas being restricted to the emperor and victorious generals. Julius Caesar wore a purple toga and legend has it that Cleopatra’s ship had sails dyed with Tyrian purple. After the fall of the western Roman Empire, the Eastern Empire, eventually known as the Byzantine Empire, continued to revere purple. Emperor Justinian I was routinely clad in purple and women of royal lineage gave birth in rooms decorated with purple fabric, giving rise to the expression, “born into purple.”

After about the 7th century, historical references to Tyrian purple faded and by all accounts the Middle Ages were characterized by drab apparel. It was in 1858 that interest in the ancient dye was rekindled when French zoologist Henri de Lacaze-Duthiers on a trip to Spain watched in amazement as a fisherman smeared his shirt with the slimy exudate of a snail, leaving a stain that at first was yellow but then turned to purple! His curiosity aroused, Lazace-Duthiers rediscovered the three mollusks that were capable of producing purple-blue dyes. He also suggested that one of these, Murex truculus, was the source of the legendary blue dye referred to in the Old Testament.

“God said to Moses, speak to the children of Israel and say to them, that they shall make for themselves tzitzit on the corners of their garments throughout their generations. And they shall place upon the tzitzit of each corner a thread of tekhelet that they shall see and remember all of the commandments of God.” Tzitzit are specially knotted ritual fringes, or tassels, on garments worn in antiquity by Israelites and today by observant Jews as a constant reminder to live according to God’s laws. According to the Bible, one thread on each corner was to be coloured with tekhelet, a blue dye. There is no mention of the source of the dye and the only information available comes from some esoteric references in the Talmud, a compilation of discussions and debates by Jewish scholars about the Torah, the Hebrew Bible. The Talmud, completed around the sixth century, suggests a snail known as “Hilazon “as the source of tekhelet and describes the dye as a colour similar to the sky and sea.

The problem is that there are no historical records of any snail that can produce a blue dye resembling the colour of the sky. However, a chance discovery in 1980 by Israeli dye chemist Otto Elsner may have solved the mystery. While researching methods that may have been used by ancient dyers, he was working with extracts of Murex trunculus. Because of the smell involved, he worked near an open window. As expected, after extracting the snail’s glands he produced a purple dye. But one day, brilliant sunshine came through the window, and when Elsner lifted his wad of dyed wool from the solution he was stunned to see it turn from purple to a brilliant blue!

As further research would show, energy from the sun’s ultraviolet rays broke the bonds between carbon and bromine atoms in bromoindigo yielding pure indigo, the same blue colour available from the indigo plant. Ancient dyers working in the sunshine of the Middle East could well have noted this reaction and used it to produce tekhelet. Baruch Sterman, an Israeli scientist who is also a scholar of Jewish history, has taken an interest in producing the blue dye he believes is tekhelet from snails. He now gives demonstrations of the method at his workshop to visitors and turns out cotton fibres that once again observant Jews can use as a component of the fringes on their garments reminding them of the ever-presence of God, and hopefully, of the wonders of chemistry.

Joe Schwarcz is director of McGill University’s Office for Science & Society ( He hosts The Dr. Joe Show on CJAD Radio 800 AM every Sunday from 3 to 4 p.m.



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