Mishneh Torah (Rambam) משנה תורה

The term techelet mentioned throughout the Torah refers to wool dyed light blue - i.e., the color of the sky which appears opposite the sun when there is a clear sky. The term techelet when used regarding tzitzit refers to a specific dye that remains beautiful without changing. [If the techelet] is not dyed with this dye, it is unfit to be used as tzitzit even though it is sky blue in color. For example, using isatis, black dye, or other dark dyes, is unacceptable for tzitzit. The wool of a ewe that a goat gave birth to is unacceptable for use as tzitzit.

How is the techelet of tzitzit dyed? Wool is taken and soaked in lime. Afterwards, it is taken and washed until it is clean and then boiled with bleach and the like, as is the dyers' practice, to prepare it to accept the dye. A chilazon is a fish whose color is like the color of the sea and whose blood is black like ink. It is found in the Mediterranean Sea. The blood is placed in a pot together with herbs - e.g., chamomile - as is the dyers' practice. It is boiled and then the wool is inserted. [It is left there] until it becomes sky-blue. This is the manner in which the techelet of tzitzit [is made].

One must dye tzitzit techelet with the intention that it be used for the mitzvah. If one did not have such an intention, it is unacceptable. When one places some wool in the pot in which the dye was placed, to check whether the dye is good or not, the entire pot may no longer be used [for tzitzit]. [If so,] how should one check [the dye]? He should take some dye from the pot in a small container and place the wool he uses to check in it. Afterwards, he should burn the wool used to check - for it was dyed for the purpose of checking - and pour out the dye used to check it, since using it for an experiment disqualified it. Afterwards, he should dye [the wool] techelet with the remainder of the dye which was not used.

Techelet should only be purchased from a recognized dealer because we are concerned that perhaps it was not dyed with the intention that it be used for the mitzvah. Even though it was purchased from a recognized dealer, if it was checked, and it was discovered that it was dyed with another dark dye which is not of a permanent nature, it is not acceptable.

How can techelet be checked to see whether it has been dyed properly or not? One takes straw, the secretion of a snail, and urine that had been left standing for forty days and leaves thetechelet in this mixture for an entire day. If the color of thetechelet remained unchanged, without becoming weaker, it is acceptable. If it became weaker, we place the techelet which changed color inside a dough of barley meal that was left to sour for fish brine. The dough is baked in an oven, and then the techelet is removed. If it became even weaker than it was previously, it is unacceptable. If this strengthened the color and it became darker than it was before being baked, it is acceptable.

One may purchase techelet from an outlet which has established a reputation for authenticity without question. It need not be checked. One may continue to rely [on its reputation] until a reason for suspicion arises. Should one entrust techelet to a gentile for safekeeping, it is no longer fit for use, [because] we fear that he exchanged it. If it was in a container and closed with two seals, one seal inside the other, it is acceptable. If, however, it had only a single seal, it may not be used.

If a person found techelet in the marketplace - even strands which were cut - it is not fit for use.If they were twisted together, however, they are acceptable. [The following rules apply when] a person purchases a garment to which tzitzit are attached in the marketplace. When he purchases it from a Jew, he may presume [that it is acceptable]. If he purchases it from a gentile merchant, it is [presumed to be] acceptable; from a non-Jew who is a private person, it is not acceptable.

When a garment is entirely red, green, or any other color [besides white], its white strands should be made from the same color as the garment itself. If it is green, they should be green. If it is red, they should be red. Should the garment itself be techelet, its white strands should be made from any color other than black, for it resembles techelet. He should wind one strand of techelet around all the strands, as one does with other tzitzit that are not colored.

The punishment given someone who does not wear [tzitzit of white strands] is more severe than that given one who does not wear techelet, because the white strands are easily accessible while techelet is not available in every time and in every era, because of the [unique] dye mentioned above.