By Gil Reynolds

Reprinted from the Stained Glass News issue 69 from December, 2004

One area that generates a lot of interest in my fusing classes is the subject of inclusions. Students may have seen a glass sink with sheet copper inside the layers of glass, or have wondered how to get a design of sparkling mica like they observed in something at a local gallery. Well, the process of fusing objects between layers of glass is actually quite simple so, if this is something you care to pursue, you might find the following guidelines helpful.

For the kiln working set, an inclusion, loosely defined, is a term that generally refers to a foreign object fused between two or more layers of glass. Here’s an example: take a piece of black glass, place a piece of thin sheet copper on top of it, add a layer of clear glass on top of that and fire everything in a kiln to 1475 F for 10 minutes. Presto! You now have yourself a genuine copper inclusion.

The list of objects that can be fused into glass might surprise you. I had one student use a bird’s wing. The resulting ash patterns produced by the bones and feathers looked very cool, but boy did they stink while they were firing in the kiln.

“These inclusions were made by using scissors to cut the leaf shapes out of thin sheets of copper and brass. The metal piece were sandwiched between the clear and black glass, then fused.”

There really aren’t any rules, although the term “inclusion” is usually reserved for sandwiching objects other than the glass you’re using. In “Fuser-Speak,” if you take System 96 frit and place it between two layers of System 96 glass, this, for some reason, is generally not called an inclusion, (Don't ask me why. I didn't come up with this vocabulary.) Yet, you can place some Fuse Master or Thompson enamels between two layers of glass and it would be correctto say, ‘Oh, I see she’s using enamel inclusions in her work.’ The key here is that the inclusion be foreign. But, you may ask, if it has to be a foreign substance, how can it be compatible?

Good question. You can use a copper penny as an inclusion, but it will usually break the glass around it when cooled. Although some people don’t mind, most of us prefer to have our finished pieces stay intact. If your inclusion doesn’t break the glass that surrounds it, then it is considered a compatible inclusion.

The PPG website defines COMPATIBILITY as: ‘The ability of two or more materials to exist in close and permanent association for an indefinite period with no adverse effect of one on the other.’ This is important for two reasons. First, non-glass objects can be compatible with glass. When you think about it, glass to metal bonds have been around a long time--think old radio tubes--so this does open up some unusual options for glass fusers.

Second, if it doesn’t break, then it’s OK. Just keep in mind that the breaking doesn’t always occur right away. Cracks caused by incompatibility can develop days, weeks, or even months after the glass is taken out of the kiln. Ideally, the majority of us want our glass to last a long, long time, so view a sample of the inclusion fused between two layers of clear glass through a polarized filter to see if excessive stress is present. If the sample shows no or low levels of stress, you should be in good shape.

The list of items people have gotten away with is surprisingly long. Some of the most popular have been copper and brass sheets or screen; gold, silver and palladium leaf; powdered mica, enamels, and fiber glass; organic matter such as leaves, paper, or ceramic fiber; and certain types of cast plaster and metallic luster. This is not an all-inclusive list, but these are the main groups people have traditionally had success with in their work.

Regardless of the nature of the inclusion, you’re going to need to be aware of how air bubbles might affect your final piece. If the top layer is a solid piece of glass, there is a tendency for that layer to soften over the inclusion and seal to the base layer of glass. Once done, any air that was present between the layers is now a permanent design element. That may or may not be desirable.

Many strive to trap the least amount of air as possible. To do this, use a modified firing schedule and edge-propping. Edge-propping is nothing more than placing supports--usually some chips of glass--along the outer edges of the top layer so the center of the glass sinks down before the edges have a chance to seal. If the supporting chips do not work with your design, make the piece oversized and cut off the support elements after it’s fused.

A recommended firing schedule would be to hold the glass at 1100 F for one hour, followed by heating up to 1350 at a rate of 60 degrees per hour. After that, heat at 600 degrees per hour up to 1475 F and hold for 10 minutes. This schedule drastically reduces the amount of air trapped between layers.

Until Next Time,
Keep a Warm Kiln,
Gil Reynolds