Class notes - Ancient art in the Modern Day
Web rescources for tips and supplies:
- http://www.bullseyeglass.com/
- http://www.bullseyeglass.com/connection/education/index.php#tipsheets
- http://www.bigceramicstore.com/index.htm
- http://www.bigceramicstore.com/Information/CeramicsInfo.htm
- http://webstore.minnesotaclayusa.com/
- http://www.clay-king.com/
The Basic Fusing and Slumping Process
http://www.warmglass.comThe Heating Phase
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
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.
The Rapid Cooling Phase
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 a risky maneuver, so it's a good idea to wear gloves and be especially careful while the kiln is open.
The major 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 above 1300 degrees Fahrenheit. 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 is 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.
The Annealing Phase
Once the Rapid 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.
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.
THE COOLING TO ROOM TEMPERATURE PHASE
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.
INCLUSIONS
An "inclusion" is a material that is trapped within the layers of glass when it fuses. Inclusions can give a piece texture, character, and color that glass alone can't achieve.
The key criterion when using inclusions is that the foreign material, which may have different expansion characteristics than the glass, is thin and weak enough to allow the glass to expand and contract normally. If it is too thick or strong, or has a dramatically different COE, it will cause the glass to crack as it cools.
The most frequently used inclusion is probably copper, but many other substances may be used. This is an area where experimentation frequently pays off with unexpected results.
Metal inclusions
Gold foil was probably the first metal inclusion used in glass. Other precious metals such as silver, platinum, and palladium can also be used, as can less expensive metals such as copper, aluminum, and variegated red (an iridized copper carried by some sign painting supply houses).
The four precious metal inclusions mentioned above tend to maintain their color in the heat of the kiln, but that's not the case with some other metals. Aluminum, for example, tends to turn black, while variegated red turns blue (with thousands of tiny bubbles). Fine silver often turns yellow from the heat of the kiln.
Copper also changes color in the heat of the kiln, but the nature of the change is not always predictable. It can range from bluish-green to vivid reddish-orange, with dozens of shades in- between.
With many of metals, best color results are obtained when the metal is sandwiched between layers and only allowed enough air to partially oxidize.
When working with metal inclusions, it is critical to use only thin foil or leaf thickness in order to allow the glass to contract and expand normally. Thin wire can also be used if desired.
It's also possible to sprinkle metal powders, such as gold and silver, on the top surface of glass before firing. Mixing these powders with glass enamels can result in interesting decorative effects.
Mica and mother of pearl
These natural substances, which are available in powder or chip form, can be used to impart a sparkle to glass pieces. For best results they should be applied sparingly (too much causes bubbles) and covered with a layer of clear glass before fusing.
If you want to use mica flakes on the surface of the glass, first spray an overglaze (any devit spray will do), then sprinkle the mica over it. The overglaze acts as a binder to help keep the mica flakes in place.
Fiberglass strands
Just as fiberglass fabric can be used to impart a texture to the bottom of a fired piece, strands of fiberglass can be laminated between layers of glass to achieve interesting effects.
To use this technique, the resin binder in the fiberglass must be allowed to burn off. Inserting a few scraps of glass "spacers" between the top and bottom layers of glass can help this happen efficiently. As the glass heats, the resin will burn off and be totally gone by the time the top layer slumps onto the bottom. This technique also has the advantage of minimizing trapped air bubbles.
The fiberglass fabric, which generally maintains its color during the firing, can also be used with enamels or with other kinds of inclusions to create interesting patterns.
Air bubbles
Rather than being a defect to be avoided, air bubbles can be encouraged and exploited for their unusual shape and appearance. This is accomplished by deliberately trapping the bubbles between layers of glass .
The basic concept behind encouraging air bubbles to form is the creation of air pockets between the layers of glass. This can be done by sandblasting, drilling, or arranging cut pieces to promote the forming of bubbles. The tendency of bubbles to form at the intersection of ridges of glass can also be used creatively.
Textured glass can also be used to promote the formation of bubbles. Two pieces of ribbed textured glass, for example, can be aligned perpendicularly to trap small bubbles at the intersection of the ridges.
Bubbles can also be created by using chemicals or solutions that generate gas that is then trapped within the layers of the fused glass. Ordinary baking soda is the most common substance used in this fashion.
