• Fusible vs “Non-Fusible” glass    All glass can be melted. However, not all glass was designed to be melted. Many of the gorgeous pieces of art glass lose their coloration when heated. Glass labeled as “fusible” has been designed to be heated. The colors will hold through a firing and the glass has been tested to melt at a specific rate and return to solid at a specific rate. 

• Coeffient of Expansion or “COE” In technical terms:  “During heat transfer, the energy that is stored in the intermolecular bonds between atoms changes. When the stored energy increases, so does the length of the molecular bond. As a result, Solids expand in response to heating and contract on cooling; this response to temperature change is expressed as its coefficient of thermal expansion”.   (this quote and the mathematical formula used to determine the coefficient of expansion can be found at http://en.wikipedia.org/wiki/Coefficient_of_expansion)  It is important to know what the “COE” of the glass that you are going to fuse because glass with different coe’s will expand and shrink at different rates when heated and when heated together, this will lead to stress cracks and fissures.  The most common COE’s in the glass hobby market are 90 and 96.  The lower the COE of glass the harder the glass is and the higher the temperature it will require to cause the glass to melt or begin to melt. Typical window glass has a COE of 82. This means that window glass will melt at a higher temperature then most of the “fusible” glass in the glass hobby market.  If you attempt to fuse window glass with a piece of COE 90 or 96 glass, your project will crack during the cooling process.  If you attempt to fuse a piece of stained glass with a piece of glass marked with a coe of 90 or 96, your project may or may not crack.  The stain glass might be coe 90 or 96. It has not been tested fusible to any specific COE.  Furthermore, the stain glass if heated to fusing temperatures may lose its coloration and surface treatment.  Many glasses created for stained glass projects were not designed to maintain their brilliance when fired.  For these many reasons, it is best to fuse with glass that has been tested by the manufacturer as “fusible” and fuse glass with exactly the same COE’s. Many artists even suggest to assure compatibility when fusing to stick with the same glass manufacturer throughout the project.

• Thick and Thin The major manufacturers of glass produce a variety of thicknesses of glass. The most widely produced thicknesses of sheet glass are standard thickness, which is approximately 3 mm thick, and thin which is approximately 1.5-1.8  mm thick.  Typically thin glass is best to use when working with many layers of glass or when the end product is most desirable light weight.  It is helpful to use thick glass when creating serving pieces or when the end product is desirable heavier and sturdier.

• Volume Control An interesting property of glass is that glass has a tendency to become 6 mm when fused.  This means that if one sheet of standard glass is fused it will shrink slightly at the edges to draw up in  height in the middle of the fused piece to reach 6 mm thickness. The glass will not always make it to 6 mm thick, but it will draw in on the edges and especially in the corners to reach this height.  Conversely, if 3 pieces of standard thickness glass are fused together, the glass will spread at the edges to shrink in height down to 6 mm thick.  This is an important thing to remember when stacking glass for fused projects.  You can see the tendency of glass to reach a 6 mm thickness if you cut 7 one inch squares of glass and fuse one on its own, two stacked together and three stacked together (leave one single piece out of the kiln) The one layer of glass shrinks slightly when fused because the glass is drawing in on the edges to reach 6 mm in height in the middle. The double layer of standard thickness of glass stays nearly the same as the unfired square in width because it is already 6 mm tall. The triple layer of standard thickness glass goes into the kiln at 9 mm tall. During the fusing process, the glass spreads slightly to lower its height to 6 mm tall. This is important to remember when designing fusing projects. If you stack 3 layers of glass on one end of the project it will bulge out in that area. If you leave one layer of glass one another end of the project, it will shrink in this area.

• Iridized Glass and Textures Another variable in glass types is based on the surface treatment the manufacture has given to the glass. Sheet glass can be given interesting textures such as ripples, fibroid lines and patterns. However, most textures will disappear when fused. Glass can also be “iridized” by the manufacturer. Iridized glass is coated with a fine metallic coating before the glass is cooled. This coating gives the glass a mother of pearl effect. Iridized glass fuses a little differently then glass that has not been iridized. It is important not to fuse two pieces of iridized glass together with iridized surfaces facing each other. The iridizations creates a slight barrier from the glass and the iridized coatings will retard fusing slightly.  Additionally, iridized coatings will diminish slightly when fused at extreme temperatures. It might be prudent to fuse projects with iridized coatings at a slightly lower temperature then the same project with non iridized glass.  This is especially true when one fuses a project iridized side up in order to maintain the iridized effect on the surface of the project.   

• Dichroic Glass A wildly exotic and enchanting surface treatment for glass is known as “dichroic coating”.  Dichroic coating is done by secondary companies that buy glass from the glass manufacturer. These “glass coaters” use a space age vacuum technology to apply extremely thin and brilliant metallic surfaces to glass.  The term “dichroic” is used because dichroic glass appears to be one color at one angle and another color at a different angle. The variety of colors available in dichroic glass is vast. Most dichroic coatings are applied to either clear or black, smooth or textured glass.  Clear dichroic  glass allows the color from the glass fused beneath it to be seen. Black dichroic glass does not.  Many dichroic coating companies also offer exciting patterns on both their black and clear dichroic glass. Because of the expensive ingredients and the advanced technology used to apply dichroic coatings to glass, dichroic glass is highly valued.  Retail prices for dichroic glass can range any where from $2 to $20 per square inch of glass.  Dichroic glass has a deep glossy finish when fused with a clear cap and a rich satin finish when fused without a clear piece of glass on top of it.  The colors change greatly when fused.  Dichroic glass can be used as a small accent on non-dichroic glass or it can be used in layers and cover the entire surface of a cabochon.  The fascinating possibilities are virtually endless.  Scoring and Cutting To begin cutting glass, it is beneficial to practice cutting on economical glass such as window pane glass.  There are a variety of glass cutting tools on the market.  The most common tools are pencil grip and pistol grip carbide steel glass cutters. The following instructions for cutting glass were provided by Diamond Tech International, a major glass tool provider:  "The Perfect Score To hold the pistol grip glass cutter, place your thumb on the top of the handle just above the cutting wheel and curl your remaining fingers underneath the handle. Line up the cutting wheel with the intended score line at a slight downward angle, apply moderate pressure to the cutting wheel and push away. A good score begins at one edge of the glass and runs to the other edge. It should look like a piece of hair on the glass. If it looks like salt, you've pressed too hard.

