Select sources on techelet

(37) And the LORD spoke unto Moses, saying: (38) ’Speak unto the children of Israel, and bid them that they make them throughout their generations fringes in the corners of their garments, and that they put with the fringe of each corner a thread of blue. (39) And it shall be unto you for a fringe, that ye may look upon it, and remember all the commandments of the LORD, and do them; and that ye go not about after your own heart and your own eyes, after which ye use to go astray;

Halacha 1 - 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. . .

Halacha 2 - How is the techelet of tzitzit dyed? Wool is taken and soaked in lime. Afterwards, it is taken and washed until it is clean, then boiled with bleach and the like, as is the dyers' practice, to prepare it to accept 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].

Halacha 3 - 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.

Halacha 8 - 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 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.

Halacha 9 - 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.