To use baking soda to make bubbles, just mix about a teaspoon of baking soda with a cup of distilled water, then brush or spray the mixture onto the top of the glass. Allow to dry, then place a layer of clear glass on top of the layer with the baking soda solution. Heat, fuse, and anneal normally. Bubbles will form randomly in an uncontrolled pattern.
Using clear glass allows the bubbles to show up best. Varying the strength of the baking soda solution can lead to different results.
Found items
Items as diverse as leaves, twigs, and cellophane wrappers, can also be used as inclusions by sandwiching them between layers of glass and using them to add unusual effects and textures to glass pieces. These inclusions will often carbonize from the heat of the kiln, sometimes leaving a fascinating "ghost image" (also called a "heat signature") behind.
For fusers with money to burn, try sandwiching a US dollar bill between two pieces of float glass and heating to 1500 degrees F. The result may surprise you.
Make your own:
DEVIT SPRAY (Borax/water solution)
Purpose: To prevent or minimize devitrification
How to use: Spray or brush lightly on top surface of glass prior to slump firing.
Ingredients: 1 teaspoon borax to one cup water. Distilled water works best. Regular borax cleaning powder (such as the "20 Mule Team brand" in the US) works well. Precise measurement not required.
Safety precautions: Don't drink. Wash hands after using. Long term exposure to borax can be harmful.
How to make: Just mix the borax with the water. If you put the two ingredients in a small glass jar with a lid, then cover and shake, you'll be assured of a good mixture and have a place to store the solution, too. Label the jar. Shake again prior to each use.
Kiln Wash
Purpose: To prevent glass from sticking to the kiln shelf or mold.
How to use: Spray or brush lightly on shelf or mold. Allow to dry thoroughly before firing.
Ingredients: The most basic formula uses equal parts of kaolin and alumina hydrate. Pottery suppliers generally carry these ingredients.
Safety precautions: Wash hands immediately after using. Wear gloves if you are sensitive. Powder can be harmful to lungs, so mix with caution. Wear respirator if you are sensitive.
How to make: Mix kaolin and alumina hydrate together. To this powder you should add four to six parts water until desired consistency is reached. Store in a tightly covered glass jar. Label the jar and shake well before using.
Additional tip: You can pre-mix kaolin and alumina hydrate and store the powder in an airtight glass jar. Then you just add water when you're ready to use the kiln wash.
Iridescent Glass
Caution: This process can be hazardous. Do not attempt it without appropriate safety equipment and ventilation.
Purpose: To produce an iridized surface on the glass. Glass iridized with a stannous chloride solution will not lose its iridescence at fusing temperatures.
How to use: Spray solution on glass at fusing temperature.
Ingredients: Stannous chloride crystals, muriatic acid, distilled water. Muriatic acid, also used to clean brickwork or increase the acidity of water, is available at hardware or swimming pool supply stores. Stannous chloride is more difficult to find, but is stocked by some ceramic/pottery suppliers.
Safety precautions: Both muriatic acid and stannous chloride can be hazardous. Wear complete safety equipment, including a respirator and gloves. This process creates significant fuming, so kilns should be well ventilated. Care should also be taken to minimize overspray from the solution onto floors and other surfaces.
How to make: Use equal parts muriatic acid and stannous chloride and from two to three parts water. Place the crystals in a glass jar and add the muriatic acid. Add the water and mix the solution thoroughly. The amount of water required can vary depending on the kind of sprayer used. Start with two parts and increase as necessary.
Use a glass sprayer. Plastic may melt in the heat of the kiln and the acid will corrode metal. Heat the glass until the temperature is around 1500 degrees Fahrenheit. Turn off the kiln, and spray the solution on the glass. Use a fine mist.
After applying, close the kiln and heat for a few moments. Then cool and anneal as normal.
Additional note: This formula and technique was adapted from Boyce Lundstrom's Advanced Fusing Techniques.
Frit
Frit is nothing more than small pieces of glass. Generally, they are sorted by size and used in a variety of ways, the most common of which are kiln casting, pate de verre, or as a design element on sheet glass. The easiest way to obtain your own frit is to buy it. Tested compatible frit is available from both Bullseye and Uroboros. It comes in a number of different sizes, ranging from powder to chunks as large as 1/4" (6 mm).
Using a hammer
For this approach, place the glass to be used between several pieces of newspaper and strike with a hammer until the pieces reach the desired size. This method is crude but effective. Make sure you wear eye protection. Wear a mask if significant dust is produced.
