furnace, furnaces, oven, ovens

Facts:

A furnace was a very large oven used for heating objects to a high temperature.

In ancient times, most furnaces were used for melting metals to make objects such as cooking pots, jewelry, weapons, and idols.
Furnaces were also used in the making of clay pottery.
Sometimes a furnace is referred to figuratively to explain that something is very hot.
An "oven" is like a small furnace, and it is used for cooking food.