κλίβανον [2 verses] (noun sg masc acc) "Oven" is klibanos, which means "covered earthen vessel [in which bread is baked in a fire], "funnel-shaped vessel [used for drawing water]", "underground channel", "vaulted passage", "hollow," and "cavern in a rock."  -

The oven is Greek for a small, clay vessel used for baking bread (see picture above).  The ovens Jesus describes are different than ours, which have the fire is on the outside and bread on the inside. These ovens are clay vessels. The fire is burned in the vessel. The dough for the bread is attached to the vessel's sides. The "grass", that is, the foliage, of "the lilies of the field" (Matthew 6:28) becomes the fuel for baking bread. This image is similar to the one evoked by the "Parable of the Weeds", where the weeds are bundled to be burned in ovens.