461. Validating Techeiles

Tzitzis 2:4

Techeiles should only be bought from a known seller out of concern that it may not have been dyed with the intention to be used for the mitzvah. Even when purchased from a known seller, if it was checked and found to have been dyed with some other dark dye that is not permanent, it is invalid.

Tzitzis 2:5

How is techeiles checked to determine if it was dyed properly? One makes a solution of straw, a snail’s secretion, and urine that stood for 40 days. He leaves the techeiles in this solution for a full day. If the color of the techeiles doesn’t change or fade, it is valid. If it faded, we put the techeiles that changed color inside barley dough that was left to sour. The dough is baked in an oven and the techeiles is removed. If it faded even more, it is invalid. If the color came back and is now darker than it was before being baked, it is valid.