What Happens When You Heat Glass? (7 Fused Glass Processes)
What Happens When Glass Is Heated?
Many things occur when one or more pieces of glass are placed into a kiln and heated at high temperatures. Most kilns that are specifically designed to be used for fusing glass have a high temperature limit of about 1800 degrees Fahrenheit (F). When you heat glass slowly through the range of temperatures—starting from room temperature and up to 1800 degrees—the glass will become softer, because the higher the temperature gets, the softer the glass becomes. By knowing what happens to the glass at different temperatures, you can control the finished look of your fused glass project.
7 Fused Glass Methods
Many interesting effects can be obtained by heating and holding glass at a specific temperature range:
- Slumping (1200–1250 degrees F)
- Heat polishing (1300–1350 degrees F)
- Tack fusing (1350–1375 degrees F)
- Contour fusing (1400–1450 degrees F)
- Full fusing (1450–1475 degrees F)
- Casting (1475–1500 degrees F
- Kiln pouring (1600–1700 degrees F)
1. Slumping Glass
Slumping a piece of glass over or into a mold requires a temperature of somewhere between 1200 and 1250 degrees F. When glass reaches these temperatures, it will generally be soft enough to bend so that it can be shaped. Slumping glass refers to shaping a piece of glass into a mold, while draping glass refers to shaping a piece of glass by allowing it to drape over the mold.
2. Heat Polishing Glass
When you heat glass to a temperature of 1300 to 1350 degrees F, the surface of the glass becomes soft enough to just start to melt. By heating to a point where just the very surface of the glass reaches this temperature, you can produce a smooth, shiny effect. Heating to this temperature range will also round off any square edges but will not change the basic shape or thickness of the finished piece.
3. Tack Fusing Glass
Tack fusing glass refers to the effect that is obtained when two or more pieces of glass are heated to approximately 1350 to 1375 degrees F. This temperature range will result in any pieces of glass that are in contact with each other fusing together, while still allowing each piece to retain its original shape, size, and thickness.
4. Contour Fusing Glass
As you pass the tack fusing temperature, your glass will start to become soft enough to begin to melt into a single piece. Contour fusing requires a temperature of approximately 1400 to 1450 degrees F. At this temperature, the glass will not quite melt but will begin to flow together into a single thickness layer of glass, leaving very little of the original shape, size, or thickness of the original pieces. Pieces of glass heated to this temperature will still not have a smooth uniform shape but will have a polished finish.
5. Full Fusing
You can fully fuse several pieces of glass into a single uniform-thickness, finished piece by heating it to somewhere between 1450 and 1475 degrees F. At these temperatures, your glass pieces will have melted enough to combine and flow together into a single piece of fused glass. This may be a finished piece or a starting point from which you could work, cut, or reshape the piece prior to another fusing. For example, I use this process to create large pieces that I then inspect, cut, and reshape into smaller pieces of jewelry with interesting combinations of color and shape.
6. Casting Glass
Casting glass is a process where numerous pieces of glass—or larger amounts of glass frit—are placed into a mold and heated to a temperature high enough to shape the glass (1475 to 1500 degrees F). The mold is similar to one that might be used to create pottery or small figures: Many unique molds for lidded boxes, small shapes, and figures are available. The small cast pieces and figures can then be tack fused to other pieces of fused glass as adornments.
7. Kiln Pouring
Kiln pouring glass is the process of heating glass to a point where it will become liquid enough to pour or drip freely. Kiln pouring requires a temperature of 1600 to 1700 degrees F, and it is usually a technique that requires a glass kiln with a deeper interior chamber. I use clay flower pots with holes drilled in the bottom to create kiln pours—or pot melts, as they are sometimes referred to. The results from this type of firing can be absolutely gorgeous. You can kiln pour or pot melt glass into a mold, or just allow it to flow onto your prepared kiln shelf, creating a large piece of glass that can be further worked and shaped into finished pieces by using any of the techniques listed above.