Starting in ancient Egyptian times, glass artists worked predominately with soda- lime glass. Soda-lime, also referred to as “soft glass” is comprised of silica (approx 70%), soda (approx 13%), and lime (approx 11%) as denoted by its name. These ingredients render an easily malleable glass that has a long work time and a relatively low melting point. Because of these working properties, this soda-lime type glass was and is commonly referred to as “soft glass.”
In the late 19th century scientists began experimenting with different glass compositions. The fruits of their labor yielded a type of glass known as borosilicate. Borosilicate glass as denoted by its name-is comprised mainly of silica (approx 80%) and boron (approx 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 because of its molecular structure is also more durable.
Thermal expansion is another property that must be considered when discussing the uniqueness of borosilicate. Thermal expansion is a measurement of linear expansion which denotes how much the glass contracts as it is cooled. Soda-lime glasses are at the highest end of the scale and expand the most as they are heated. This means that as they cool, they contract or shrink at the greatest rate. To prevent cracking, this rate of cooling must be carefully controlled using an annealing oven.
Borosilicate glass, however, is on the lower end of the thermal expansion scale. It expands and contracts at a much lower rate than soda-lime glass and is not as susceptible to cracking problems, even when cooled quickly without kilning. This is why borosilicate is used in the scientific industry as well as for cookware.
What this means to the artist is when placing borosilicate glass in the flame a rod can be directly inserted into the heat without having to be slowly warmed, as is the case with soda-lime glass. Because of this unique property, borosilicate can be used for large sculptures in which a small section can be worked without the entire piece being hot. It also allows the artist to selectively heat sections of a work or reheat portions without cracking. With soda-lime glass, a large sculptural work can be very challenging, especially when multiple reheats are required.
FACTORS THAT AFFECT COLORED BOROSILICATE
The most unique facet about working borosilicate is the striking color palette. Traditionally, striking colors were those that returned to their original hue after a period of kilning. Borosilicate striking colors on the other hand can produce an entire range of textures and hues, simply from a single rod. The strikes are repeatable and the colors do not burn out, as is the case with certain soda-lime colors. They are extremely versatile, but take careful attention to produce certain effects.
In this users manual we will help you tackle these issues and make the process of creating these alluring colors repeatable and straightforward. In addition to the broad overview of working our colors we will focus in on three of the most interesting and challenging color families to work:
First, we will discuss the major factors that affect colored borosilicate: