Shulchan Aruch/Yoreh Deah/94
One who inserts a milk spoon into a meat pot, or the opposite, we estimate based on is the total amount of what was inserted from it into the pot (if the spoon is ben yomo, that is, it was used in a vessel that was on the fire within the last 24 hours). And there is one who says that a metal spoon, we measure based on all of [the spoon], because when part of it gets warm, it all gets warm. (And the first view is core [i.e. correct], and such is the practice.
If he insterted a spoon into the pot twice and it did not become known in the meantime, he needs 60 times [the amount of the spoon] twice. And there are those who say that it 60 times once is enough, and this is the custom.
If there is 60 times the amount of the spoon, then the pot and the food are allowed, but the spoon is forbidden whether with meat or with dairy, because it absorbed from a meat and milk mixture. And even post facto, the spoon forbids if they returned and dipped it into either meat or dairy if within 24 hours. But if there is not 60 times the amount, then it is forbidden to derive benefit from any of them, even the pot. However, you are allowed to put in it fruit or cold food because you are not benefiting from the actual forbidden item.
If the spoon has not been used within 24 hours, the pot and the cooked food are allowed, but the spoon is forbidden ab initio, whether with meat or with dairy. But in any case, post facto the spoon does not forbid because it had not been used within 24 hours.
If you boiled water in a new pot and dipped a dairy spoon in it, then boiled more water and dipped a meat spoon in it, and the two spoons had been used within the last 24 hours and in neither of the cases had there been enough water for 60 times [the spoon], you may not use the pot for meat or dairy, but can cook anything else in it because it was new and you had never cooked anything in it. Hagah – But if you did cook meat or dairy in it, then it is allowed because it is a case of second degree transference. A pot in which you cooked vegetables or water and dipped in a spoon that had been used within 24 hours, and the pot had not been used within 24 hours, or the opposite, or if there was 6o times in the food, then everything is allowed. But we have the custom to be strict and eat the food in the way of the dish (meat or dairy) that had been used within 24 hours and to forbid the dish that has not been used within 24 hours, but this is only a stringency because according to the law, all of these are allowed.