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What is kilnformed glass?

The fused glass items on this web site are made using a process called Glass Fusing or Kilnforming .This process may be referred to as working with "warm" glass, because the working temperatures of kilnformed glass lies midway between the extremes of glass blowing ("hot" glass) and stained glass ("cold" glass). On the other hand, some feel that this is a wimpy, unexciting, term for a very exciting type of glasswork. I prefer the term kilnforming.

The glass artwork presented here is made by using several processes:

  • Glass Fusing -  joining pieces of glass together by melting them in a kiln. This is done at temperatures around 1500F.
  • Slumping - using a mold to shape glass in a kiln and make three dimensional items like bowls and plates.
  • Sandblasting -  this is a process where particles of an abrasive (in this case aluminum oxide) are propelled toward the glass by compressed air.  This can be used to frost the glass, carve the glass,  and create patterns on the iridized coatings.
  • Cold Polishing - the edges of the fused and slumped pieces can be left in their natural, soft, curved configuration; or they can be shaped and polished outside the kiln with various types of abrasive sanding belts.
  • Hotworking - some of the multicolored pieces were made in a glory hole furnace at temperatures approaching 2000 degrees F. These were melted on the end of a punty rod, manipulated and stretched to get the desired effects.
  • High Firing - heating the glass as hot as 1700 degrees F to take advantage of the tendency of the glass to flow and move of its own accord at these temperatures. This is used in the creation of aperture pours.
The glass for kilnforming is purchased already colored. The coloring is created at the glass factory by adding various chemicals and elements to the silica mixture that is heated to create the glass. There are four main types of glass used in glass fusing:
  • Opalescent glass - opaque glass that you can not see through.
  • Cathedral glass - transparent glass.
  • Iridized glass - either opalescent or cathedral glass that has been coated with a metallic coating made of tin. This coating can be silver colored, gold colored, or have a rainbow transition from gold and silver, to purple, blue and green. There are also patterned and textured iridized glasses. Different effects can be obtained depending on whether the iridized coating is facing the bottom of the piece, the top surface of the piece, or is sandwiched between layers of glass.
  • Dichroic glass - Thin layers of metallic oxides, such as titanium, silicon, and magnesium are deposited upon the surface of the glass in a high temperature, vacuum furnace. This creates a brilliant coating on the glass that displays more than one color, especially when viewed from different angles. This is most often used for jewelry.
The glass is purchased in sheets of varying thickness, crushed glass (frit), powdered glass, thin rods (stringers) and thin shards of glass (confetti).

Any glasses fused together have to be tested "compatible" or they will crack upon cooling. Glass has to be heated slowly, and can be brought to specific temperatures to achieve different effects. It then has to be cooled slowly to complete the  annealing process. The kilns for glass fusing are usually computer controlled to achieve consistent results. However, in spite of this each hand made glass item will be different, and certain inconsistencies or imperfections are part of the mystery and beauty of this process.

For those of you who are glass workers, there are links on the left that demonstrate some interesting kilnforming techniques.

In addition, I highly recommend Bullseye Glass Company's online video tutorials, which are a great value, and can be seen here.

Bullseye Kiln-glass Education Online

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