Tack fusing
Place the glass in the kiln. One arrangement that works well is to heap smaller chips of glass on a larger sheet. Heat until the glass tack fuses — around 1400 degrees F. Once the glass has fused, turn off the kiln, remove the glass (use gloves and tongs — steel fireplace tongs work well), and drop immediately into a bucket about 2/3 full of cold water. The shock will crack the glass into many small pieces. Glass broken this way will be in relatively large chunks. If you want finer pieces, you can use the hammer technique described above to break it down some more.
Melting in a crucible
This technique requires more caution than the first two. Place the glass in a crucible, a ceramic container made for withstanding the heat of the kiln. Heat it to around 1700 degrees and soak to allow the glass to melt.
Then turn off the kiln and use tongs to remove the crucible. (Wear gloves and eye protection and take special care.) Slowly pour the molten glass into a bucket of cold water. The glass will break into finer particles than in the tack fuse approach discusses above. Make sure you return the crucible to the kiln and let it cool slowly to prevent thermal shock.
Pipe-crushing
Obtain two hollow pipes, one slightly larger in diameter than the other so that one pipe fits inside the other. Close off one end of the smaller pipe, fill it with rocks or similar heavy items, then close off the other end.
Now place the larger pipe upright on a hard surface like cement and fill it part of the way with the glass you want to break. Slide the smaller, heavy pipe into the larger one, letting it drop full force onto the glass. (You will probably need a second person to help you hold the larger pipe.) Raise the smaller pipe and drop again and again until you are satisfied with the size of the particles. If you use this technique, wear eye protection and a mask or respirator to protect you from the silica dust. Also, you may want to use a magnet to extract any metal chips that may be caught in the frit.
Frit-making machines
It is possible to buy frit-making machines, called "glass crushers." Alternatively, you can rig up your own machine using a garbage disposal, heavy duty blender, or similar item. Boyce Lundstrom's Advanced Fusing Techniques describes a crusher built from an old garbage disposal and a large steel drum. Kervin and Fenton's Pate de Verre and Kiln Casting of Glass also has information about building your own frit machine.
If you make your own frit, you will probably want to separate it into sizes and store it in jars or plastic bags until needed. You can separate the glass manually or you can use wire mesh screens, which are available from ceramic supply stores.
Stringer
Although the resulting stringer will probably not be as perfectly formed as those made commercially, making your own stringer can be accomplished in two different ways.
The first involves traditional lampworking techniques. Cut a strip of glass about 1/4" wide and 10" long. Grab one end of the glass with a pair of pliers and heat the other end in your flame. Heat until the end becomes molten and starts to ball up. Then quickly grab the molten in with a second pair of pliers and stretch the glass to form the stringer.
This technique takes practice, but after a few attempts you should be able to form longer and longer threads. If you do not have access to a lampworking setup, you can use a propane torch (relatively inexpensive and commonly available at hardware stores) .
A second technique involves the use of a kiln and a crucible. Place glass frit or chunks in the crucible and heat until molten. This will likely require a temperature of approximately 2000 degrees F, which may be beyond the capability of your kiln.
Once the glass is molten, turn the kiln off and use a metal rod to reach into the crucible and remove some glass. A 1/4" steel bar works well. With the glass on the end of the bar, marver the glass on a metal or graphite surface. (Marvering simply means to roll the glass until it become smoother and forms a nicely shaped bar.)
Once marvered, return the metal rod to the crucible and pick up some more glass. Marver a second time, rotating the rod to keep the glass from falling off the end.
Now have a second person grab the molten glass with a pair of pliers and slowly walk away from where you are holding the rod. It may take some practice to learn the correct pace for walking away. Too slow and the stringer will be short and fat. Too fast and it will be too brittle. But if you walk at the right pace the glass will stretch into a long stringer.
This process can be repeated as many times as necessary to use up the glass and form the desired stringer.
Confetti (or Shards)
Confetti (also known as shards), paper-thin slices of glass generally used as design elements, are most frequently made using glassblowing techniques, but they can also be made in the kiln.
There are several ways to use your kiln to make confetti, but all are based on the concept of slumping glass until the stretched sides become very thin. To achieve the necessary thinness, it's generally necessary to suspend the glass high in the kiln and allow it to slump and stretch until it reaches the floor. When the glass cools, the thin sections of the glass can be broken into shards and used in future fusing projects.
