The Melting Process

Pieces of glass can be melted together or “fused” in a glass kiln to make an entirely new single-piece glass art image. This process is referred to by various names: kiln worked glass, kiln formed glass, kiln fired glass, fused glass, and warm glass. The “warm glass” term locates fused glass between the “hot glass” of blown glass with a gas furnace “Glory Hole” (or the lampworker’s torch on a smaller scale), and the “cold glass” of “Stained Glass” that is not heated once the glass is originally made, but rather cut into shapes and then conjoined with lead or copper foil.

When pieces of glass or ground-up glass (“frit”) that range from the size of large sea salt to powder are placed on top of a sheet of glass and fused in a kiln, they create a new glass “painting”—or a painting entirely composed of pieces of glass. Internationally renowned glass artist Narcissus Quagliata was one of the originators of this technique and coined the term “Painting with Light” to describe the process.

If the artist wants the fused glass to have a three-dimensional shape rather than be flat, it can next be placed on top of a mold in the kiln and fired again. This additional firing would be to a lower temperature that would allow the glass to gently melt or “slump” into the desired shape.

Painting

Painting with glass (using pieces of glass) is a different process from painting with paints or enamels on the surface of glass, which are then usually fired to create permanency. When using ground glass to create a painting, both the length of time the kiln is held at what is called the “process” (or highest) temperature and the process temperature itself can be varied for different results.

More texture and roughness is created by using a lower temperature. Taking the kiln all the way to a “full fuse” temperature will create an entirely smooth, flat surface. The surface that was on top of the glass in the kiln will become shiny with full fusing, while the surface that was on the bottom next to the kiln shelf will have a matte finish; so a choice can be made on which is preferred for the final result.

Advanced Glass Education

I have been attending national glass conferences and classes with national and international instructors since 2002. Classes with Richard La Londe, Jody Danner Walker, Tom Jacobs at Bullseye Glass, Roger Thomas and Catherine Newell, in particular, have been very helpful in the honing of my techniques for “Painting with Light” in the glass paintings where I use ground glass or “frit.”

I feel particularly privileged to have also had the opportunity on a couple of occasions to take classes from renowned international artist Narcissus Quagliata, whose very inspiring glass creations are found across the globe.

The Challenges

One of the challenges with kiln fused glass is that the various colors of the glass itself must be originally manufactured to all have the same coefficient of expansion. Otherwise when fused together, they will be brittle and crack because they expanded with the firing heat at different temperatures. This special manufacturing process is available from only a few glass companies, and it all means that the glass product the fused glass artist uses is much costlier than window glass.

There are also challenges on the artist’s time for creation. Not all colors I might want to use in a glass painting are readily available in the palette of colors that my glass manufacturer produces. When planning a glass piece, significant time can be spent in creating and firing samples of various other colors that I want to incorporate into the piece. Because of the reactivity of chemical compositions of some colors with others, colors cannot always simply be achieved by mixing, as with oil paints. 

Colors that can react to each other must be separated by a clear layer, so they do not turn brown with their reaction. Layered colors like this have a different appearance from mixed colors, so some desired colors remain quite difficult to achieve. There is an additional time factor to always assuring this separation, too, with constant application of clear between reactive colors as the artist progresses in the creation of the image across the glass.

The pre-fired colors of the frit are not true to their post-firing colors, either, (a number are just white, as is the clear frit for separating reactive colors) so a mental image must be kept of the intended composition at all times while applying the frit. The design does not appear at all as it will once kiln-fired. The thickness of the frit, which can only be estimated while applying, also determines the depth of its color. Most landscapes require multiple firings with multiple applications of frit or multiple layers in order to achieve the appearance of depth and the colors desired.

I frequently use a more impressionistic technique that involves piling on the frit a bit irregularly, which produces a glass surface of irregular thickness. If the glass is quite thick, or just as importantly, if it is of variable thickness, each firing can require a couple of days of slow incremental changes in the kiln temperature to complete. A slower firing schedule prevents the glass from cracking as it heats and cools. Thick glass, or glass of different thicknesses, allows parts of it to heat or cool at different rates and, therefore, will break (thermal shock) if the temperature of the kiln is not changed quite slowly. During this firing time, the kiln cannot be opened to have anything added or removed.

In contrast to painting, frit artists cannot see if they are achieving their desired effect as they work, but only after their frit application and kiln firing are completed (which can be days worth of work). As the frit melt with firing, they retract to often leave spaces between colors or appear less dense in color than desired and the composition must be achieved by the application of additional frit with additional firing.