Waste glass is called cullet. The word cullet comes from the art of glass blowing. Every time a blown item is separated from the blowing pipe there remains some glass at the blowing pipe and also the connecting part at the blown item has to be removed afterwards. The last is also called the little neck of the blow piece or collet. These two pieces of glass, the collet and the blow pipe left-over are going back into the glass oven, they are recycled. The word cullet is probably derived from collet.
Some of the waste glass is collected. After collecting all the glass products are crushed and this crushed glass bears the name cullet. There is no typical size for the broken pieces of glass to be called cullet. Broken windows, broken bottles, broken glassware for instance, belong all to the category cullet.
There are companies which are specialized in selling cullet and offer a wide range of different colors and glass types.
Reusing waste glass is good for the environment
The main purpose to collect broken glass is recycling. Glass is an ideal material for recycling. Broken glass can be heated up in the glass oven and from the glass melt new items can be made, over and over again and virtually nothing is lost.
Every metric ton or 1,000 kg of cullet recycled, saves 315 kilograms of carbon dioxide from being released into the atmosphere during the creation of new glass.
For the production of glass, from the raw materials soda ash, limestone and sand, every 1000 kilograms of cullet used replaces 1.,200 kilograms of the raw materials.
By adding 10% of cullet to the melting glass batch instead of the raw materials, 2.5% of energy is saved.
Since glass makes up a large part of household and industrial waste, due to its weight and density, collecting glass waste reduces the volume of waste sent to landfill.
In layman’s terms: Recycling of glass is good for the environment (less carbon dioxide output, reducing volume of waste landfill), it safes natural resources (less raw materials necessary, less fuel for melting necessary) and it safes costs (lower energy costs).
Collection of waste glass
Most people around the world are used to the recycling of glass. Already forty years glass containers can be found in many municipalities or there is some sort of organisation around the collection of waste glass.
At the collection points usually clear glass is separated from the green and the amber glass. Otherwise the glass is separated afterwards at a cullet company.
This separation of colors is very important. Glass of different colors have usually a different chemical nature and this influences things like melting temperatureand viscosity. Another problem is the color because clear glass will be slightly colored if only a little green, amber or blue glass is mixed in the used cullet.
The household waste glass is mostly packaging glass, which is all made of the normal soda-lime glass. Heat-resistant glass has a totally different chemical nature. If only a little bit heat-resistant glass is present in cullet then it has a big effect on the viscosity during the melting process and this has to be avoided. That’s why heat-resistant glass may not be thrown into a waste glass container.
Metal caps and plastic parts can usually be separated by the waste glass processor. However every non-glass part that slips to the separation process, like the Tungsten wires from the good old light bulb, can give rise to inclusions which make the new glass not only less beautiful but it also makes the glass less strong (locally) and this can be a safety issue.
Products made out of cullet
Because it is a bulk industry, the highest value product that is made of recycled glass are new glass packaging materials (and also household glassware). Another high-value end product is fiberglass which can be used for insulation materials or in composite materials. The cullet used for glass packaging and fiberglass needs to be color separated and free from any foreign material.
Cullet that doesn’t meet these specifications can be found in many other “secondary” applications like glass countertops, tiles, abrasive products and filtration products.
Much research has been done to mix the glass cullet with concrete for which the cullet doesn’t need to have high specifications. The concrete made in this way is stronger and has a higher insulation capacity than concrete without cullet. The cullet used for concrete is called glass aggregates and is a mixture of cullet with different colors.
These glass aggregates are also increasingly used, as a replacement for gravel or crushed rock, for pipe bedding for sewer pipes and drinking water pipes.
Many glassware factories make use of cullet for their products. For high-end glassware this is usually up to 10% because the clarity of the glass can be affected by using more cullet.
Some glassware companies make full use of cullet and use only cullet for the production of their glassware. If only cullet is used to make a new glass batch, the glass is not as homogeneous as it is made with the usual raw materials. This results in an end product with some local flaws and more air bubbles. Some glassware manufacturers make use of the imperfections of this “cullet-glass”, because the products can look antique and more nostalgic. A good example is the company Guajuye in Mexico which makes beautiful glassware from this type of glass.
People improvise all the time and there are artists and even companies who use waste glass before it is crushed. Good examples are the Green Glass Company and the French artist Laurence Brabant. They use old bottles and make all kind of decorative products, but also useful products out of it.