How Glass Is Made
Most glass is a mixture of a large amount of silica that comes from fine white sand or pulverized sandstone, combined with smaller amounts of an alkali like soda (sodium bicarbonate) or potash to lower its melting point, and lime (from limestone) to help stabilize the mixture and ultimately make the glass stronger and water-resistant. These elements are naturally occurring throughout North America.
The silica, soda and lime are fused together at extremely high temperatures. By adding other substances during the process, the properties of the glass can be altered, including its color, how reflective it is, how brilliant or sparkling it looks, how well it acts as an insulator and more. Bits of old or broken glass from previous manufacturing, called ‘cullet’ is usually recycled into the mix. However, it is not just these additives alone that affect the final piece of glass, but also the way in which it is heated, cooled and formed.
First, the silica/soda/lime mixture is heated in melting furnaces to approximately 2,500° Fahrenheit, sometimes for as long as 24 hours. The molten glass is then cooled several hundred degrees, to a temperature at which it can be worked. At this stage the glass is an orange-red color and looks like a thick liquid. The glass has to be worked quickly to form it into the desired shape, because as the glass cools it becomes hard. The glass can now be blown, pressed, drawn or rolled. Finally, when the glass is formed into the desired shape, it is placed in a ‘lehr’ oven to be ‘annealed.’ This special process eliminates areas of stress in the glass to strengthen it, by cooling it down at controlled temperatures.
To blow glass, a blob of molten glass is placed at the end of a long, hollow iron blowpipe. Air is blown in and causes the glass to form into a pear-shaped bulb, which is then rolled on an oiled slab, shaped with tools and sometimes re-blown into a mold. To keep the glass from hardening during this process, it is periodically re-heated in small ovens. If the glass is to be engraved, copper wheels are used, and if it is to be etched, hydrofluoric acid does the job.
For glass bottles, a molten glass bubble is employed. It is placed in a mold, and the air pressure in the bubble forces the glass against the side of the mold. Once the glass cools and hardens, the mold is opened and a newly-made glass bottle removed.
Pressed glass, which offers better control of the glass’s density than blowing, was the first glass to be manufactured on a large scale with the invention of a glass-pressing machine in the U.S. in the 1920s. This worked by taking the molten glass from the furnace and dividing it up into small sections. These would be placed in molds made of iron or brass. A plunger would press the glass down into the mold, and after a few seconds it was ready.
Because the cold metal of the mold would produce wrinkles in the hot glass, intricate patterns were used to disguise the flaws. It wasn’t until twenty years later that a technique was developed to heat the molds to very high temperatures before dropping in the molten glass, which eliminated the wrinkles. The decorative patterns accordingly became simpler.
Drawn glass is the process used to manufacture tube and rod-shaped glass, as well as some sheet glass. To make tubes, the molten glass gets drawn over a hollow cylinder or cone that has air blowing through it, to keep it from collapsing until the glass hardens around it. The tube can be drawn out horizontally, vertically, or at an angle. Conversely, to make glass rods, the air inside the cylinder or cone is eliminated.
Rolled glass is used for some flat glass, and means that the sheet of glass rolls along the assembly line as it is manufactured. Because of the type of metal used in the rollers, they aren’t damaged by the heat of the molten glass. However, the glass can wind up with a rough surface. When this process was first developed, molten glass was poured onto large tables and then rolled flat onto plates. After it cooled, the glass was ground and polished.
Then a series of innovations began at the turn of the last century and continued through the first world war, improving the quality and economy of both drawn, sheet glass and rolled glass.
Float glass is the most widely used type of flat glass today. It was first developed after the second world war by a British company but not introduced to the market until they felt it was perfected, in 1959. Its manufacture is unique because the molten glass is formed by floating it on a bed of molten tin kept at high temperature. The glass spreads out and flattens, and is then drawn out into a continuous glass ribbon. The surface of the glass winds up being extremely smooth, with a brilliant finish like sheet glass, plus the optical quality of plate glass.
Cutting and drilling glass is a fine art. If the glass needs to be cut it must first be scored with a glass-cutter wheel. Pressure is then applied across the score to force a break. Another way to cut glass is thermally. This can be done with a focused flame heating a narrow strip of the glass. A water jet is then directed on the heated strip to break the glass. Or, a ring of focused flames are used to heat a particular area of the glass until it becomes soft enough to pull apart. Glass can be drilled, with either a steel drill, a tungsten-carbide drill, or for the best control, quality and speed of production, a diamond core drill.
Glass is categorized by its composition. The most common – ninety percent of manufactured glass – is so-called soda glass, the combination of silica, soda and lime. Although it’s the cheapest to make, it is also the least resistant to high temperature, or sudden changes in temperature, or chemicals.
Lead glass will usually have at least twenty percent lead oxide content. It looks brilliant in the light, especially when cut and faceted, and is more expensive that soda glass. But similar to soda glass, lead glass will also not do well with high temperature or sudden changes in temperature.
For better resistance to temperature changes and chemicals, the more expensive borosilicate glass is used. It has a minimum of 5% boric oxide, and is used to make light bulbs, sealed-beam headlights, bakeware and labware. And even more durable than borosilicate glass is aluminosilicate glass.
Fiberglass and foam glass are mostly used for insulation purposes. To make fiberglass, the molten glass is formed into continuous, hair-like glass filaments. Foam glass is made by trapping gas bubbles in the glass, creating an almost spongy consistency.
Colored glass is made by adding chemicals into the mix, with particular chemicals creating specific colors. For example, the amber or brown glass you see used in beer bottles gets its color from iron sulphide. Iron-chromite creates shades of green, while cobalt makes beautiful shades of blue.