What Is Borosilicate, And What Is Glassblowing?

Borosilicate glass is a type of glass that contains boron trioxide which allows for a very low coefficient of thermal expansion. This means it will not crack under extreme temperature changes like regular glass. Its durability has made it the glass of choice for high-end restaurants, laboratories and wineries.

Borosilicate glass was first developed by German glassmaker Otto Schott in the late 19th century in Jena. This early borosilicate glass thus came to be known as Jena glass. After Corning Glass Works introduced Pyrex in 1915, the name became synonymous for borosilicate glass in the English-speaking world (in reality, a sizable portion of glass produced under the Pyrex brand has also been made of soda-lime glass since the 1940s). Borosilicate glass is the name of a glass family with various members tailored to completely different purposes.

Borosilicate glass is made up of about 15% boron trioxide, which is that magical ingredient that completely changes the behavior of glass and makes it thermal shock resistant. This allows the glass to resist extreme changes in temperature and is measured by the “Coefficient of Thermal Expansion,” the rate at which the glass expands when it is exposed to heat. Thanks to this, borosilicate glass has the ability to go straight from a freezer to an oven rack without cracking. For you, this means you can pour boiling hot water into borosilicate glass if you wanted to say, steep tea or coffee, without worrying about shattering or cracking the glass.

Glassblowing is a glassforming technique that involves inflating molten glass into a bubble (or parison) with the aid of a blowpipe (or blow tube). A person who blows glass is called a glassblowerglassmith, or gaffer. A lampworker (often also called a glassblower or glassworker) manipulates glass with the use of a torch on a smaller scale, such as in producing precision laboratory glassware out of borosilicate glass. 

As a novel glass forming technique created in the middle of the 1st century BC, glassblowing exploited a working property of glass that was previously unknown to glassworkers; inflation, which is the expansion of a molten blob of glass by introducing a small amount of air to it. That is based on the liquid structure of glass where the atoms are held together by strong chemical bonds in a disordered and random network, therefore molten glass is viscous enough to be blown and gradually hardens as it loses heat.

To increase the stiffness of the molten glass, which in turn makes the process of blowing easier, there was a subtle change in the composition of glass. With reference to their studies of the ancient glass assemblages from Sepphoris of Israel, Fischer and McCray postulated that the concentration of natron, which acts as flux in glass, is slightly lower in blown vessels than those manufactured by casting. Lower concentration of natron would have allowed the glass to be stiffer for blowing.

During blowing, thinner layers of glass cool faster than thicker ones and become more viscous than the thicker layers. That allows production of blown glass with uniform thickness instead of causing blow-through of the thinned layers.

A full range of glassblowing techniques was developed within decades of its invention. The two major methods of glassblowing are free-blowing and mold-blowing. 

Free-Blowing. This method held a pre-eminent position in glassforming ever since its introduction in the middle of the 1st century BC until the late 19th century, and is still widely used as a glassforming technique, especially for artistic purposes. The process of free-blowing involves the blowing of short puffs of air into a molten portion of glass called a "gather" which has been spooled at one end of the blowpipe. This has the effect of forming an elastic skin on the interior of the glass blob that matches the exterior skin caused by the removal of heat from the furnace. The glassworker can then quickly inflate the molten glass to a coherent blob and work it into a desired shape.