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I have been reading the various questions and answers about techeilet but I still don't understand the exact demand (and apologies if I missed this amongst the other questions). Is the goal to have a thread of a certain color or sourced from a particular origin. The source seems important because people discuss the talmudic description of the Chilazon (eg. "According to the Tosefta (Men. 9:6), the Ḥillazon is the exclusive source of the dye." from wikipedia) but there is also discussion of the particular shade.

If the issue is the specific color, then do techeilet strings have to be checked against a standard on any schedule to ensure that there has been no fading (like how one checks a mezuzah)? If the issue is the source then does the color not matter ultimately (as per the comment on this question)? If the requirement is a combination of a particular species and shade then the same issues related to checking the color would still exist.

  • Since the same dye can take both loosely and strongly in the material depending on use and length of the process, it would seem difficult to say that the requirement for a specific color is restricted by source or, conversely, that the requirement for a source must lead to a distinct color... – Isaac Kotlicky Mar 28 '16 at 14:36
  • @IsaacKotlicky, it's a machloket Rishonim whether the "sky blue" of techeilet refers to the sky at noon or night, I believe that Rashi holds like the former while the Rambam holds like the latter. – Noach MiFrankfurt Mar 28 '16 at 15:19
  • related judaism.stackexchange.com/a/12596/759 – Double AA Mar 28 '16 at 15:50
  • @NoachmiFrankfurt Where do they say that? – Double AA Mar 28 '16 at 15:50
  • @DoubleAA, I may have misconstrued the comments here: judaism.stackexchange.com/a/4507/4504 – Noach MiFrankfurt Mar 28 '16 at 17:23
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The word "tekheiles" actually refers to a kind of blue wool; not the dye itself, but the dyed wool. Most hold that tekheiles must come from the chilazon.

There is the opinion of the Tif'eres Yisrael (stated in his introduction to Seder Mo'eid, kelalei Bigdei Qodesh "hinei") that tekheiles is any colorfast blue wool of the right hue. According to him, the Tosefta's and gemara's insistence on using the chilazon is a historical artifact of that being the only dye of that hue which is colorfast when applied to wool that they knew of in their day. But according to him, with today's chemistry, other dyes would be kosher options.

As far as I know, the Tif'eres Yisrael is unique in holding that opinion. Although some (including the TY) do read the Rambam as saying as much about tekheiles for kohein's clothing, they would still agree that for tzitzis the Rambam requires the dye come from a chilazon.

The "space" of possible colors is three dimensional, so a color cannot be described with fewer than three numbers. Monitors mix primary colors, so you may have seen RGB (red-green-blue) numbers to describe the desired color. Dyes use a very different set of axes: hue, saturation and value (or light). Hue tells you where on the spectrum it is, and is usually described as a circle. (Closing the spectrum by connecting the bottom end and top end at purple / magenta.) Saturation is how much color, and value (or light) is how dark (or light) it is.

Picking a given dye would mean that the hue should be constant. But not the other factors. The amount of dye used would change the saturation and value of the resulting color.

BUT: being told the dye must be extracted from a certain kind of marine animal doesn't mean that all marine animals of that species produce exactly the same chemicals in the same proportions. Just as there is slight color variation among the greens of maple leaves. And perhaps there is a recipe by which the dye is extracted, or attached to the wool. So, it's not even clear that the dye is always exactly the same hue, either.

Nor that there is one kosher hue rather than a range of hues. For example, the Rambam's "color of the sky on a clear day" is not that specific.

As for fading.... The dye must be colorfast. The gemara even offers a test for colorfastness to distinguish between true tekheiles and indigo dyed wool (from the indigo plant, in the tea family of bushes). So, if you see fading, either the dye was not applied correctly, or it's not the right dye. Either way, any fading at all would mean the wool isn't real tekheiles.

As an example of what we expect when we say "colorfast": R/Dr Moshe Tendler reports that when he took murex-dyed wool and left it in bleach, the wool eventually dissolved, leaving the dye as a sediment on the bottom.

  • followup -- would "colorfast" be based in the technology of back then or the technology now? (If we have harsher chemicals now than existed, does the dye have to stand up to what they knew or what we know?) – rosends Mar 28 '16 at 16:16
  • The question of "colorfast" and technology is not arbitrary -- what I have read mentions that "colorfast" indicates "resistance to fading" and with newer technologies like artificial light and chemical baths fading might still happen. I don't know the result of the experiments jstor.org/stable/1506527?seq=1#page_scan_tab_contents – rosends Mar 28 '16 at 16:27
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    @Danno: People treat the gemara's test for distinguishing indigo (from the eponymous plant) from tekheiles as thought it were giving the test of what is sufficiently colorfast. I do not know where that comes from, though; it's not like I saw a formal source or anything before the current murex dying that makes that identification. It requires considering your question, which I agree is valid. Still, something less likely to oxidize or dissolve than the wool itself MUST be sufficiently colorfast for the halakhah. – Micha Berger Mar 28 '16 at 19:35

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