Glass – A Closer Look
Did you know?
When glass is formed naturally it is called obsidian, it is created when certain rocks are melted then cooled and solidified quickly, this can happen as result of lightning strikes, (fulgarite) volcanic erruptions and meteorites(Moldavite). During the Stone Age this type of glass was used to make tools such as arrow heads. Even today they are still razor sharp. A testament to this material’s unique properties is that no man-made material can cut as cleanly as obsidian.
We think glass was first created by humans about 4-5,000 years ago, probably by accident when fires were set on sand. The combination of wood-ash, sand and intense heat caused the sand to melt. The earliest method of forming glass is known as fusing, this virtually died out when around 50 BC the Romans discovered that glass could be shaped if it was gathered on the end of a hollow blowing pipe, and inflated like a bubble. It could be blown into a hollow mold or freely shaped with simple tools on the end of the blow pipe. For the first time, a person could mass-produce dozens of objects a day by glassblowing.
Types of glass
There are six main types of glass
1. Soda-lime glass is the most common (90% of all glass made), and the least expensive form of glass. It usually contains 60-75% silica, 12-18% soda, 5-12% lime. Most window and bottle glass is made from this.
2. Lead glass has a high percentage of lead oxide (at least 20% of the batch). It is relatively soft, and its refractive index gives a brilliance that may be exploited by cutting. Thermometer tubing and art glass(Waterford crystal) is made from lead glass.
3. Borosilicate glass is any silicate glass having at least 5% of boric oxide in its composition. It is resistant to temperature change and chemical corrosion. Pipelines, light bulbs, laboratory ware, and bake-ware (Pyrex) are made from borosilicate glass.
4. Alumino-silicate glass has aluminum oxide in its composition. It is similar to borosilicate glass but can withstand higher operating temperatures.
5. Ninety-six percent silica glass is a borosilicate glass, melted and formed by conventional means, then processed to remove almost all the non-silicate elements. This glass is resistant to heat shock up to 900°C.
6. Fused silica glass is pure silicon dioxide in the non-crystalline state is used to make fibre optics for the telecommunications industry.
How Glass is Made?
Glass is made by reducing the individual ingredients (listed above) to a powder through a grinding process. The correct proportions are then added together and heated until they melt, it can then be cast, moulded, extruded, stamped, blown and rolled to create sheet glass.
Glass can be textured by casting it into a textured mould, etched with acid, sand- blasted and cut with diamond tools.
Depending on what is added to the glass it can become transparent or opaque, clear or coloured. Some colouring materials for glass are widely known. Perhaps the best example of this is cobalt blue, produced by adding cobalt oxide to the glass melt. “Vaseline glass” is a fluorescent yellow-green glass that contains small amounts of uranium oxide. “Ruby gold” and “cranberry glass” are red glasses produced by the addition of gold. “Selenium ruby” is a red color caused by the addition of selenium oxide and “Egyptian blue” is produced by the addition of copper.
Annealing- Is the process of slowly cooling a finished glass piece
Batch- The mix of raw materials used to make glass
Cullett- Is glass that has been crushed and is ready to be remade- recycled
Engraving- A method used to create designs onto glass, the design is cut into or scratched onto the glass
Gather – A ball of molten glass taken from a furnace on the end of a rod
Nipt-diamond-waies- Is a type of decoration on glassware, where the glass is shaped using pincers to produce diamond shapes
Punty- Is an iron rod used to hold and shape molten glass
Slumping-Is when a piece of glass is re-heated and flows and is put into a mold
© Michael Ray 2011