The simplest way to make kiln-formed confetti is to use a drop ring. By supporting the ring high in the kiln and allowing the glass to slump beyond the point of forming a simple vase, the sides very thin. When the glass cools, they can be broken in shards.
One consequence of the drop ring technique is that the confetti has highly curved sides. If this is a problem, you could make a square drop ring to lessen the curvature, or you could make the confetti by stretching nichrome or other heat-resistant wires across a metal frame constructed in your kiln. By laying a sheet of glass across the wires and then slumping, the sides of the stretched glass will be very thin. When the glass cools, they can be broken into nearly flat shards that can be used in future fusing projects.
Although the colors of the confetti made in this fashion are sometimes not as intense as the colors of commercially available shards, starting with tested compatible glass virtually guarantees compatibility, so that confetti can be layered heavily if your design dictates.
Kiln cast glass:
Model Creation or Casting
Kiln casting starts with a model of the item to be transformed into glass. This item can be a "found" item, such as a pine cone or a department store manikin, or it can be an image that has been formed from wax, clay, or another substance. Some models require virtually no work before they can be used, while others require extensive forming and elaborate carving.
Choosing the material for the model
The most common substances for models are clay and wax. Both are relatively inexpensive, easily found, and can be used for both simple and elaborate items.
Any kind of clay can be used for making the model, so long as it can easily be shaped and will maintain its shape once worked. Use throwing or modelling clays rather than stoneware or porcelain clays, which tend to be less malleable. Also, water-based clays (rather than oil-based ones) are generally easier to remove from the mold.
Working with clay is straightforward. Major techniques include pinching the clay into shape with your hands, coiling (forming shapes with rope-like coils of clay), and slab working (rolling out thin slabs of clay). Special tools are also available to simplify and expedite the process of working with clay. Many ceramics books, such as Kathy Triplett's Handbuilt Ceramics, discuss these and other clayworking techniques.
One key characteristic to keep in mind when working with clay is that clay tends to shrink slightly as it dries. The degree of shrink depends on the type of clay used, but it is not unusual for clay models to shrink five to ten per cent (and as much as fifteen to twenty per cent) as they dry.
Another factor to consider is that if the clay model is extremely complicated or elaborate it may be difficult to remove from the mold. For this reason it is most often used if you are making a shallow or open faced object like a bowl or simple medallion.
Wax, the second main material used for making models, is especially useful for more elaborate shapes. This is because it can be heated and melted out of the mold; this process, called "lost wax casting," is described in more detail later in this chapter.
Like clay, wax can be sculpted with special tools. Many ordinary tools, such as files, drill bits, and knives, can also be used to model with wax. Moreover, wax can be melted and poured into a mold to form replicas of a molded shape. The process of doing this, which usually involves the use of a master mold to make wax replicas of a model, is described in the box on the next page.
There are many different kinds of wax, including parafin, beeswax, and specialty casting and carving waxes. Each type of wax differs in terms of moldability and ease of use, but just about all types may be used. Many artists recommend the use of microcrystalline wax, a petroleum by-product that is carried by some art supply stores.
In addition to wax and clay, models may be made from "found" items such as vegetables, paper mache, and other organic substances. The advantage some of these materials is that they can simply be burned out of the mold.
For other materials, such as glass, metal, or clay, the method of removal from the mold needs to be considered prior to encasing in mold material. Sometimes this is done by making a master mold from the found item and creating a model in wax using latex or a similar material. Click here to read about this process in more detail.
One reason why wax is a popular mold material is that it is easy to remove from the mold. Many other models cannot easily be removed because they contain "undercuts," projections that interfere with easy removal from the mold.
Mold Manufacturing and Preparation
The traditional mold material used for kiln casting is a 50/50 mixture of plaster and ground silica. Molds for slumping can also be made using a ratio of two parts plaster to one part silica. Generally, a mixture of 200-mesh silica and gypsum plaster, both of which are available from pottery suppliers, are used.
Mold may be made using materials other than plaster and silica. In some formulations, the basic mixture is augmented with binders or other substances. Commercial preparations, such as Mold Mix 6, are also available. If you decide to use one of these alternative formulations, follow the manufacturer's recommendations for preparation and mixing.
