Primer article - because sometimes we just want to know a little bit first!
Fusible Glass that is not labeled as “fusible” has not been manufactured to be heated into a molten state. Many times the surface colors will fade and the rate at which the glass will expand is unknown. Glass labeled as “fusible” has been designed to be heated and fused together The colors will hold through a firing and the glass has been tested to melt at a specific rate and return to solid at a specific rate.
Coefficient of Expansion or “COE” is the rate at which glass will shrink and expand when heated and cooled and is represented by “COE”. It is essential to fuse with glass that has been tested by the manufacturer as “fusible” and fuse glass with exactly the same COE’s. Many artists even suggest to assure compatibility when fusing, to stick with the same glass manufacturer throughout the project.
Iridized Glass can be “iridized” by the manufacturer. Iridized glass is coated with a fine metallic coating before the glass is cooled. This coating gives the glass a mother of pearl effect. This coating can be selectively removed to reveal the plain glass surface. Iridized glass fuses a little differently then glass that has not been iridized. It is important not to fuse two pieces of iridized glass together with iridized surfaces facing each other. The iridizations creates a slight barrier from the glass and the iridized coatings will retard fusing slightly. Additionally, iridized coatings will diminish slightly when fused at extreme temperatures.
Dichroic Glass coating is done by secondary companies that buy glass from the glass manufacturer. These “glass coaters” use a space age vacuum technology to apply extremely thin and brilliant metallic surfaces to glass. The variety of colors available in dichroic glass is vast. Most dichroic coatings are applied to either clear or black, smooth or textured glass. Like Iridized glass, the dichroic coating can be selectively removed to reveal the plain glass surface for contrast.
When loading a kiln with glass, it is not advisable to stack kiln shelves with glass projects in a glass kiln where the elements are only in the lid. Ideally, glass projects should be placed in the center of a kiln shelf with the shelf placement as close as possible to the pyrometer or thermocouple in the kiln.
Annealing Glass - To anneal glass is to allow it to slowly return to a solid state from a fluid state. If glass is not allowed to anneal properly, the glass will break due to stress. Annealing is achieved by holding a body of glass at the temperature in which the glass will return to a solid for the amount of time required for ALL of the glass molecules to return to a stable solid state. The temperature that COE 96 returns to a solid is approximately 960 degrees F. The time required to for all of the molecules to return to a solid state depends on the size of glass. Larger projects require more hold time than smaller projects.
Firing schedules - The schedule that the kiln will be fired at depends upon the size of the project being fired and the effect desired. A typical firing schedule for a small (3” x 3” or smaller) COE 96 project to be fused completely is below:
Segment 1 – rate 500 – temp. 1150- hold 10 min
Segment 2 – rate 9999 – temp 1465 - 1470 – hold 12 min.
Segment 3 – rate 9999 – temp 960 - 970 – hold 30 min.
The firing schedule above can be entered into the average digital kiln by pushing the “Ramp/Hold” button on the panel. The directions to go from there can be found in the kiln user’s manual.
Segment 1 of this sample firing schedule ramped the kiln at 500 degrees per hour until the kiln reached 1150 degrees and held at 1150 degrees for 10 minutes. This slow heating at the beginning stages of the firing help the entire body of glass to start to melt at one time. This will enable air bubbles to be eased out and not to be trapped in the glass. Larger projects may require a slower ramp (350 or even lower - 100) to avoid bubbles. Segment 2 tells the kiln to heat from 1150 to 1470 as fast as possible (9999) and hold for 12 minutes. Larger projects will require a longer soak time (hold time) to reach a full fuse. Segment 3 tells the kiln to go as fast as possible from 1470 to 960 or 970 degrees and hold for 30 minutes. This is the annealing stage for the glass. Larger projects will require a longer hold time to anneal properly.