6mm Rule When heated to full fusing temperatures, glass naturally wants to assume a thickness of 6mm. This means that if your piece is thinner than 6mm, it will 'suck in' from the sides, so that the middle of the piece is 6mm thick. On the other hand, if your piece is thicker than 6mm when it is put in the kiln it will spread out when fired, to reach the optimal 6mm thickness. To combat this problem always make sure that your piece is always 6mm thick to start with by stacking 2 layers of 3mm glass, or 3 layers of 2mm.

AnnealingThe controlled cooling of glass in order to minimise undesired stress.

AFAP – FULL - 999°C As Fast As Possible. Used for describing the desired rate of heating or cooling in a firing schedule.

Binder A binder is an important ingredient used when making your own mould mixture (or Investment), as well as serving other purposes. It unites all the different ingredients, primarily encouraging adhesion and cohesion of particles within the mixture.

Bubble Squeeze An adapted firing schedule used for pieces that are at risk of trapping large air bubbles. A hold at 677°C allows air to be squeezed out from between layers of glass.

Casting The process in which glass objects, usually 3-dimensional and solid, are created by filling a mould with crushed glass, then heating until the glass is molten so it fully takes the shape of the mould. This method is often used to make glass replicas of 3D objects, as the original object is not damaged in the mould-making process. Lost Wax and Pâte-de-Verre are both examples of casting, with very different results.

CoE – Coefficient of expansion Coefficient of expansion (CoE) is the rate at which a material expands when heated. For kilnformed glass artists, the CoE is shorthand for describing glasses that are compatible with each other. For example, Spectrum's System 96 glass (as well as compatible glass that is manufactured by Uroburos) is often described as simply "96 glass". Similarly, Bullseye's compatible glass is frequently referred to as "90 glass". See also Compatibility.

 Cold Working Any cutting, grinding or polishing of glass is known as cold-working. It is called "cold" to distinguish it from techniques in which the glass is formed with heat. Common cold-working methods include sandblasting, bevelling, sanding, engraving and acid etching.

Compatibility Glass that can be fused together without excessive residual stress is considered compatible. See also CoE.

Devitrification A white, frosty or matte surface on glass that can appear after fusing. Often described as "scum" and referred to as "devit". Glass molecules are normally arranged randomly. Devitrification (literally "to become unglass-like") is the organization - or crystallization - of glass molecules.

Dichroic Dichroic glass is made by coating a sheet of glass with many very thin layers of metal, or metal oxides. This results in a shiny surface that can display different colours depending on whether light is reflected or transmitted from the surface. A dichroic coating can be applied to both textured and non-textured glass, and can be applied in different patterns as well as block colours. This results in each piece of dichroic being completely unique, which makes this glass a very popular choice with jewellery makers.

Firepolish The process of heating a glass object in a kiln to a temperature high enough to impart some level of gloss to the surface.

Firing Schedule When firing a piece of glass art, it is important to use the correct firing schedule to ensure that your piece is fired correctly. This schedule is programmed into the kiln and determines the ramp, the top temperature and the hold/soak period, all of which affect how the piece turns out. Different techniques require different firing schedules, as do different kilns.

Fuse - Fusing The technique of joining two or more pieces of glass by heat in a kiln.

Hold - Soak Maintaining the temperature in a firing for a select amount of time. This is important as it unifies the temperature of the whole piece, minimising any stress on the glass and therefore reducing the risk of cracking.

Hot Glass The term 'hot glass' refers to the process of manipulating glass under very high temperatures. Glassblowing and lampwork are both examples of this technique, where the glass is manipulated and changed when it is very hot, as opposed to warm glass work, which takes place in a kiln at slightly lower temperatures.

Inclusions Inclusions are any item that can be fused between layers of glass. These include metal foils, Glassline paper and Mica powder.

InvestmentAn investment is another name for a mould mixture, usually created to specific recipe. All investment moulds are composed of three basic ingredients: a binder, a refractory and modifiers.

Lost Wax Casting Lost Wax Casting is the process by which a glass duplicate of a 3D object is made, using wax and casting plaster. This process has 4 basic steps. Firstly, a negative mould is taken of the object being cast, using a rubbery material from which the original object can be removed without damaging the impression. This mould is filled with wax to create a copy of the original object, which is then covered with a mould mixture (or Investment) to create another negative mould, leaving a hole for the wax to be melted out of. Finally, the wax is removed using a wallpaper steamer or similar, leaving a void that can be filled with crushed glass and fired to create a solid glass replica of the original object.

Modifier A modifier is added to a mould mixture (or Investment) to change its characteristics, depending what properties the mould mixture is required to have. Different modifiers are available which all serve different purposes, including reducing the weight of a mould, making a mould stronger or allowing water to drain.

Pâte-de-Verre Pâte-de-Verre is a kilncasting method that literally means ‘paste of glass’. It involves mixing frit granules with a binder, applying this mixture to the inside of a negative mould and firing, which results in an interesting and unique piece of work.

Ramp The rate of heating or cooling in a kiln, measured in degrees per hour.

Reactive Glass Reactive glass is more likely to react with the chemistry of other glasses or (usually metal) inclusions in the kiln, giving interesting visual results. Check the Working Notes of individual glass styles to finds what, if any, glass styles or inclusions it will react with. Whilst sometimes unwanted, reactions can be used to create beautiful and unique works. Visit our Kilnforming Guides section for tip sheets on using reactivity to your advantage. 

Refractory A refractory is an important ingredient used when making your own mould mixture (or Investment). It is a material that is difficult to melt or change, can withstand high temperatures, and is used as a strengthener and base for the mixture.

Shift Tint The hues of 'shift' colours change depending on the thickness and/or lighting of the piece: natural, incandescent, or fluorescent. The same piece can appear as a completely different colour as the light changes through the day, lending an ethereal quality to a piece. The difference is more pronounced the thicker the piece, making it ideal for cast glass objects. 

Slump - Drape To heat glass sufficiently so that the glass bends under its own weight, taking the shape of the mould on which it rests.

Stress Internal force or tension within a piece of solid glass. Stress can be caused by glass incompatibility or improper annealing.

Striker Some Bullseye glasses appear pale or colourless in the cold sheet form, but “strike” or mature to a much different colour when fired.

Suckers Suckers are unsightly depressions or wrinkles that can appear when casting. They are caused by differences in temperature during the cooling process, as thinner areas will cool more quickly than the rest of the piece. To prevent suckers, keep your glass at a uniform temperature during cooling by soaking/holding the glass at 677°C to thoroughly unify the temperature, then cooling gradually to the annealing phase.

Tack Fuse – Contour Fuse Tack fusing is a fusing technique in which the pieces of glass are heated until they are just hot enough to stick together and retain many of their original characteristics.

Thermal Shock When glass is heated or cooled unevenly, it expands or contracts unevenly. Since glass cannot stretch, this change can cause the glass to break. This is called Thermal Shock and is often caused by heating or cooling glass too quickly.

Top Temperature – Process Temperature The hottest temperature in a firing schedule. The temperature at which the glass is melting to achieve your desired look.

Warm Glass Warm glass is the technical term for the working of glass by heating in a kiln. It is referred to as 'warm', as the glass does not reach temperatures as high as 'hot' glass work. In warm glass work, the piece is manipulated and arranged when the glass is cold, and then the piece is fired in a kiln, as opposed to in hot glass work where the glass is manipulated at very high temperatures. Warm glass mostly consists of fusing, slumping and casting.