The Traditional Plaster/Silica Mixture
There are two main criteria for a casting mold. First, it should be strong enough to hold the melted glass. Second, it must be resistant to the heat of the kiln. Many mold materials are also resistant to glass sticking, so no kiln wash or other barrier is usually needed.
- Preparing a box for the mold. Once the model is completed, you will need to build a box to surround it to hold the mold material until it dries. The box should be about one to two inches larger than the model on all sides. It can be made from cardboard, linoleum scraps, or a similar inexpensive substance.
The box should be affixed to a firm surface such as a sheet of glass or cardboard base. Hot glue works well for this purpose. You will also need to affix the model to the bottom of the mold box before covering it with your investment material. - Mixing the investment. The plaster/silica mixture, often called an "investment," should be mixed in a clean container. Using a container that has not been thoroughly cleaned may contaminate the mixture or cause it to set up more quickly than desired.
There are many different investment formulas, but the simplest uses equal parts of silica flour (around 200 mesh), pottery plaster, and water. These ingredients can be weighed out prior to mixing, or (if such precision goes against your nature) you can mix by feel, adding as you go. With a bit of practice, you'll be able to estimate the correct amount of mixture you need to make.
It's best to finish the mold in one pouring, but if you mix too little you can let the mold set up and add more once it dries. Don't be discouraged if your first few attempts don't come out exactly right; with a little practice you'll soon become an expert.
Mix the two dry ingredients together in a separate container. You should wear a respirator or approved mask when working with silica to keep from inhaling the potentially harmful silica dust.
Put the water in the mixing container. Use clean, room temperature tap water. Some people believe the results are better if you allow the water to sit for a few hours before using.
Once the investment is mixed, begin adding it to the water. (Always add the dry ingredients to the water, not the water to the dry plaster and silica.) Add the dry mixture a little at a time, stirring by hand between additions. Some people use a sifter to evenly spread the dry mixture on the surface of the water; others simply sprinkle across the surface.
Continue adding the investment until it "peaks." Peaking occurs when the mixture sinks slowly and dry investment islands appear in the container. (If you measured out your ingredients properly, the mixture should "peak" just as you use the last of the dry ingredients.)
Once the investment peaks, give the mixture one final stir to mix in all the dry ingredients. Then stop mixing and allow the mixture to sit undisturbed for about five minutes. This process, called "slaking," helps ensure that the investment particles become saturated with water. - Pouring the mold. After allowing the investment to slake for five minutes, slowly pour it into the box that surrounds your model. To minimize bubbles and distortion of the model, pour the investment along the edges of the box, rather than directly on the model. Do not pour leftover mixture down the sink, as it will harden and ruin your plumbing.
- Removing the model. After the mold has air-dried (this may take several days), you should be able to remove the model. Clay models can generally be worked loose with a stick or removed by hand. They can be difficult to remove from deep inside crevices, so are best used for more open molds.
Wooden or paper models can be burned out, although it may be necessary to remove ashes and any other possible contaminants from the mold after firing.
Wax models will need to be melted out. Once this has been done, the remaining cavity can be filled with frit or cullet and then fired. This process, which is adapted from the jewelry-making field, is called "lost wax casting." - Drying the mold. Air drying the mold is a necessary step, but it is generally not sufficient to totally dry (or "cure") the mold. You also need to heat the mold in a kiln to remove any lingering moisture or contaminants from your model.
The key to kiln drying is to take it slow. The slower you heat, the stronger the mold will be. Soak the mold at 225 degrees F and again at 350 degrees F. This is to drive off any remaining water and help stabilize the mold. Continue to heat until the temperature reaches about 1200 F. In general, the slower you heat the better. Allow the mold to cool slowly back to room temperature, then it's ready to use.
Cracks sometimes appear in the mold after curing; if these are small they won't present a problem, but large cracks may require repair or even starting over again. These cracks are more likely to occur if you fired too quickly or if the mold is large.
One interesting aspect of casting with molds is that you are far more likely to crack the molds than you are the glass. This is contrary to slumping, in which the glass is more at risk. In casting, the mold tends to protect the glass, but its tendency toward cracking requires that heating and cooling take place more slowly than if you were simply fusing or slumping glass.
Filling the Mold with Glass
Once you have removed the the model from your mold and cured the mold so that it is ready for firing, you are ready for the next step in the kiln casting process, filling the mold with glass.
