Let’s say that a board was laid over the mouth of a new (i.e., unfired) oven and it protruded on all sides by the space of a handbreadth. If there’s ritual impurity under the board, utensils above it remain clean; if there’s ritual impurity above the board, utensils beneath it remain clean. In the case of an old oven, any utensils are rendered unclean, though Rabbi Yochanan ben Nuri rules them clean. If the board was laid across the mouth of two ovens, then if there’s ritual impurity between them, they are rendered unclean. Again, Rabbi Yochanan ben Nuri rules them clean in such a case.
If a screen was placed over the mouth of an oven, enclosed by a tight seal, then if there’s ritual impurity below or above the screen, everything is rendered unclean but what’s directly opposite the oven’s air space remains clean. If there’s ritual impurity directly opposite the oven’s air space, then everything above it is rendered unclean all the way up to the sky.