One who wishes to prepare tea using a tea bag must do so in a kli shlishi. This means that he must first pour the hot water into a cup (which becomes a kli sheni). Then, from this cup, he must pour the water into another cup, which becomes a kli shlishi. He may then place a tea bag in the water.
At first glance, it would seem that one should be allowed to do this even in a kli sheni, since the rule is that a kli sheni cannot cook. However, as we established, even a kli sheni can cook kalei ha-bishul, and some are concerned that tea leaves fall into this category. Additionally, others believe that there is a rabbinic prohibition on placing uncooked food into a kli sheni because it resembles cooking (MA; MB 318:34). Therefore, one who wishes to prepare tea must do so in a kli shlishi.
Some are even more stringent. They maintain that since we see that a tea bag releases color and flavor into the water even in a kli shlishi, this is an indication that tea cooks very easily, and therefore one may not put it even in a kli shlishi (AHS; Ĥazon Ish). However, according to the majority of poskim, the rule that a kli shlishi does not cook is absolute, and one may always put uncooked food in a kli shlishi. Additionally, just because a tea bag releases color and taste in a kli shlishi does not prove that it has been cooked. The fact is that even if you put a tea bag in water that is only 40°C and not hot enough to cook at all, the bag will still release color and taste. Therefore, one may put a tea bag into hot water in a kli shlishi.
If one prepared tea essence before Shabbat, one may pour it into hot water in a kli sheni. This is because the essence is not considered kalei ha-bishul, so pouring it into the cup does not resemble an act of cooking.
Those who are especially meticulous, and avoid coloring foods le-khatĥila (see 12:10 below), should pour the essence into a kli shlishi and pour water onto it from a kli sheni. In this manner, the essence does not play an active role in coloring the water. All agree that in such a case there is no issue of coloring.
If the tea essence gets used up on Shabbat, and only tea leaves remain with no liquid, one may prepare new essence by pouring hot water from the urn into a cup, which becomes a kli sheni, and from there pouring the water onto the tea leaves. This is not considered cooking, because the tea leaves were already cooked before Shabbat.
. As we learned at the end of the previous note, one should not place tea leaves in a kli sheni, out of concern that it resembles cooking. Similarly, we must take into account the possibility that tea leaves are kalei ha-bishul, which may not be placed in a kli sheni. Even pouring onto them from a kli sheni is prohibited. But if the leaves are inside a tea bag, one may pour onto them from a kli sheni. This is because pouring only cooks the outer layer, so the leaves inside the bag will not be affected (Or Le-Tziyon 2:30:3; Yalkut Yosef 318:41).As I wrote above, if the liquid of the tea essence prepared before Shabbat is running out, one should pour hot water from a kli sheni onto the cooked tea leaves. Since the leaves were already cooked before Shabbat, there is no longer a prohibition of cooking them. Pouring water directly from the urn into the container of the essence is prohibited, since the liquid of the essence has already cooled down, and many poskim maintain that one may not heat them to yad soledet bo in such a case (see section 5 above; MB 318:39). Furthermore, perhaps the tea leaves did not cook entirely before Shabbat, in which case pouring the water on them will complete their cooking. But irui from a kli sheni is certainly not cooking in this case. Even if we are concerned that the leaves are considered kalei ha-bishul, since they were already cooked before Shabbat there is no concern that pouring from a kli sheni will continue to cook them. One also does not need to worry about cooking the cold water left with the leaves, because water is definitely not included in kalei ha-bishul.