The mishna in today’s daf (Menachot 63a) stated that a mincha that is ma’feh tanur, an oven-baked mincha cannot be made in a kupach. R. Yehudah disagreed and said that a kupach could be used for such a mincha. What, exactly, is a kupach, and how did it differ from a tanur?
Rashi, here, states that a kupat is a small oven and can only fit one pot (s.v. kupat). Rashi’s comments are more spelled out in the third chapter of Shabbat, where the mishnayot distinguish between three instruments for cooking: the kirah, the kupach, and the tanur. Here is what Rashi says about these three items (I have reordered the statements to appear in ascending order of the size of the items):
כופח – בגמרא מפרש לה: עשוי חלל ככירה, אבל ארכו כרחבו, ואין בו אלא שיעור שפיתת קדירה אחת, וכירה יש בה שפיתת שתי קדירות. (לח:)
כירה – עשויה כעין קדירה, ונותנין קדירה לתוכה. (לו:)
תנור – מתוך שקצר למעלה ורחב למטה, נקלט חומו לתוכו טפי מכירה (לח:)
Kupach is explained in the Gemara: it is made with a hollow, like the Kirah, but its length is equal to its width, and it only has the size to hold one pot. The kirah has the size to hold two pots. (Shabbat 38b)
Kirah is shaped like a pot, and one puts a pot inside it. (Shabbat 36b)
Tanur – an oven, because it is narrow above and wide at the bottom, its heat is concentrated inside it more than is the case with a kirah (Shabbat 38b)
The differences between these three items can be seen visually in the pictures below. These are all taken from the book כלי חרס בתקופת התלמוד, “Earthenware Vessels from the Talmudic Period,” by Dr. Yehoshua Brand, and a pdf of the sections on these ovens can be downloaded on our Resources page under Diagrams and Maps.
The first picture is that of a kupach, the next of an earthenware kirah. The kupach’s hollow would be filled with wood and fuel and the pot would be put on its top (not inside it). The holes on the floor of the kupach which would allow the ashes to fall away into the bottom chamber. In contrast, the earthenware kirah looked like a pot-holder with walls. It could have four walls or, as the picture below shows, three walls. The woods and fire was put in between the walls, and the pot supported above. It could be wider than the picture shown and support two pots.
The last picture is that of a tanur, and – like Rashi described – it is pyramidal in shape, and as can be seen was used for baking bread by sticking the bread on its walls.
It is not clear why our mishna did not consider the question of using a kirah for baking bread. The Gemara in Shabbat states, in its explanation of the greater serverity afforded to a kupach over a kirah, that the heat of a kupach was greater than a kirah (Shabbat 35b). Brand assumes that this means that the kupach was taller (it is assumed that the kira was wider, since it could hold two pots). It is also possible that its heat was greater because the kupach was not as wide, and because it had walls all around without ventilation. This could explain why R. Yehuda only mentions a kupach for baking. Its greater heat would allow for more effective baking than the kirah.