Could you explain to why, in Talmudic times, did they have to do teima? Is it still necessary?

“teima” refers to the procedure of taking a sample of the dye solution on to a test piece of wool to determine if the dye in the vat has reached the proper state to start dyeing wool. The need for such a procedure seems to indicate that vat dyeing was the method employed since in vat dyeing you don’t know what color you will get until the dye solution is oxidized in the fabric (i.e., in the vat it is in a “colorless” [really its yellowish] chemically reduced state). In ancient times this was probably very important since the process was less controlled, they threw in many snails and fermented them for a week; it wasn’t known exactly when the solution had reached the proper point for dyeing. Today, after much fine tuning, we weigh out fairly exact quantities of snail glands and add precise measures of specific chemicals – all of which combine to make the dye solution within a matter of minutes. We have reached a relatively stable process for which we are not in need of performing samples to insure the proper color outcome. (As far as I am aware, teima was performed out of a technical necessity not a halachic one). – Mois Navon.