CALCAR, in glass-making, the name of a small oven or reverberatory furnace, in which the first calcination of sand and salt of potashes is made for the turning them into what is called frit. This furnace is made in the fashion of an oven, ten feet long, seven broad in the widest part, and two feet deep. On one side of it is a trench six inches square, the upper part of which is level with the calcar, and separated only from it at the mouth by bricks nine inches wide. Into this trench they put sea-coal, the flame of which is carried into every part of the furnace, and is reverberated from the roof upon the frit, over the furnace of which the smoke flies very black, and goes out at the mouth of the calcar; the coals burn on iron grates, and the ashes fall through.