Glossary of Glass Fusing Terms
| Alumina Hydrate | A fine, white powder manufactured from bauxite. A major ingredient in shelf primer. |
| Aluminum Oxide | A crystalline compound used for sandblasting glass. |
| Anneal |
Controlled cooling and soaking of glass to relieve the stress in the glass which was developed during the fusing / slumping process. This is done while cooling that glass from 1000 degrees F down to 600 degrees F.
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| Annealing Point |
The temperature at which the internal stress in glass is reduced to an acceptable limit. For most glasses, the annealing point temperature is 35°F to 40°F above the strain point.
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| Anneal Soak |
The stage in cooling when the glass is held at a constant temperature for a time that is sufficient to relieve existing stress and stabilize the glass. |
| Base Glass |
The bottom layer of glass, or parent glass, to which other glasses are fused. |
| Beveling |
The grinding and polishing of a glass edge at an angle other than 90-degrees. |
| Bisque |
Clay ware that has been fired but not fired to maturity. Bisqueware is often used to make molds for sagging and slumping. |
| Blister |
A relatively large bubble occurring in the glass during firing. |
| Cathedral Glass |
A white powder occurring naturally as chalk, limestone, and marble. It is also known as Whiting.
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| Cathedral Glass | A term applied to transparent colored sheet glass. |
| Coefficient of Expansion (COE) | A number that indicates the rate of expansion, per degree of temperature increase, of glass as it is heated. Glass with the same COE can be successfully fused together. Bullseye glass is COE 90, while Uroboros manufactures both COE 90 and COE 96. See Compatibility. |
| Cold Working | Working with or changing glass at room temperature. This could involve sanding, grinding, polishing, drilling or sandblasting. |
| Combing | After different layers of colored glass are assembled on a kiln shelf, the glass is heated to well above full-fuse temperatures (1650°F) until the surface is liquid enough to be moved (combed / raked) with a metal hook. This results in a pattern looking like feathers. |
| Compatibility | Compatible glass has the same Coefficient of Expansion (COE), and after firing and annealing properly, remains relatively free from internal stress. |
| Confetti | Small eggshell thin shards of glass used for creating random patterns, shading and color blending in glass fusing. |
| Crazing | A fine network of cracks in the surface of glass. |
| Crucible | A ceramic pot in which glass can be melted. Common fire clay crucibles are adequate for most fusing studio applications. Other materials like graphite can be used for a crucible. |
| Cullet | Scraps of glass for re-melting. |
| Decal | A picture or design printed on special paper for transferring to glass by means of heat in a kiln. |
| Devitrification | Crystallization in glass which looks like scum on the surface of the glass. This crystallization takes place when glass is held at temperatures slightly below the liquid temperature for glass; this temperature is approximately 1400 °F. |
| Dichroic Glass | Glass that has been coated with multiple layers of oxides in a highly sophisticated process utilizing a vacuum chamber. Dichroic glass is unique in that it has completely different transmitted and reflective colors. The colors will change depending on the angle from which the glass if viewed, causing an iridescent effect. |
| Double Rolled Glass | Molten glass is passed between a pair of rotating metal rolls to form the sheet. |
| Fiber Paper | An alumina or silica fiber paper that may be used as a fusing surface in place of shelf primer or to provide surface relief. |
| Fire Polishing | Heating glass to the point where the edges round and it has a shiny appearance. |
| Firing Down | The addition of heat in the kiln during the cool down phase of firing in order to slow the
cooling rate and prevent thermoshocking of the glass.
