Keyword: "harden" | ClipArt ETC

"A muffle furnace, employed for annealing, hardening, and tempering." —Encyclopedia Britannica, 1910

Automatic Oil Muffle Furnace

"A muffle furnace, employed for annealing, hardening, and tempering." —Encyclopedia Britannica,…

A furnace used to harden glass.

Furnace

A furnace used to harden glass.

Inside Length of Furnace, 12 Feet.

Furnace

Inside Length of Furnace, 12 Feet.

Furnace

A furnace used to harden glass.

"Flint-Glass Manufactory." — Encyclopedia Britannica, 1893

Glass Manufactory

"Flint-Glass Manufactory." — Encyclopedia Britannica, 1893

"Interior of a crown-glass house." — Encyclopedia Britannica, 1893

Glass Manufactory

"Interior of a crown-glass house." — Encyclopedia Britannica, 1893