460. How Techeiles is Made

Tzitzis 2:2

The techeiles of tzitzis is dyed as follows: first, wool is soaked in lime (the calcium type, not the fruit). Next, it is washed until clean, then boiled with bleach as dyers do to prepare it to hold the dye. The chilazon is a shellfish from the Mediterranean Sea that is the color of the ocean and whose blood is black like ink. The blood of the chilazon is put in a pot with herbs like chamomile, as dyers do. This is boiled and the wool is inserted until it turns sky blue.

Tzitzis 2:3

When dyeing tzitzis blue, one must have the intention that it is to be used for the mitzvah. If one did not have this intention, the techeiles is invalid. If one puts wool into the pot of dye in order to see if the dye is good, the entire pot may no longer be used. So how can one check the dye? He must take some dye from the pot and put it into a small container. He may then put the wool he will use to check into this sample of the dye. When he is finished, he burns the wool that was used to check the dye and pours out the rest of the dye sample that was tested. This is because using it for this examination invalidated it for use in the mitzvah. Afterwards, he may use the rest of the dye from the pot, which was not used for the dye test.