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| Firing Schedule | Written details of the times and rates of heating and cooling during a particular fusing cycle. |
| Float Glass | The proper term for window glass. Float glass is manufactured by floating a bed of molten glass over a bed of molten tin. |
| Flux | A substance that promotes fusion or aids and induces flow. |
| Frit | A material which consists of a glass that has been crushed. Frit is available in a variety of sizes, usually varying in size from table salt to rock salt. |
| Full Fuse Temperature | The temperature (approximately 1350 to 1650 °F) at which glasses melt together to form a flat surface. (Note: This temperature may vary for different individual glasses) A hard glass (float glass) will fully fuse at 1660 °F whereas a soft glass (Bullseye – Spectrum – Uroboros) will fully fuse at 1350 °F. |
| Fusing Glass (Warm Glass) | A process that involves placing different pieces of glass and/or frits in contact with each other in an arranged design, then heating them in a kiln until they reach a temperature at which they bond together into a single piece. |
| Haike Brush | A natural fiber, fine bristle brush used in the application of shelf primer and other suspended materials. |
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Heat Soak or Soak Temperature |
To maintain a specific kiln temperature for a given time. |
| Incandescence | The emission of visible light by a hot object. |
| Inclusion | Any foreign body enclosed in glass. |
| Initial Heat Ramp Rate | The first stage of the idealized Fusing Cycle during which the unfused layers of glass are heated from room temperature to just above the strain point. It is important that this stage be performed slowly to avoid cracking the glass. |
| Iridescent | A term used to denote the effect that is caused by spraying a specialized chemical application onto a sheet of molten glass. This causes the surface of the glass to reflect a rainbow like color. |
| Kaolin | This type of clay is used as a major ingredient of shelf primer. |
| Kiln Wash (Shelf Primer) |
This material is a mixture of hydrated alumina and kaolin clay and may have some binders and/or suspension agents. Its purpose is to keep glass from sticking to a kiln shelf or other surface.
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Kiln Ventilation System
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This is a fan which draws air through a kiln to create an oxygenated environment and remove burn off gases. If one is using petroleum based paints, enamels, or lusters a vent system will remove the toxic burn off gases. It is also useful if one uses shelf paper which gives off a vapor which may stick to the glass in the kiln.
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| Kiln Forming | Using a kiln to heat and form glass. |
| Luster | A suspension of metallic oxides in an organic solvent. Upon firing they leave a thin coating of color on the glass. Usually need to be fired to 1100 °F. |
| Marvering | The controlled forming of hot glass, usually with a graphite paddle or wet wood paddle. |
| Metallic Overglaze | A metallic overglaze contains very fine particles of metal such as gold, silver, or copper which is fired onto glass. |
| Millefiore Glass | Intricately arranged bundles of colored cane (frequently a floral pattern) fused together; pieces of this fused bundle (rod) are then used in other hot processes (e.g. fusing or the forming of the traditional millefiore paperweight). |
| Mullite | Material with a high alumnia content which is commonly used for making kiln shelves. |
| Noodle | A fettuccini-like glass shape used as a decorative element in kilnforming or torchwork. |
| Opal or Opalescent Glass | Glass in which a material has been introduced during production which causes a degree Opalescent Glass of crystallization to occur that creates an opacity in the glass. This glass has little or no translucence and will not transmit light.Intricately arranged bundles of colored cane (frequently a floral pattern) fused together; pieces of this fused bundle (rod) are then used in other hot processes (e.g. fusing or the forming of the traditional millefiore paperweight). |
| Opaque | Impenetrable by light; neither transparent nor translucent. |
| Overglaze | A material composed of finely ground glass of a specific composition and applied to the pre-fused surface of the glass to prevent devitrification and to produce a very glassy surface. It is generally applied as a powder or suspended in a spray medium (such as Spray “A”). |
| Pate de verre | A process whereby crushed glass is mixed with a special media making it into a paste like material and then fused in a mold. |
| Peephole | Any hole in a kiln with a plug which can be removed for observation. |
| Polishing | Cold polishing is done by applying a polishing material to a revolving buff which may be
