What’s the difference? Kiln-forming Techniques

Ever wonder what is meant by kiln-forming techniques?  What the difference is between kiln-forming and warm glass? Between fusing and kiln casting? What temperatures do these all take place in? Hopefully, this review will help.

Kiln-Forming

Kiln-forming is the process of making glass and shaping it using heat and gravity in a kiln. Glass fusing, slumping, and kiln casting are all part of kiln-formed or warm glass.

Specialty glass manufactured for use in the kiln is called kiln-glass. One of its properties is compatibility, allowing the glass to combine with other compatible glass into one piece.

As glass is heated, it becomes soft and sticky. As the temperature increases, the glass becomes a liquid. Once the glass is molten, physical processes like gravity allow it to fill the space in which it is contained and join it (fuses it) with other compatible glass in the same area. Once this has happened, the temperature is lowered through an annealing process so that the glass resumes its original structure. Glass is a material like no other!

Slumping

Slumping is the kiln-forming technique where glass changes into a three-dimensional form by bending over, into, or through a mold. Slumping can occur at a lower temperature than tack fusing. This change happens because gravity and mass allow the glass to bend, stretch, and conform to a mold.  Typically slumping occurs to a fused glass piece made by one of the fusing techniques.

Fusing

Fusing is the kiln-forming technique where two pieces of kiln-formed glass are joined together by heating them in a kiln to make fused glass.

There are different glass fusing techniques based on the temperatures obtained. The lowest temperature process is “tack fusing.” In this technique, two or more pieces of glass stick together as the temperature rises, but the heat is not enough to change their shape, texture, or other original characteristics. The next process on the temperature scale is called a full fuse. Here, the two or more pieces of glass become one, and the corners soften. The hottest fusing technique is high heat flow fusing, where you melt the glass until it flows and fills all the available spaces in a damed area.

Kiln Casting

Kiln casting is the kiln-forming technique of creating a glass object in a kiln by heating glass above or inside a mold until it flows to fill all the voids. The lowest temperature where kiln casting occurs is 1500 degrees F. One of the biggest challenges to this technique is creating the mold. Open-faced and closed or semi-closed molds are the two basic types used in kiln casting. Using closed or semi-closed molds allows for the creation of three-dimensional sculptural pieces.

Temperatures of various Kiln-Forming Techniques

Fahrenheit Centigrade Process Definition
1100-1500 593-816 Glass Painting Using glass frit, glass enamels, or vitreous paints to paint on glass
1200-1300 649-704 Slumping Shaping glass by bending it over, into, or through a mold
1300-1400 704-760 Fire Polishing Heating glass to round the edges and make it shiny
1350-1450 732-788 Tack Fusing Fusing until glass sticks together, each piece retains its texture and details
1450-1550 788-843 Full Fusing Joining pieces of glass until they flow together with seams
1500-1600 816-871 Pate de Verre Fusing paste made with powder or fine glass grit inside a mold
1500-1700 816-927 Kiln Casting The heating of glass inside an open or closed mold until it starts to flow and fills all the available space
1600-1800 927-982 High Heat Flow / Glass Casting Melting glass until it flows into all the spaces of a damed area
1650-1750 899-954 Combing (Raking) Manipulating glass by raking a tool across the surface when it is in the molten form

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