Art Clay 650, with its low firing temperature, can be combined with glass, glass beads, and dichroic glass to create beautiful effects.

The main component of glass is silicon dioxide, often called silica: found naturally and plentifully as sand. When it melts, at about 1700°C, it's like syrup on a cold day. When it cools, it forms a rigid and brittle glass called quartz glass.

To lower the melting point, and the cost of melting, chemicals are added, typically sodium carbonate and calcium oxide: although other chemicals, and different cooling processes, produce a wide range of mechanical properties and colours.

Chemically, glass is classed as an amorphous solid: not a liquid, as is widely believed. As it's heated, it becomes softer then fluid, allowing it to be blown, moulded, poured, pressed, coated, decorated, engraved, or heat-treated.

A form of glass occurs naturally within the mouth of a volcano when the intense heat of an eruption melts sand to form Obsidian, a hard black glassy type of stone.

If two or more pieces of glass in contact are heated, they begin to soften and fuse together. With careful heating and cooling, the separate pieces of glass become one.

If glass is put on a mould and heated, it begins to soften and collapse, or slump, onto the mould. This is a common technique for making bowls and plates.

Sagging or slumping are often thought of as being the same. Correctly: during sagging, heated glass, supported at its edges, sags down in the middle to conform to a mould; during slumping, heated glass, supported at its middle, slumps down at its edges to conform to a mould.

Dichroic glass has two different colours: a transmitted colour and a reflective colour, both of which change depending on the angle of view. For example blue-red will be blue in transmission and red in reflection.

During manufacture, quartz and metal oxides are vaporized onto the surface of the glass as a multi-layer crystal structure using a sophisticated vacuum deposition process.

The metric system, used in the EU and most other countries, uses centigrade. The US and a few other countries use fahrenheit. There are simple ways of converting:

centigrade = (fahrenheit - 32) divided by 9 then multiplied by 5

fahrenheit = (centigrade divided by 5 then multiplied by 9) + 32

Using the centigrade scale, water freezes at 0°C and boils at 100°C. Using the fahrenheit scale, water freezes at 32°F and boils at 212°F.

From room temperature up to about 540°C, it stays rigid and brittle. It's expanding slowly but, depending on the size and shape, may crack or break if the temperature increases too quickly: called thermal shock.

By 540°C, any surface contaminants burn off. It begins to soften slightly and the surface looks glossy. It shouldn't crack or break.

Around 730°C, it glows a bright yellowish-red and becomes soft enough to mold. The edges soften and round off, and two pieces of glass that are touching will begin to stick together. This is where slumping can begin.

Above 730°C and towards 820°C, it glows more red. It will have slumped completely and starts to stretch out of shape.

Full fusing, the complete merging of two or more pieces of glass into one, takes place at around 820°C. Above that temperature, glass becomes increasingly liquid. This is when kiln casting and pate de verre can begin.

Above 820°C, it glows bright red. Bubbles may move toward the surface of the glass and pop. By 925°C, it's buttery and can be moved with a tool, a technique called combing.

Soaking generally occurs at the highest temperature in the cycle, around 820°C for fusing and 680°C for slumping, but it can be higher or lower for different processes such as fire polishing, combing, or casting. The soak time can also vary.

When fusing, longer soak times cause the piece to become flatter and smoother. When slumping, longer soak times cause the glass to conform more closely to the mold. How long to soak also depends on other factors, such as thr type of glass, the thickness of the glass, the final shape desired, and how long the kiln has taken to go 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 Rapid Cooling phase. This involves cooling the glass as quickly as possible until the red color goes away and the natural color starts to come back.

Generally, rapid cooling is accomplished by lifting the lid of the kiln for a few seconds and allowing some of the hot air to escape. This can be risky, so it’s a good idea to wear gloves and be very careful while the kiln is open.

The main reason for the rapid cooling phase, 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 between around 600°C and 820°C. 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 the glass molecules start to crystallize: you usually see a whitish scum. 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 around 750°C; 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.

Once the Rapid Cooling phase is complete and color has started to return to the glass, the kiln has cooled to approximately 560°C and the Annealing phase begins. During annealing, the stress in the glass is relieved and the molecules 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 centres 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 stain 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.

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 shotgun 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 glass is cooled to room temperature. 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 30cm diameter 3.0 mm thick glass can safely cool from 400176;C to room temperature in 40 minutes. Doubling the thickness to 6 mm doubles the time to 80 minutes and 10mm thick glass needs at least two hours.

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.

If a glaze or hot glass sticks to the kiln shelf, it's very difficult to remove without pulling away part of the shelf surface. To protect the shelf and make separation easy, you need to brush on a coat of glass separator or use a protective layer of kiln paper.

Glass separator, often called kiln wash, consists of finely ground minerals that don't fuse at normal firing temperatures. It's mixed with water and painted on using a soft-bristle haik brush. Usually, several thin coats are applied in different directions.

A few tips: don't use glass separator on ceramic fibre; don't get the powder on your hands or breathe it in; stir the mixture every time you dip the brush in; and store it in a glass jar.

You need to dry the shelf before firing, or the water will turn to steam and the pressure increase may crack or shatter the shelf. It's unlikely, but it could explode, so wear safety glasses when you open your kiln.

You can let it dry naturally in a warm place overnight, put it on a central heating radiator, or stand it on kiln posts and heat it, with the kiln vent open, at 150°C for 30 minutes.

Generally, glass separator will last for several firings: the lower the temperature, the longer it lasts. Before applying another layer, smooth the shelf surface with some wet+dry paper. However, for most small pieces, kiln paper is easier to use.

A SUCKER FOR VACUUM

Before using kiln wash, you need to remove dust from the kiln. Instead of blowing it everywhere or brushing it onto the floor, use a hand-held vacuum cleaner.

Kitiki uses a hand-held battery-powered mini-vac that runs off 4 AA batteries. It has an extendable brush nozzle and a removable washable dust collector. It's also useful for getting the biscuit crumbs out of your keyboard.

It doesn't need a manual, but the manufacturer's sheet says: do no put sheet metal and so on in vent or it will be troubled, and do not put brush to electrical outlet or it will be leakage of electricity. So, now you know.

Kiln paper is consists of compressed ceramic fibres held together with a binder. It can be cut to size with scissors, but don't get the fibres on your hands or breathe them in.

Before using it, fire it to about 650°C for five minutes to burn out the binder. The kiln, and the room, should be vented, as the burning binder sometimes smells and releases smoke. The paper can be re-used, if you can peel it off successfully once your work has cooled.

To avoid getting bubbles in your glass, make sure that kiln shelves that have been kiln washed are completely dry. Any moisture remaining will turn to steam and might form an air pocket. Increasing the hold time will sometimes let ait escape.