felt, leather, cork, etc. This may be done by starting with a coarse polishing material and working up to a very fine polishing material. There are also diamond wheels / plates which must be used with water to create a smooth surface and reduce heat build up in the glass which may crack the glass. (Also see Fire Polishing)
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| Pressing | The shaping of glass by forcing hot glass into a mold. |
| Punty | A solid stainless steel or special hard glass (borosilicate) gathering rod which is used to gather molten glass. Commonly used for glass blowing. Cld polishing is done by applying a polishing material to a revolving buff which may be felt, leather, cork, etc. This may be done by starting with a coarse polishing material and working up to a very fine polishing material. There are also diamond wheels / plates wich must be used with water to create a smooth surface and reduce heat build up in the glass which may crack the glass. (Also see Fire Polishing) |
| Pyrex | A trademark name of borosilicate glass manufactured by Corning Glass Works. Pyrex has a low coefficient of expansion, approx. 33. Very resistant to thermoshocking. |
| Pyrometer | A device to measure temperature. Composed of three parts: a thermocouple, a mechanical or digital temperature indicator, and a connecting lead wire. |
| Rebound | The phenomenon of returning heat (stored in the kiln brick, insulation, or shelf) to the kiln after the rapid cool stage. |
| Sagging | The downward sinking or bending of glass caused by its own unsupported weight while being heated; the cross section of the glass changes noticeably due to stretching. |
| Seed | An extremely small, gaseous inclusion in the glass. ‘Seedy’ glass contains many bubbles, creating visual texture. When fusing above 1550 degrees F. seed bubbles rise to the surface and may cause blisters. |
| Single Rolled Glass | Molten glass is poured onto a metal table and a single metal roll is used to flatten it into a sheet. |
| Slumping | The controlled bending or downward sinking of glass while being heated. Unlike sagging, the cross section of a slumped piece does not change noticeably. |
| Strain | The deformation (change in dimension) of a body of glass due to stress. |
| Strain Point | The lowest annealing temperature. If there is any stress in the glass at this point, it is permanent. ( See Annealing Point). |
| Streaky Glass | Two or more colors of glass mixed together to create a multi-colored sheet. |
| Stress | The force which tends to deform or strain a glass body. Stress is a force per unit area, usually expressed in pounds per square inch. Strain in glass is the result of stress. The two main types of stress existing in glass are compressive and tensile stress: tension resulting from stretching; compression resulting from squeezing. |
| Stressometer | A light source with one diffusing lens and two polarized lenses, which when properly used, will give a visual indication of the amount of stress existing in glass which has been fused or torch worked. |
| Striker | Glasses that appear pale or colorless and “strike” (change) to the target color when heated to a specified temperature. |
| Stringer | Threads of glass pulled into a spaghetti-like shape. Used as a decorative element in kilnforming or torchwork. |
| Tack Fuse | Fusing at the lowest temperature possible while still having separate pieces of glass stick together. Glasses retain all of their individual character and the edges round slightly. One may use a temperature about 100 °F lower than a full fuse temperature. |
| Thermal Shock | Shock caused by heat or cold. Thermal cracking occurs in glass as a result of too-rapid heating or cooling below the strain point temperature of the glass. |
| Thermocouple | This is the device that sticks inside a kiln and measures the temperature. It is connected to the pyrometer which displays the temperature. |
| Translucent | Transmitting light, but causing sufficient diffusion to eliminate perception of distinct images. |
| Transparent |
Capable of transmitting light so that objects or images may be seen as if there were no intervening material.
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| Vent | An opening, serving as an outlet for air, smoke, fumes and the like. Kilns are vented to allow fumes from organic material to escape during early heat-up and to allow excess heat to escape after reaching fusing temperature. (Note: A peephole is not a vent). |
| Warp | A slight bend or twist in a straight or flat form. Warping in fused glass occurs due to improper annealing or the use of incompatible glasses. |
| Working |
A temperature range in which glass may be formed. The “lower end” refers to the temperature at which glass is sufficiently viscous to hold its formed shape, while the “upper end” refers to an elevated temperature at which glass may be readily moved with tools. Slumping takes place at the lower end of the working range, while combing is possible at the middle to upper end of the working range. |