To hold the pistol grip glass cutter, place your thumb on the top of the handle just above th cutting wheel and curl your remaining fingers underneath the handle.Line up the cutting wheel with the intended score line at a slight downward angle, apply moderate pressure to the cutting wheel and push away. A good score begins at one edge of the glass and runs to the other edge. It should look like a piece of hair on the glass. If it looks like salt, you've pressed too hard.

• Breaking Glass: For long strips or curved scores, use Running Pliers. Line up the center of the curved jaw with the score line and squeeze gently. The score should break apart. For narrow strips or small scores use a Breaker/Grozer Pliers. Hold the glass in one fist, with fingers curled under and into palm. Hold the pliers in the other hand, and with the flat jaw on top, place the nose of the pliers parallel to the score line (approx. 1/8" distance from score). Using equal force, pull both hands up and away (like a drawbridge). Be sure not to pinch the glass with pliers.

• Fusing Techniques and Firing Schedules The information in the table below was taken from a very helpful book: Introduction to Glass Fusing by Petra Kaiser 2003 Wardell Publications. You may notice that the temperature ranges are pretty wide. This is because the Author doesn’t know what type of glass you are using. COE 96 glass fuses nicely at 1480.

Each glass manufacturer produces a firing and annealing guidline for the glass they produce. You will find one such guidline produced by Uroboros Glass enclosed. The important thing to remember when firing glass is that the slower you go, the better off you are. Glass is going through a marvelous transformation when we fire it from a solid to a liquid and back to a solid. Great care must be made to help it through the transition periods without cracking and with as few bubbles as possible.

·         Loading a kiln with glass  It is unadvisable to stack kiln shelves with glass projects in a kiln. This is why glass kilns get wider instead of taller when they get larger. Ideally, glass projects should be placed in the center of a kiln shelf with the shelf placement as close as possible to the pyrometer in the kiln.

·         Firing glass with cones When using cones, one should fire as slowly as one can tolerate. The larger the glass project in the kiln, the slower it should be fired. It is possible to successfully fire a 4 x 4 two standard glass layers tile in a ceramic kiln using a cone 016.  If the kiln  large (18” or larger) it should be fired from low to med to high in hour intervals. After the cone triggered the kiln to turn off, one can quickly peeked into the kiln. After it is clear that the project has been completely fused, the kiln lid should not opened again until the kiln was at room temperature. If the project is approaching over-firing (the edges are concave or convex) it may be necessary to “flash” the kiln. This is done by carefully opening the kiln lid and allowing 300 – 400 degrees to leave the kiln. One can view the glass and watch for the “red hot” color to fade. After the kiln is flashed, the lid should not be lifted again until the kiln has reached room temperature. If the project is severely under-fired when the cone triggers the kiln to turn off, one can trick the kiln to fire without the cone rest for ten  or 15 minutes longer to achieve full fuse. CAUTION!! The kiln will run continuously when tricked! One should stay near the kiln and view at 5 minute intervals so that one remembers to turn the kiln off.

The following are approximate temperatures for firing at medium speed of 270 degrees per hour for self supporting cones. Your temperature may vary by the condition of the kiln.

cone 022- 1094   cone 021-1143   cone 020-1180  cone 019-1283 cone 06-1855     

cone 05-1911     cone 04-1971   cone 03-2019

The following are approximate temperatures for firing at medium speed of 270 degrees per hour for small bars and cones. Your temperature may vary by the condition of the kiln.

cone 019-1279    cone 018-1350   cone 017-1402  cone 016-1461 cone 06-1852     

cone 05-1915     cone 04-1958    cone 03-2014

Cone temperature ranges as taken from Ceramic Arts Supply of Ontario website (http://www.ceramicartspace.com/OrtonCones.php)

    As you can see, a cone 019 would work nicely to slump a glass project in a cone operated     kiln. Again, the kiln should be fired as slowly as possible.

·         Annealing glass It is important that the entire body of glass returns to a solid state at the same time. If the extremities of glass turn to a solid before the middle of the glass cracking will occur. 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. This will typically occur between 975 and 900 degrees depending on the glass. To anneal a glass project, one must hold the glass project at or near the annealing temperature long enough for the annealing to occur. The length of time required depends on the thickness and size of the glass being annealed. A good rule of thumb is to anneal the glass for an hour or more unless the project is thicker then 3 standard pieces of glass or wider then 10” in diameter. One should add annealing time to larger projects. If one is attempting to anneal glass in a non-programable kiln. One can add kiln shelves and posts below the glass project shelf to slow the natural heating and cooling of the kiln. Many large ceramic kilns cool slow enough to kiln many glass projects by virtue of their size. It is advisable to use a pyrometer with any kiln that is not digital.