Avodah Zarah, Chapter 5, Mishnah 11. The subject of the Mishnah: how to make a winepress fit for use - the press in which grapes were trodden in order to extract the juice for making wine. You can picture the press as a large bathtub or a shallow pool: the grapes are laid on the floor, they are trodden on, and the juice that flows out is collected for making wine.
Background: the pitch on the press:
The press itself is made of stone, wood or earthenware. In order to seal it against liquid, the practice was to coat it with pitch - a substance produced from processing wood, a kind of thick resin with a strong, foul smell. In the time of the Mishnah, the way to restrain that odor and keep it from ruining the wine was to mix a little wine into the pitch itself. Hence the problem: when it was gentile workers who spread the pitch on the press, that wine in the pitch became yayin nesech, and it is liable to render forbidden the wine produced in the press.
What is the law of a press that was coated with pitch by a gentile? The Mishnah distinguishes according to the material the press is made of:
"Gas shel even shezifsah oved kochavim - menagvah vehi tehorah" - a press built on stone, like a bedrock foundation, that was sealed with pitch by a gentile, requires only wiping down. The literal meaning of niguv is drying, and in practice it refers to rinsing with water and drying with ash, and then repeating this a second time. In this way all the wine on the surface is rinsed away and is also absorbed into the ash, and once this has been done twice it is enough, and there is no concern that additional wine will seep from the pitch into the grape juice. It is worth noting that "vehi tehorah" does not mean purity as opposed to tumah, but rather fitness for use.
"Veshel etz" - regarding a press whose body is made of wood, the Tannaim disagree:
"Rabbi omer: yenageiv" - in Rabbi's view its law is the same as that of a stone press, and he is not concerned about what has been absorbed into the wood. Rinsing and drying twice is enough.
"VaChachamim omrim: yekaleif es hazefes" - in the view of the Sages, sealing a wooden press against liquid requires a thick layer of pitch containing a large amount of wine, and rinsing does not suffice against what has been absorbed; yayin nesech is liable to leak into the grapes. Therefore all the pitch must be peeled off and the press coated anew.
"Veshel cheres - af al pi shekilef es hazefes, harei zu asurah" - earthenware absorbs a great deal, and it is difficult to the point of impossible to draw out of it what it has absorbed. Therefore, even if he peeled off all the pitch that lined the press, that is not enough: we are concerned that the earthenware absorbed from the pitch the wine that became yayin nesech, and that the wine will later be discharged into the grape juice. Such a press is forbidden, and it cannot be made fit either by peeling off the pitch or by rinsing and drying.
How to make an earthenware press fit: milui ve'irui:
Even so, there is a way to make it fit: after peeling off the pitch, one uses the method called milui ve'irui - milui means filling, and irui means pouring out. The press is filled with water, which is left standing for twenty four hours in order to draw out of the earthenware everything it absorbed, and then the water is poured out. This is done a second time, and a third time. After three rounds in which the water stood for twenty four hours and drew out of the earthenware walls all the wine absorbed in them, we may assume that all the wine is gone, and from then on the press is fit for use.