The Glass Menagerie, Part One

The new year is always a good time to take care of housekeeping. This is as true for glass shops as it is for neon-sign fabricators.

Glass is a far more complicated subject than it may seem. More than 40,000 different types of glass are known today — ample reason to shed light on the properties and handling techniques of the integral material of neon tubes.

Glass genesis

The oldest known manufactured glass objects are approximately 3,000 years old. More than 2,000 years ago, the Roman historian Pliny the Elder wrote about the invention of glass: "A caravan of mineral traders once camped on the white sands of Phoenicia’s Belus River. Lacking rocks, they made a fireplace out of lumps of soda from their cargo. Arising the next morning, they found among the dead coals a translucent substance that shattered into fragments with razor-sharp-edges.

"Historians confess that Pliny didn’t hesitate to use his facile imagination when spinning a tale. However, in this instance, he enjoys a measure of support from modern glass technology." (from Creative Glass Blowing, by James E. Hammesfahr: W.H. Freeman, San Francisco, 1968).

Glass basics

But the principle hasn’t changed much since. Sand, soda (also known as lime) and potash still comrprise window glass today. However, gas, instead of coal or wood, fires today’s glass furnaces, and the process has been optimized for large quantities. Modern, mammoth glass ovens can melt up to 30 tons of glass per hour. In today’s glass factories, the process is so well shielded that hot glass can hardly be seen. Special glasses, as well as classic colors, are still made traditionally.

Glass ingredients are pulverized and mixed in precise amounts; this mix is called a batch (Fig. 1). Glassmakers have developed recipes, which are well-kept secrets, through lengthy experimentation. Because of ample variety, manufacturers don’t name glasses; rather, they number them for reference. You don’t order "sodalime glass;" instead, you order a specific product, such as Corning 0080.

The fresh batch is mixed with broken glass, or cullet, and fed into the melting pot, or crucible. The mixture depends upon a batch’s particular process and parameters. The amount of cullet will range from 30% to 75%, which prevents the powdery batch mixture from clogging together. Without it, fine grains of fresh material would stick together, forming large lumps that would never fully melt, thus creating non-uniform glass.

Crucibles, which accommodate between 100 lbs. and approximately 2 tons of glass, are made from special, high-temperature clay (Fig. 2). The melting process — and the ongoing chemical reactions between the ingredients — requires strict temperature and atmospheric profiles. After preheating and drying, the sodalime will melt, which dissolves the potash, and this compound dissolves the silica.

The basic glass-forming process ends at approximately 1,750