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Fused Glass - What Happens When You Heat Glass

Updated on August 28, 2009

What Happens When You Heat Glass

Many things occur when a single or multiple pieces of glass are placed into a kiln and heated to high temperatures. Most Kilns that are specifically designed to be used for fusing glass have a high temperature limit of about 1800 degrees F.

When you heat glass slowly up through the range of temperatures staring from room temperature to 1800 degrees the glass will become softer, 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.

Many interesting effects can be obtained by heating and holding glass at a specific temperature range.

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.

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. 

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.

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 of 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.

Full Fusing

You can fully fused 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 will have melted enough to combine and flow together into a single piece of fused glass. This piece may be a finished piece or a starting point from which you cold work, cut or reshape the piece prior to another fusing.

I use this process to create large pieces which I then inspect, cut and reshape into smaller pieces of jewelry that have interesting combinations of color and shape.

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 into a mold similar to a mold that might be used to create pottery or small figures.

Casting Glass into a mold requires a temperature of 1475 to 1500 degrees F.

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.

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 require a temperature of 1600 to 1700 degrees F, and 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 referredto. 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 above listed techniques.


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      Stan 20 months ago

      where is the name of the author?

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      RON 4 years ago


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      Artdollie6 7 years ago

      we used to operate a porcelain doll shop but have been experimenting with glass wanting to make original jewelery. this article has answered several of my questions. We have several Skutt kilns. so we have done some bottle slumping and ended up with lots of questions. You have been very helpful. Thanks.

    • Coolpapa profile image

      Coolpapa 8 years ago from Florida

      Great Hub! Glass folks should remember that temperatures will vary depending on your kiln!