The basic fusing and slumping process has five main stages:
- Heating phase - where the temperature increases from room temperature to the temperature where fusing and slumping takes place
- Soaking phase - where the temperature is maintained at a given point for a period of time
- Cooling phase - when the temperature falls from its highest point to just above the annealing range
- Annealing phase - the critical step that relieves the stress in the glass
- Cool to room temperature phase - where the glass gradually becomes cool enough to touch
The "heating" phase, which takes place between room temperature and around 1200 to 1700 degrees F (depending on the process you are performing), is where the glass makes the transition from a solid to a more flowing form. As glass is heated and moves through this phase, it goes through three separate states. First, from room temperature up to about 1000 degrees F (540 degrees C), glass remains rigid and brittle. It is expanding slowly, but will still crack or break if the temperature increases too rapidly . This kind of temperature related fracture is called "thermal shock."
How rapid is rapid enough to cause thermal shock? The answer depends on several factors, but the most important are the thickness of the glass and the width of the piece of glass.
By the time the temperature of the glass gets above 1000 degrees F, any glue, moisture, or surface contaminants have burned off. The glass begins to soften slightly and the surface of the glass will look glossy. Thermal shock will not occur at this temperature.
When the temperature reaches around 1300 to 1400 degrees F, the glass gradually becomes soft enough to conform to a mold. It starts to glow a bright yellowish-red. The edges may soften and round and two pieces of glass that are touching will begin to stick together. This is the temperature range where slumping takes place.
If heating continues above 1330 degrees F and moves toward 1500 degrees F (820 C), the color of the glass deepens and becomes more red. Glass in this range has slumped completely and even starts to stretch out of shape.
Full fusing, the complete merging of two or more pieces of glass into one, takes place at around 1500 degrees F. Above that temperature, glass becomes increasingly liquid. Kiln casting and pate de verre take place in this range.
As the temperature moves above 1500 degrees F, glass also glows bright red. Bubbles may move toward the surface of the glass and pop. By the time the temperature reaches around 1700 degrees F (925 C), the glass is buttery and can be moved when prodded with a tool. The technique of manipulating molten glass with a tool is called "combing" or "raking". Glass manipulation techniques should be undertaken with care and only after you have some experience with fusing and slumping.
The "Soaking" phase generally occurs at the highest temperature in the cycle. This temperature is around 1500 degrees Fahrenheit for fusing or around 1200 - 1300 degrees for slumping, but it can be higher or lower for different processes such as fire polishing, combing, or casting. The length of the soak time can also vary.
When slumping, longer soak times cause the glass to conform more closely to the mold. When fusing, longer soak times cause the piece to become flatter and smoother. How long to soak also depends on other factors, such as type of glass, the thickness of the glass, the final shape desired, and how long the kiln has taken to make it through the heating phase. Soaking can last as short as a minute or as long as an hour or more.
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