Borosilicate Glass: A brief history
From the laboratory standpoint, there are three main tyeps of glass used today; borosilicate, quartz and soda lime or flint glass. What is borosilicate glass, how is it different from other glasses, and why is it used so widely in laboratory research? The formation of glass and glass products goes back to the beginnings of recorded history. The invention of borosilicate glass is usually credited to Otto Schott, a renowned late 19th century German glassmaker. It was apparently first sold under the name 'Duran' in Europe in 1893. In the United States, 'Pyrex' became synonymous with borosilicate glass after Corning Glass Works inventor Joe Littleton introduced it to the English speaking world in 1915.
Though borosilicate glass was at first widely used for kitchen glassware, thermometer glass and railroad lanterns, there are now very few companies using it for that purpose, simply because it is more difficult to produce, and hence more expensive. And in the kitchen, the costs to the consumer more or less outweigh the benefits gained. There are, however, several small companies in the U.S. and abroad still offering colored borosilicate glass for the art glass market, and that market is still viable because of the durability and flexibility of borosilicate glass. It is also easy to work at low temperatures.
Because borosilicate glass melts at a higher temperature than other silicate glass, it was more complicated to bring into industrial production, and new techniques were required. New burners, using oxygen and natural gas, were developed. And for the most part, the manufacturing process has remained the same for the past 100 years or so. Borosilicate, because of its wide temperature range, strength and chemical resistance became 'The Glass' for the laboratory.
The unofficial U.S. 'capital' of glass blowing is Vineland, NJ. The reason Vineland got this distinction is very simple: The first production factory in the United States started just a few miles away in 1723. It was located there because of the proximity of necessary natural resources such as sand, seashells, and the evergreen trees of the New Jersey Pinelands. And of course, the subsequent proliferation of manufacturing firms and jobs in the area brought large numbers of glassblowers here. And they have remained throughout the years, the glassblowing techniques handed down from grandfather to father to son.
Nowadays, very few glass companies actually 'melt' borosilicate glass in the U.S., even in Vineland. Instead, virtually all area companies, such as Chemglass, Ace Glass, Wilmad-Lab Glass and Kimble-Kontes, are fabricators, purchasing 'blanks' from large firms like Schott, Corning and Kavalier Glassworks and creating custom laboratory glassware such as round-bottom flasks, distillation apparatus, condensers, and much more.
'Soft' glass is traditionally made from sand, sodium carbonate and calcium carbonate, thus the name 'soda lime'. Boron is added to a much purer type silica sand in the manufacture of borosilicate glass. The composition of borosilicate glass is generally about 80%% silica, 10% boron oxide, 8% sodium oxide, 8% potassium oxide, and 1% aluminum oxide.
Borosilicate glass used in labware has a very low thermal expansion coefficient (32.5-33), which is about 1/3 that of 'soft' glass. This reduces material stress caused by changes in temperature, and hence, the glass is more resistant to breakage. It is NOT invincible, though -- it will break or crack if dropped or otherwise impacted. Fortunately, it is much more likely to crack or snap rather than to shatter. It has a softening point (at which the glass may slump under its own weight) of 800 degrees C is where glass blowers normally work it. The annealing point (the temperature at which residual strain or stress will relieve itself) is around 560 degrees C.
Under 'normal' laboratory conditions, borosilicate glass can withstand extended temperatures of around 230-240 degrees C. For very short-term use, it will generally survive temperatures around 400°C. But the absolute maximum operating temperature for the formed glass is 450 degrees C. Temperature changes from very hot to very cold should be also be avoided -- a maximum thermal shock differential of 160 degrees C should be observed.
Borosilicate glass is very resistant to chemical corrosion, which makes it perfect for laboratory use. It can handle extremely volatile chemicals, and even nuclear waste! Everything has an Achilles heal, and for borosilicate glass it is flourines. Hydroflouric acid will etch borosilicate glass.