The basic fusing and slumping process has five main stages. All are important to get the job done.
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.
After soaking, when the glass has taken on the desired shape, the process enters the “Cooling” phase. This involves cooling the glass until the red color goes away and the natural color starts to come back.
Historically, cooling was accomplished rapidly by lifting the lid of the kiln for a few seconds and allowing some of the hot air to escape. However, in recent years the tendency has been to allow the kiln to cool naturally, without opening the lid. This approach avoids the risk of burns or exposure to heat from holding the kiln lid open at high temperatures.
The major argument for cooling rapidly (as well as for the rapid temperature increase at the end of the heating phase) is to reduce the amount of time the glass spends above 1300 degrees Fahrenheit (700 Centigrade). Glass left too long in this zone has a tendency to devitrify, or take on a scummy, generally unattractive surface appearance that is difficult, if not impossible, to reverse.
Devitrification occurs when glass molecules start to crystallize. It usually takes the appearance of a whitish scum on the top edge of the glass being fired. Most glass artists consider it to be a nuisance to be avoided, but some like the effect and use it in their glass projects. It is most likely to occur above 1300F (usually around 1350 to 1400 degrees F); for this reason, it’s a good idea to minimize the time glass spends around that temperature.
Some glasses are more prone to devitrification than others and some, such as the “tested compatible” glass manufactured by Bullseye, Uroboros, and Spectrum, have been especially formulated to resist devitrification. You can also minimize devitrification by spraying or brushing on a “devit spray” prior to firing. This spray is available commercially under several different names (Spray “A”, Clear Coat Overglaze, Super Spray). It’s also possible to make your own version of the spray using borax and water.
Since most glasses made for fusing in a kiln are formulated to resist devitrification, it’s usually possible to allow the kiln to cool naturally during the phase, thus avoiding the risks of burns or exposure to heat from holding the kiln lid open at high temperatures.
Once the Cooling phase is complete and color has started to return to the glass, the kiln has cooled to approximately 1050 degrees Fahrenheit and the “annealing” phase begins. Annealing is a process by which the stress in the glass is relieved and the molecules in the glass are allowed to cool and arrange themselves into a solid, stable form. Successful annealing is the key to creating glasswork that will remain stable once it cools to room temperature.
Unlike many substances, glass does not melt or harden at a single temperature. Instead, it gradually softens and hardens as the temperature changes. The phase during which this transition from liquid to solid occurs is called the “annealing zone.” There are three critical points within this zone.
- The Upper Annealing Point – this is the upper end of the annealing zone, where the glass begins to return to solid form.
- The Annealing Point – this is the temperature where the molecules in the glass optimally realign themselves evenly throughout the glass. It’s always between the upper annealing point and the strain point.
- The Strain Point – this is the lower end of the annealing zone. It’s the place where the glass solidifies. The stress (or strain) remaining in the glass at this point is unlikely to be changed or relieved unless the glass is heated up again and annealed again.
The concept of annealing glass centers on the notion that soaking the glass at a point in the annealing zone can relieve stress. In theory, you can relieve the glass of strain and anneal at any temperature in the annealing zone, but the closer you are to the actual annealing point, the more efficiently annealing will take place.
After soaking at the annealing point, you should slowly reduce the temperature until it is below the strain point. The purpose of the initial soak is to allow the glass molecules time to adjust as the glass moves from liquid to solid. Slowly dropping from the annealing point to the strain point helps ensure that stress is not reintroduced before the strain point is reached.
Every type of glass has a different annealing temperature and a different annealing zone. Tests can be performed to determine these points, but even for the same type of glass they will differ slightly depending on the color or other variables in the glass. If your fused item uses many different types of glass, it may have many different annealing points and annealing zones, making the annealing zone soaking and cooling process extremely complicated.
Constant linear annealing (sometimes called “shotgun” annealing) is a method of annealing that does not require you to know the annealing point of the glass. Instead of soaking at a given point, the constant annealing approach simply allows the temperature to drop very slowly over a range that is large enough to encompass many different annealing zones. The idea is that you will be able to anneal at a number of different annealing points as the temperature drops through the range.
Once annealing is complete, the Cooling to Room Temperature phase begins. Often this is no more complicated than simply allowing the kiln to cool naturally, but thicker pieces of glass and kilns that cool rapidly require a bit more attention. The key is to slow down the rate of cooling so that thermal shock is prevented and the glass cools without cracking.
Probably the most important factor in how quickly you can cool the glass is the overall size and thickness of the glass being cooled. Very small pieces can generally be cooled as rapidly as desired, but larger pieces need more time to cool. For example, a 12″ (30 cm) diameter 1/8″ (3 mm) thick glass can safely cool from 750 degrees F to room temperature in 40 minutes. Doubling the thickness to 1/4″ (6 mm) doubles the time required to 80 minutes and 3/8″ thick glass requires at least two hours to cool to room temperature.
If your kiln retains heat very well, the natural cooling rate of the kiln may be sufficiently slow. In some cases, however, you may need to intermittently fire the kiln to slow down the rate of cooling. It’s a good idea to keep records so you learn how quickly your kiln cools.