This article is for newcomers to glass fusing. When I first began fusing glass the only thing I knew was that glass would fuse at high temperature but that was it. I kept on melting pieces of glass in an enamelling kiln until I had a whole collection of disasters. If I had known about two things, coefficiency of expansion and fusing temperatures, I would have saved myself a lot of heartache. Once you know about these two topics you have got it cracked (not literally)!

What is glass? It is a mix of silicas with chemicals and different combinations of these create different glass types. The most common glass is soda-lime silica, a mix of these three chemicals makes glass suitable for bottles, home wares, picture glass and window glass. Most of the glass we use for fusing is soda-lime silica that has been cast or pressed. The molten glass is poured and rolled to create an even surface. Hand rolled glass has a bumpy and irregular surface that is lovely for stained glass. Floated glass has been poured onto a bed of molten tin and passed through mechanical rollers to make a flat surface, as seen in picture glass and window panes.

Coefficiency of expansion. So imagine that you have, in front of you, a selection of glass. Your stash includes float glass (window glass), a mixture of pieces of stained glass (art glass) and pieces of commercial fusing glass such as Bullseye, Uroboros and Spectrum. Sadly, you can’t take this pile of goodies and cut them to create that perfect fused dish without knowing one thing about them – their coefficiency of expansion.

All glass, when heated slowly in a kiln, goes through the same process of expanding when hot. It reaches its stress level at a temperature that is hot enough to dry your eyeballs when it will turn white-hot. (If the glass goes over its stress level it will start to draw itself up into a shapeless blob). When it has reached its stress level the edges of the glass will be rounded and then, as the kiln begins to cool, the glass slowly relaxes itself and becomes solid again. The glass needs to go through an annealing process where it is held at a certain temperature, to de-stress, before it can be allowed to cool to room temperature.

Different glass expands at different temperatures and contracts at different temperatures, depending on the glass type. For example, float glass is coe 82 – 84, Bullseye and Uroboros is coe 90 and Spectrum is coe 96. You can’t fuse float with Bullseye glass as they will both expand at different rates and cause fractures within the glass piece. Glass has to be compatible with other glass to fuse successfully. What does coe actually mean? I refer you to Brad Walker of Warmglass.com. The coefficiency of expansion is the change in the molecular structure of glass as it is heated. The numbers (coe 90) refer to the number of millionth of an inch that the glass will expand for each degree of centigrade or 1.8 degree Fahrenheit. Brad Walker gives the example of Bullseye glass that expands 0.0000090 inches for each degree it is heated.

Testing for compatibility. Luckily, the commercial fusing glass companies have done the hard work for you and told you the coe of their glass but what about that unidentified stained glass that you have in front of you? You will need to test it yourself to find out which of your glass types it is compatible with. This is a simple job but time consuming as it involves placing pieces of unidentified glass onto a glass you know the coe of and fusing it in the kiln. When you have fully fused the glass and it has cooled, remove it from the kiln and hold it up to the light. If you see a small line around any of the glass pieces then these will not be compatible. A sure-fire test method is to buy polarising film or a stressometer that will allow you to inspect the glass for stress halos.

What if your tested glass only has a faint halo round it? After all it looks solid so can’t you get away with a bit of stress? Unfortunately, no, because one day your customer will be admiring the glass dish in his hand and then CRACK! It will shatter before his astonished eyes. Maybe not tomorrow but some day soon…Take this from someone who tried to fuse millefiore eyes (coe 104) onto a float glass fish (coe 82).

I hope this gives new fusers some insight into the technical stuff of glass fusing. If you want to ask me a question, point out an error or talk to me about other stuff please convo me at http://glassprimitif.etsy.com

Glassprimitif