Source: Northstar
From ancient Egyptian times, glass artists worked primarily with soda lime glass. Soda lime, also called “soft glass,” consists of silica (about 70%), baking soda (about 13%) and lime (about 11%), as the name implies. These ingredients make for an easily moldable glass with a long processing time and a relatively low melting point. Because of these processing properties, this soda lime glass was and is commonly called “soft glass.”
In the late 19th century, scientists began experimenting with different glass compositions. The fruits of their labor produced a type of glass known as borosilicate. Borosilicate glass, as the name suggests, consists mainly of silica (about 80%) and boron (about 13%). This type of glass melts at a higher temperature and has a shorter working time. It is an extremely versatile glass that is less dense than soda lime and is also more durable due to its molecular structure.
Thermal expansion is another property to consider when discussing the uniqueness of borosilicate. Thermal expansion is a measure of linear expansion that indicates how much the glass shrinks as it is cooled. Soda lime glass is at the highest end of the scale and expands the most when heated. This means that the glass contracts or shrinks most rapidly during cooling. To prevent cracking, this cooling rate should be carefully controlled using an annealing furnace.
Borosilicate glass, on the other hand, has a lower coefficient of thermal expansion. It expands and shrinks much less quickly than soda lime glass and is less prone to cracking problems, even when cooled quickly without an oven. Therefore, borosilicate is used in the scientific industry and for cookware.
For the artist, this means that when borosilicate glass is placed in the flame, a rod can be inserted directly into the heat without having to be heated slowly, as is the case with soda lime glass. Because of this unique property, borosilicate can be used for large sculptures in which a small part can be worked without the whole piece becoming hot. It also allows the artist to selectively heat parts of a work or reheat parts without cracking. With soda lime glass, a large sculptural work can be quite challenging, especially if it requires reheating several times.
