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Some 2000 years ago, a group of Phoenician merchants used blocks of "natron", an alkali, to support their cooking pots over the fire while preparing dinner. When the fire burned out, they discovered a clear residue. This has been credited with being the first human-made glass.
The story has a nice imaginative appeal. However, it would not have been possible for the heat from the fire to fuse natron and sand into a glass-like substance. This would have required a temperature over 1100 degrees centigrade. What likely happened is that the sand in the region was, in fact, powdered obsidian -- natural glass. It would then have been possible for the heat from the fire, assisted by the fluxing action of the natron on the sandy particles, to produce a glass residue.
Glass containers for food, beverages, cosmetics and medicines have been with us a long time. Human-made glass is thought to be the oldest manufactured substance in the world. Archaeologists have found glass bottles and jars dating back to 1500 B.C.
Since then, glassmaking has had a long and fascinating development, right up to its present multitude of uses by artisans, craftspeople and manufacturing firms.
How Container Glass is Made
Container glass is made of simple, natural ingredients: silica sand, soda ash and limestone. These materials are mined all over North America and there is a plentiful supply available. The following chart illustrates the basic list of ingredients for container glass.
A variable percentage of recycled glass (cullet) can be added to the raw materials to reduce the amount of virgin raw materials used. The variable percentage is based on the consistency of supply and quality. The use of recycled glass conserves raw materials, and the energy used to make glass, while at the same time reducing solid waste.
Glass Colour
Container glass can be made in many different colours, with the most common colours being flint (clear); amber or brown, produced by the addition of iron-sulphide; various shades of green, produced by adding iron-chromite, and light blue, produced with cobalt. These are shown in the pie chart as minor ingredients.
Different colours can serve a specific function for the companies which fill the glass containers with foods, beverages and other products. One colour might be chosen for marketing purposes, while another is used for technical reasons. For example, amber glass used by the brewery industry protects the contents against damage by ultraviolet light.
Glass Container Manufacturing
The glass container manufacturing process begins when raw materials are received and stored in silos. These materials are then transferred through a gravity feed system to a weigher and mixer, where the materials are mixed with cullet (recycled glass) to ensure a homogeneous melting. The mixture is conveyed to a batch storage bin where it is held until dropped into the feeder to the melting furnace.
The furnace most commonly used is a continuous regenerative furnace capable of producing between 90 and 360 tonnes of glass per day. As the glass melts at 1500 degrees centigrade, it passes to the front of the furnace, eventually flowing through a throat to the refiner. In the refiner, the molten glass is heat conditioned for delivery to the forming process.
After refining, the molten glass is channeled through forehearths and delivered to the forming machines. There, the glass is cut into sections (gobs) by a set of mechanical shears and formed using blank and blow moulds. Once the glass is shaped into a container, it is fed to a lehr oven for annealing, which removes unwanted stress areas in the glass. Then it is inspected and prepared for shipment to market. Any damaged or defective glass is transferred back to the batch plant to be used as cullet.
Recycled glass in new production
Only container glass can be used to make new containers. The reason is that, although most glass is made from silica sand, soda ash and limestone, container glass has a distinct and different formulation. Window glass, drinking glass, crystal or laboratory glass, if put in for recycling, will cause quality problems for the glass container industries using recycled glass.
The amount of recycled glass used by Consumers Glass in its glass containers increased each year since 1988, when it stood at 7%, to a high of 34% in 1994. In 1995, less recycled glass meeting Consumers requirements was available and a 31% recycled content was achieved.
Copyright ® 1999, Glass Works
