Sarah Dionne


Functional and Decorative Warm Glass Art

glass PROPERTIES

Most of us might remember learning some of the properties of glass while in chemistry class-- I will go over the basics here. It is important to know these physical and chemical properties when working with glass so that you can better predict your results.


There are several types of glass currently under manufacture. These include Soda-Lime glass (this is most of the glass you come in contact with, including fusing and stained glass), lead crystal (used for those beautiful wine glasses your mother won't let you touch) and Borosilicate (like Pyrex and other heat-safe glass used in cooking, baking, and in the laboratory). We will focus on Soda-Lime glass, since most fusing glass is of this type. It is a synthetic material composed of silica (silicon), sodium or potassium, lime, and colourants (usually metallic oxides like copper, iron, sulfur, cobalt, or others).


Remember the three states of matter? (Gas, liquid, solid) Because of its composition, glass displays properties of both a liquid and a solid while at room temperature. This means it can be called vitreous (the term used to describe matter between states), amorphous (it does not have a crystalline structure like a solid), and a super-cooled liquid (it has the molecular structure of a "stiff" liquid at its room-temperature solid state). As glass is heated, it behaves more and more like a "regular" liquid, meaning it will flow and take the shape of its container.  


In glass fusing inside a kiln, we usually heat the glass to a maximum temperature of 1700 degrees F. This is hot enough for the glass to flow. Once flowing, gravity and surface tension will cause the glass to seek a thickness of 1/4 inch (6 mm) if left to flow freely. If thicker glass is needed, dams can be used to stop the flow and contain it to a designated space.


Generally, fusing glass is manufactured in two different COEs (co-efficients of expansion). This number relates to the magnitude of expansion and contraction that the glass undergoes as it is heated and cooled again. The COE is the ratio (expressed as a percentage) of the change in volume. So, after firing, a glass with a COE of 96 will only be 96% of its original size. Spectrum Glass manufactures their System 96 line that has a COE of 96. Bullseye Glass manufactures fusing glass with a COE of 90. In general, one cannot combine glass with different COEs in a single piece. The stresses due to the different degrees of expansion and contraction would cause the glass to be under strain. This strain would then cause the glass to crack, break, or even explode!


A Note: There are several other fusing glass manufacturers in the United States, including: Uroboros, and Wissmach. CBS (Coatings by Sandberg) and DichroMagic coat fusing glass made by other manufacturers with dichroic coating. Most glass retailers sell glass made by all these companies. 


WHEW!--And that is just the beginning...There are a few great books (and at least one periodical) that explain this and more in great detail. I highly recommend them to anyone serious about glass fusing:


Warm Glass: A Complete Guide to Kiln-Forming Techniques: Fusing, Slumping, Casting. Philippa Beveridge, Ignasi Domenech, Eva Pascual. 2003. Lark Books, New York, NY.


A Beginner's Guide to Kiln-Formed Glass: Fused, Slumped, Cast. Brenda Griffith. 2007. Lark Books, New York, NY.


Paul Tarlow of Helios Glass has created several e-books that cover a range of techniques. His website, www.FusedGlassBooks.com has every title available to purchase as a download or paper copy. 


Glass Patterns Quarterly is a quarterly magazine featuring stained and fused glass projects, pointers, and related articles. They also offer magazines on hot glass and the glass art industry. You can purchase a paper or digital subscription.


There are many great Facebook groups for "glass people". Some are just a place to display photos and some are more for asking the group questions or troubleshoot a problem. As with anything you see on the web, take any advice from strangers with a grain of salt! It might not always be correct...