In Numbers chapter 15, God instructs Moses: “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 sky blue to the fringe at each corner.” (Numbers 15:38) In the next two verses, we are told the reason: “That shall be your fringe; look at it and recall all God’s commandments and observe them, so that you do not follow your heart and eyes in your urge to stray. Thus you shall be reminded to observe all My commandments and to be holy to your God.” (Numbers 15:39–40)
These verses are included in the third paragraph of the full Shema prayer, recited each morning and night. Typically, Jews who wear a tallit, a four-cornered prayer shawl, kiss the fringes when the word tzitzit (fringes) is recited. The Hebrew word for sky blue is techelet, a special hue that was also used for the high priest’s clothing and to dye tapestries displayed in the Temple. On today’s daf, we learn the recipe for making techelet.
Abaye said to Rav Shmuel bar Rav Yehuda: How do you dye this sky-blue wool ? Rav Shmuel bar Rav Yehuda said to Abaye: We bring blood of a hilazon and various herbs and put them in a pot and boil them. And then we take a bit of the resulting dye in an eggshell and test it by using it to dye a wad of wool to see if it has attained the desired hue. And then we throw away that eggshell and its contents and burn the wad of wool.
You now most likely have the same question that the rabbis had: What is a hilazon? Two days from now, on Menachot 44, we will get an answer:
Its body resembles the sea, its form resembles that of a fish, it emerges once in 70 years and with its blood one dyes wool sky-blue (for ritual fringes). It is scarce, and therefore it is expensive.
This description isn’t terribly helpful in identifying the hilazon. If the creature only appeared once in 70 years, it’s likely most of the rabbis never saw it with their own eyes. That’s why it was so important to test it: A dyer might have extracted a blue dye from a fish they thought was a hilazon, only to find out that it didn’t produce the specific sky blue needed for the ritual fringe.
But say the dyer is successful and the test batch produces the exact color needed, demonstrated by dipping a cord of wool in an eggshell filled with the dye. Why, then, would they need to throw away the eggshell and burn the test wool? If techelet is so rare and expensive, why not just make tzitzit threads out of it? The Gemara explains:
Learn from this statement three halakhot: Learn from it that wool dyed for the purpose of testing is unfit (for ritual fringes). And learn from it that we require dyeing for the sake of the mitzvah. And learn from it that using dye for testing renders all the dye in that vessel unfit.
According to the Gemara, although testing is required to make sure that it’s really techelet, the batch can’t be used for anything else. The test wool also can’t be used to make tzitzit because, as the Gemara explains later, thread dyed to make tzitzit must be done for that sole purpose. Therefore, a test batch, while needed to ensure quality control, can’t be used for the actual garment.
But if you look at a typical tallit today, you’re likely to see fringes that are only white. What happened to the blue thread?
For centuries, even though Jews had the recipe we just read, they couldn’t find the key ingredient: blood from a hilazon. Over the years, various communities tried and failed to come up with the correct blue dye but invariably it wasn’t the right shade, or it wasn’t color fast. And so, it wasn’t possible to observe that particular mitzvah, resulting in tallitot with all-white fringes.
Until very recently, that is. Over the past century, scientists and scholars have identified the murex trunculus as the hilazon. Its blood produces a purple dye that, when exposed to sunlight, turns a perfect shade of sky blue that does not wash out. On the color absorption scale, techelet tests at a delightfully apt wavelength of 613.
And so, once again, some Jews have begun to attach fringes with a string of blue to their tzitzit, refreshing this mitzvah for our own time.
Read all of Menachot 42 on Sefaria.
This piece originally appeared in a My Jewish Learning Daf Yomi email newsletter sent on February 22, 2026. If you are interested in receiving the newsletter, sign up here.
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