Guide to Glass Fusing, Slumping, and Kiln-forming Techniques

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The Tutorial

The best way to use this tutorial is to start at the beginning and move through at your leisure, stopping and re-starting when you wish to.  When you've completed the various sections, you'll have a solid background in fusing and slumping and a basic understanding of topics such as glass compatibility and the concept of annealing.  Jump around if you must, but the sections build on each other so it's a good idea to at least briefly hit them all.

If you are already familiar with kiln-forming basics, you can use the links below to jump directly to the section you wish to review.   

What is warm glass? What is kiln-forming?

The term warm glass refers to fusing, slumping, and other glass processes which take place at temperatures between 1100 and 1700 degrees Fahrenheit (600 to 925 Celcius).  That doesn't sound warm, but it is when you compare it a glassblower's working temperatures, which often exceed 2000 degrees Fahrenheit.  

Another term for these processes is kiln-forming.  It's not unusual for the two terms -- "warm glass" and "kiln-forming" -- to be used interchangeably, although in recent years "kiln-forming" has become the preferred term, while "warm glass" has increasingly been used to refer to this website.

What are fusing and slumping?

Glass fusing is the process of using a kiln to join together pieces of glass. If you apply heat to glass, it will soften.  If you continue to apply heat, the glass will become more fluid and flow together.  Two or more pieces of glass will stick (or "fuse") to each other.  When the right kind of glass is heated and then cooled properly, the resulting fused glass piece will be solid and unbroken.

Many people also use the word "fusing" to include bending and shaping glass using the heat of a kiln. This manipulation can take many forms, but the most common is slumping, where a mold is used to cause already fused glass to take on the shape of a bowl, a plate, or similar object. Other kinds of manipulation done with fusing techniques are combing, which involves using a tool to distort the shape of the glass while it is hot, and fire polishing, which uses a kiln to heat the glass just enough to make it shiny and smooth.

Another category of kiln-forming activity involves the use of molds to form glass into more complex shapes.  Virtually any shape that can be formed in clay or wax can also be made in glass.  These more advanced kiln forming processes include kiln casting (melting glass into a mold inside a kiln, pate de verre (forming shapes by heating a "paste of glass" inside the kiln), and glass casting (pouring molten glass into a mold).  These processes tend to be more complicated than basic fusing and slumping. 

The table below summarizes the major kiln-forming techniques.

     KILN-FORMING PROCESSES AND TEMPERATURES

Process

Definition

Fahrenheit

Celsius

Full fusing

Joining two or more pieces of glass by heating until they flow together 1450 to 1550 788 to 843

Tack fusing

Fusing until the glass just sticks together, with each piece retaining its individual character.  1350 to 1450

 

732 to 788

Slumping

Shaping glass by bending it over or into a mold 1200 to 1300 649 to 704

Combing

Manipulating glass by raking a tool across the surface of  molten glass   1650 to 1750 899 to 954

Fire polishing

Heating glass just enough to round the edges and give it a shiny appearance 1300 to 1400 704 to 760
Kiln casting Fusing small pieces of glass (called "frit") inside a mold 1500 to 1600 816 to  871 
Pate de verre Fusing a paste made with small pieces of glass inside a mold 1300 to 1500 704 to  816
Glass casting Melting liquid glass into a mold 1500 to 1700 816 to 926

Note that the temperatures given are for typical fusible art glasses. 

Other kinds of glass may require different temperatures.

 

This tutorial is a condensed version of copyrighted material from Brad Walker's book, Contemporary Warm Glass: A Guide to Fusing, Slumping, and Related Kiln-forming Techniques.

Please click here to learn more about the book.

Click here to go the next part of this tutorial, "Getting Started."

 

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