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Why Is Glass Green and How Many Types of Glass Are There?

Modern day glass as compared to the glass manufactured in the past is considered optically clear even though it has a distinct green hue which varies from each batch made.

Standard glass is manufactured from high-grade silica sand in furnaces using such high temperatures that most plants run them non-stop.

There are really only two types of standard glass. The first is plate-glass, which is high-end architectural glass used for premium glazing applications in the building and construction industry.

The second type is float or sheet glass; which provides the same quality as plate-glass but has a measurably lower production cost. As a result it is now the most commonly used on modern high-rise building exteriors.


This is float glass that has been tempered or laminated to make it safety glass. Float glass is almost distortion and defect free making it perfect for windows and cladding panels. This product gets its name from the production process. Floating is letting the molten glass 'float' on a bed of molten metal such as tin, over which the liquid flows easily due to gravity. This method gives the sheet uniform thickness and very flat surfaces. It was developed in the 1950s by British glass manufacturer Sir Alastair Pilkington, who pioneered the technique.

Glass Composition

High grade silica sand mixed with soda ash, dolomite, limestone, salt cake and cullet, is fed into the furnace which melts the lot and produces liquid molten glass. The liquid is then subjected to one of three techniques depending on what is required.

Each batch has between 15% to 30% cullet, which is crushed glass that is recycled. Cullet helps to save on raw materials and energy consumption.

The ratio of each part is calculated using the 'glass batch calculation method' to decide the desired results. So why then is it that every glass batch manufactured from the same raw materials, often from the same factory and even from the same furnace is ALWAYS different in hue?

I should know this is true. Having been in the decorative glass industry supplying coated glass for big projects can quickly become a nightmare, first if there is breakage which warrants a replacement or secondly if the project is delivered in phases and the customer wants glass colors to be same.

The main reason for the difference is that one big variable in the calculation is the cullet. This is crushed or broken glass from earlier batches, thus there is no way to control the varying degrees of iron content in the cullet. Iron is the mineral that gives glass its green color.

When I have a sizable project I order an extra two to five pieces of glass per batch in case of breakage. In cases where we are doing a large project in phases then we make sure the customer is educated on this issue. We also organize the glass into numbers of panels needed per project section, such as, five panels per lift interior or toilet cubicle, or 20 panels per corridor and then order the glass so. In this way slight to moderate differences in color will not matter as much because the panels are not installed next to each other or on the same floor.

We can also use low-iron content glass which is especially manufactured to produce crystal to ultra clear float glass. If the project requires a pure white coated glass installation or pastel and light shades of color coatings, then this is a good solution too.

Next I will share information on the types of treatments done to float glass to adapt it for use for a variety of purposes and industries.


Article Source: Ashley J John

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