Halakhah for Shabbat 3:2
תַּנּוּר שֶׁהִסִּיקוּהוּ בְקַשׁ וּבַגְּבָבָא, לֹא יִתֵּן בֵּין מִתּוֹכוֹ בֵּין מֵעַל גַּבָּיו. כֻּפָּח שֶׁהִסִּיקוּהוּ בְקַשׁ וּבִגְבָבָא, הֲרֵי זֶה כְכִירַיִם, בְּגֶפֶת וּבְעֵצִים, הֲרֵי הוּא כְתַנּוּר:
A tanur (an oven) which was heated with straw or with gevava [Because a tanur is narrow above and broad below, its heat is more concentrated than that of a kirah, so that even if it were heated with straw or with gevava, we fear that he might stir the coals, for he never removes his mind from it] — a dish may not be placed in it, both within or on the side. A kupach which was heated with straw or gevavah is like a kirayim; with gefeth or wood, like a tanur. [A kupach is made like a kirah, but it is as long as it is broad, so that there is room for only one dish. The fire passes beneath it and its heat is greater than that of a kirah (because a kirah is open above the space of two pots, whereas the kupach is open only the space of one pot), and less than that of a tanur.]