How is Glass Made

Discover how glass is made.

  • The first glass was made by nature, not man.
    Photo by AdobeStock/Leigh Prather
  • “Ask a Science Teacher” by Larry Scheckel answers over 200 common questions about science.
    Cover courtesy The Experiment

Ask a Science Teacher (The Experiment, 2011) by Larry Scheckel is sure to resolve the everyday mysteries you’ve always wondered about. You’ll learn how planes really fly, why the Earth is round, how microwaves heat food, and much more—before you know it, all your friends will be asking you! This section answers how glass is made.

Glass is a hard, brittle, and transparent solid. About 90 percent of the glass we use is soda-lime-silica glass. It is 75 percent silica, or just plain sand. The “soda” refers to sodium carbonate, and the “lime” comes from limestone, which is mostly calcium carbonate. The ingredients are put into a gas-fired furnace. Window glass is made by having the molten, liquid glass flow over a molten tin bath in a continuous sheet. Nitrogen gas pushes on the top surface to make it smooth.

Glass melts, or turns to a liquid, at a temperature between 2,500–3,000 degrees Fahrenheit. The melting temperature depends on the composition of the glass. Glass does not melt at an exact temperature.

Unlike ice, which melts at a precise temperature of 32 degrees Fahrenheit, glass goes through a phase-change transition. Glass gets softer and softer as its temperature increases, until it can start to flow. That’s a nice property of glass, because glass can then be molded into any shape we desire. Ingredients are added to the soda and lime to change the properties of the glass. Lead or flint causes glass to sparkle. Boron is added to glass to change its thermal properties; this makes borosilicate glass suitable for cooking. We also use a lot of borosilicate glass in the science lab. It has a very low thermal expansion rate and does not break when a very hot or very cold liquid is put in it. Pyrex is a brand name for the same kind of glass. Lanthanum oxide has light reflective properties and is used in eyeglasses. Iron is put in window glass to absorb infrared energy. Take a look at the edge of ordinary window glass, and you can see the green tint. That’s from the iron.

Glass used in optical equipment is generally made from low-dispersion crown glass or denser, high-dispersion flint glass. The glass is selected based on how light is bent, or refracted, through it. Dispersion is a term that describes how much light is bent or refracted as it passes through the glass.

Tempered glass is stronger than regular glass, but if it does break, it shatters into small pieces rather than the big sharp shards of ordinary glass that can cause major bodily harm. Regular annealed glass is placed on a table, which is rolled through a furnace; then the surfaces are quenched or quickly cooled with air, while the interior of the glass remains liquid, or molten. This process sets up stresses in the glass. Tempered glass is used in side and rear windows of cars and in glass doors that are not in a frame.

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