Tips from the Glass Academy: What is Borosilicate Glass?


Borosilicate – What is it?

(thanks to Northstar Glassworks for the info)

We have a relatively new addition to our product line up. It is called Boro! It’s new to us with the addition of larger bench burners in our classroom and with customers asking for it. We now stock clear rods and tubes, lots of colored rods and a few colored tubes.

Starting back in ancient Egyptian times, glass artists worked predominately with soda-lime glass.  Soda-lime, also referred to as “soft glass” is comprised of silica, soda and lime as denoted by its name. These ingredients make an easily malleable glass that has a long work time and a relative low melting point. Because of these properties, the glass became known as soft glass. In the late 19th century, scientists began experimenting with different glass compositions. This yielded borosilicate which is made of silica and boron. It melts at a higher temperature and has a shorter working time. It is extremely versatile and is less dense than soda-lime and because of its molecular structure is also more durable.

Another thing that makes boro different is its thermal expansion. 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 boro glass in the flame, a rod can be directly inserted into the heat without having to be slowly warmed, as in 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 piece can be very challenging, especially when multiple reheats are required.

~ Janet

To view our extensive inventory of boro glass (33 COE), please click here.

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