A little about the making of glass - Part IV

by Evelyn M. Allen
Issue #20 - December 1974

We have got all our glass mixtures to the hot metal room melting, fusing the batch at high temperatures, 2400 to 2700 degrees, in individual clay crucibles or pots in furnaces or in large single furnaces or tanks. It normally takes twenty to thirty hours to melt the batch into clear sparkling glass. Glass furnaces are fueled with natural gas or fuel oil. When the melting is complete the glass is cooled down to working temperature - 1900 to 2100 degrees Fahrenheit, and highly skilled hot metal workers, sometimes called "flints" can begin to shape and process, blow or press, the glass into a finished item.

In most oases, clear glass or crytal is melted in a single unit or tank containing over one hundred tons of glass. Colored glass, lead glass and other special kinds of glass are melted in individual pots, each containing about one ton. The glass boils up violently, but eventually settles down to a clear transparent mass of about honey consistence. In shaping, the use of molds to secure uniformity of shape has become standard practice. Most moulds are made of cast iron, carefully tooled and shaped by skilled mould makers. A mould for a cup and saucer could take up to eight weeks to make, so you can imagine the cost of one single mould.

The oldest method of forming articles is done with an iron "blow-pipe" in the hands of a skilled glass worker. The molten glass is gathered on the end of a "blow-pipe" by the gatherer; removed from pot, somewhat cooled, carefully shaped, and the "gob" of glass is blown (a few years ago this was all done by the mouth and lungs) into a hollow body which the gatherer hands to the blower. The blower shapes the glass bubble further, and in most cases, blows it into a shape in a mould. The glass blower is responsible for the finished shape of the artic1e when it emerges from the mould. As soon as blown items have progressed to this point, they are ready to be severed from the blow pipe and placed into the annealing lehr. Annealing means careful and gradual cooling; for almost any piece of glass, if left to cool by itself will shatter into many pieces. Annealing lehrs are bake ovens with continuous belt conveyors traveling at very low speeds. Certain heavy items may go through the oven several hours.

More next month.