The Mentor · Glass and Glass-Making · Page 11

and the nature of the product, constitute the art of the glass-blower." Glass is shaped, as a rule, by one of three methods-- blowing, pressing and casting.
To give glass strength and durability it is necessary that it be very gradually cooled, after it has been shaped. Great care is required during the process of cooling to avoid the uneven closing of the distended pores of the glass. In order that the mass may cool to the same degree from its surface to its core, ovens are employed in which the substance remains for days subjected to a graduated temperature. This process is called annealing. Two types of annealing kilns or ovens are used-- the intermittent and the continuous.

MAKING A STAINED GLASS WINDOW

The first is heated and filled with glassware, then sealed up and cooled by regulated drafts of air. The kiln most in use at the present time is the continuous oven, which is in the form of a long passage with a fire at one end. An endless chain carrying pans that contain glass objects passes slowly from the heated end of the oven to the cold end. In this way, the temperature affecting the ware is lowered little by little, and the pores of the glass contract evenly. To increase the toughness of glass it is sometimes immersed while hot in a vat of oil.

SUPPLEMENTARY READING

GLASS MANUFACTURE	By Walter Rosenhain	GLASS AND GLASS MANUFACTURE	By P. Marson
THE STORY OF GLASS Written for Young People.	By Sara Ware Bassett	DECORATIVE GLASS PROCESS	By A. L. Duthie
THE GLASSWARE DEPARTMENT	By Mary A. Lehmann	ENGLISH TABLE GLASS	By P. Bate