Page 10 - Curriculum Visions Dynamic Book

Making glass is not just a matter of producing glass with suitable properties. The cost of glassmaking also has to be taken into account. So, scientists look for a compromise between good all-round properties and low cost in manufacture.
The proportions of 75% silica, 10% lime, and 15% soda produce a glass with good all-round properties. But variations from this standard formula are also important for specific common uses. For example, sheet glass (used for windows) has 6%
lime and 4% magnesia (magnesium oxide, or MgO). Bottles, on the other hand, contain 2% alumina (aluminium oxide, or Al2O3).
Prince Rupert’s Drop
In the 17th century this toy used to amaze the court of James the First
of England and amuse his grandson Prince Rupert. It consisted of a curved droplet of glass made by dripping molten glass into cold water. As a result of rapid surface cooling, the surface tends to shrink fast while the inside is still hot. That leaves the glass under great stress.
The thick end of the drop can stand up to this stress and can even be struck with a hammer without breaking. However, the thin
tail of the drop is much more fragile. If it is snapped off,
that releases all of the stresses stored up in the drop, and the whole drop instantly shatters
into a powder.
The prince would ask someone to hold the bulbous end of the drop, and then he would break off the thin tail. The glass would then explode in their hand, startling them. Because it disintegrated into a powder, this breakage was safe!
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