We will begin by reviewing the terminology and basic halachot. A pot sitting on the fire is termed a kli rishon she’al gabei ha’eish. When the pot is removed from the fire, it becomes simply a kli rishon, which is assumed to be capable of cooking as long as the food therein is at least the temperature of yad soledet bo,[1] which may be as low as 45°C (113°F).[2] A utensil into which hot food is transferred from a kli rishon is called a kli sheni. The gemara[3] states that a kli sheni is not capable of cooking, even when its contents are yad soledet bo.[4] However, this is likely not the case regarding foods that are particularly easily cooked,[5] and it is not always clear which foods are in that category.[6]
The physical difference between a kli rishon and a kli sheni is that the former’s walls start off hot and will keep its contents hot for longer than the kli sheni pot will, since the latter’s walls start off cold and thus hasten the cooling of its contents.
There are some borderline cases in which it is difficult to determine whether a utensil is a kli rishon or a kli sheni. One of those is your question, where the kli in which the food is now found (the ladle) spent some time in a kli rishon, thereby becoming hot not only from the food within it, but also from heat outside it. There is a machloket on the matter,[7] and it is generally viewed as an unresolved issue that should be treated as a safek.[8] The Chazon Ish[9] posits that if the ladle is in a pot while it is on the fire, it certainly turns into a kli rishon. (In general, one should not use a ladle in a pot on the flame, because of the halachic problems of stirring.[10]) The consensus is that if the ladle sits in the kli rishon for a long time, it takes on halachic characteristics of a kli rishon.[11] How long this takes is arguably affected by how hot the soup is and the heat conductivity of the material. However, poskim generally do not distinguish in these contexts between the types of material the ladle is made of. It is only in tangentially related contexts that a possible distinction between metal and other materials exists.[12]
How we deal practically with the safek about the status of a ladle depends on what other factors are involved, and we will mention a few. The Mishna Berura[13] implies that one should not pour from the ladle onto something that is forbidden to be cooked. However, even according to the ruling of the Rama[14] that one may not put bread into a kli sheni, the Mishna Berura[15] allows putting bread into a bowl into which soup was ladled. Acharonim explain that the leniency in this case is due to the fact that many do not agree that it is forbidden to cook bread that was already baked.[16] Another case in which the Mishna Berura[17] is lenient is pouring from a ladle onto the leftover soup in a bowl (e.g., when taking a second helping), even if the leftover soup already cooled down completely and is thus usually forbidden to reheat. The apparent logic here is that since it is not unanimously agreed that it is forbidden to reheat a liquid that was cooked and cooled off,[18] we can be lenient to treat the ladle as a kli sheni.
If one wants to avoid the situation of the ladle becoming a kli rishon according to the consensus of poskim, he should leave it out of the soup pot between uses. However, if one leaves it out enough for the residue to cool off totally, there is a problem returning the ladle to the pot when one wants to serve more soup, unless one first cleans off the liquid residue. There are indeed some who recommend cleaning the ladle before putting it back into the soup pot.[19] However, we feel that (at least Ashkenazim) may rely on the view of the Igrot Moshe[20] that insignificant quantities of already cooked liquid may be heated up again.