Glass Fusing – Kilnformed Glass

What is Glass Fusing, Warm Glass or Kiln-formed Glass?

The term kiln-formed glass or warm glass refers to fusing, slumping, casting and other glass manipulation processes which take place at temperatures between 1100 and 1700 degrees Fahrenheit (600 to 925 Celcius).  It may not sound “warm” to you, but it is when you compare it a glassblower’s working temperatures, which often exceed 2000 degrees Fahrenheit.  

Glass fusing is the process of using a kiln to join together pieces of compatible glass. If you apply heat to glass it will soften, and by continually applying heat, the glass will become more fluid and flow together and become one, or “fuse”to each other.  When compatible glass (all the glass used has the same coeffient of expansion and contraction) is heated and then cooled properly (annealed), the resulting fused glass piece will be solid and contain no internal stress. 

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 kiln-forming techniques are combing or raking, which involves using a tool to distort the shape of the glass while it is hot by raking the tool through the glass, and fire polishing, which uses a kiln to heat the glass just enough to make it shiny and smooth after the piece has been coldworked….more on coldworking later.

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” pressed inside a mold within the kiln), and hot casting (pouring molten glass into a mold).  These processes are definitely more complicated than basic fusing and slumping.  

Anyone thinking of starting to work in kiln-casting, pate de verre, hot casting processes , or any advanced techniques should definitely take our beginners fusing workshop and have worked with kiln-formed glass in their own studios before venturing out into the complicated and multi-process world of kiln casting, etc.  Understanding the basic properties of glass and their limitations, understanding your kiln, and knowing your coldworking options definitely works to your advantage when exploring more advanced techniques…..less frustration on your part in the long run.

Each type of glass has it’s own particular cooling range of temperatures and that process is called annealing.  The annealing process gives the cooling glass enough time to allow any stress introduced into the molecules of the reconfigured glass during the fusing processes to relax and become friendly with each other once again.

Annealing is a process of slowly cooling glass to relieve internal stresses after it was formed. The process is carried out in a temperature-controlled kiln.  Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change or mechanical shock. Annealing glass is critical to its durability. If glass is not annealed, it will retain many of the thermal stresses and significantly decrease the overall strength of the glass.  After the glass has gone through the higher temperature fusing processes it must be cooled until it  reaches a stress-relief point, the annealing temperature, at a viscosity which the glass is still too hard to deform, but is soft enough for the stresses to relax. The piece is then allowed to heat-soak until its temperature is even throughout. The time necessary for this step varies depending on the type of glass and its maximum thickness. The glass is then slowly cooled at a predetermined rate until its temperature is below the strain point.  Following this, the temperature can safely be dropped to room temperature at a rate limited by the heat capacity, thickness, thermal conductivity, and thermal expansion coefficient of the glass.

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