The first major consideration is to determine which size of glass to use. As was noted before, size can range from very small particles to large chunks. In general, the larger the chunks of glass the clearer will be the final casting.
In addition to size, you should also determine the type and color of glass to use. The type of glass is significant for two main reasons: first, you must use compatible glass; second, you should use a glass that is not prone to devitrification. The longer soak times typically used in kiln casting make controlling devitrification even more important than in fusing and slumping.
Color of glass is primarily an artistic choice. However, if you desire a clear casting, there are a few basic principles to keep in mind. Use larger chunks of glass, rather than smaller. Help prevent devitrification by using glass that has been specifically formulated for casting. Note also that many "clear" glasses actually have a greenish tint; if this is not desirable, seek out a special "casting clear" (sometimes called "Colorless Clear") that has been formulated without the green tint.
Once you've determined the color, type, and size of the glass to be used, you'll need to decide the method you will use to fill the mold with glass. There are three major alternatives, each of which results in a different final appearance.
Filling the Mold with Frit or Chunks
The easiest way to kiln cast is to simply fill the mold with frit. Unfortunately, this is not as straightforward as it sounds. That's because frit compacts as it melts and settles down into the mold. How much it compacts depends on the size of the frit (or chunks) used. The smaller the individual pieces of glass, the less it will compact. A good general guideline is that it will compact to about two thirds of the original size.
There are several ways to deal with the settling of the glass. The most straightforward is simply to top off the mold with fresh frit as needed. This method is commonly used in connection with a reservoir at the top of the mold. This approach requires that you fire the glass, soak a while to allow for settling, then open the kiln to add more glass to the top of the mold. Obviously this has the potential to be an unsafe operation, so care must be taken to prevent burning or electrocuting yourself. Always turn the kiln off before reaching inside and always protect yourself with safety equipment and common sense.
If necessary, topping off a mold can be done several times. Frit is small enough that thermal shock will not be a problem. Moreover, at casting temperatures (about 1500 to 1550 F) frit will melt quickly, so just let it soak for 15 to 30 minutes, then check to see if more is required. You can keep adding fresh frit until the mold is full.
Another way of dealing with the settling of the glass is to take advantage of a reservoir to hold extra glass at the top of the mold. If your casting allows it, you can use large chunks of glass that sit on top of the mold and drip into the mold as the temperature inside the kiln increases. This is easier (and can be done with frit of any size) if you build a reservoir into your mold to hold glass until it melts and falls down into the main part of the mold.
Finally, if you plan to cast by filling the mold with frit or chunks of the same color (or mixed indiscriminately) you can estimate how much glass will be required by using this simple technique.
First, get a jar and fill it with water. On the outside of the jar, mark the top level of the the water inside the jar.
Second, pour water from the jar into the mold until the mold fills. The mold may not be watertight, so you'll need to pour quickly before the water wicks away into the mold.
Third, fill the jar (which should still have some water remaining) with the glass frit you want to use until the water level in the jar reaches the original mark you made in step one. The amount of frit required is the amount needed to fill the mold cavity.
Finally, allow the mold and glass to dry thoroughly before firing.
(This method can also be used to estimate the amount of glass needed for crucible casting, but you will need to add some additional glass to account for the glass that will stick to the crucible.)
Regardless of the method you use to fill the mold with frit, you'll follow the same steps once the mold is finally full. Soak a few moments to allow the casting to fully form, then begin the annealing and cooling process.
Using a Crucible Drip
For situations where you want an exceptionally detailed, clear kiln casting with as little distortion or demarcation lines as possible, you might want to try the crucible drip method of kiln casting. This approach, which requires a kiln with relatively high clearance, involves placing the glass to be melted into a crucible with a hole in the bottom.
Most crucibles are made out of clay or a similar material. You can buy crucibles, or you can use a clay flowerpot (which already has a hole in the bottom) as a crucible. Note that clay flowerpots will only last a few firings. In most cases, crucibles are not coated with kiln wash. This allows some glass to stick to the crucible, but it avoids potential contamination from the kiln wash.
To help ensure a clear casting, the crucible should be filled with larger, rather than smaller, chunks of glass. Since not all of the glass will flow out of the crucible, you should use a bit more glass than it will take to just fill the mold. When loaded, the crucible should be suspended above the mold, so that as the glass melts it drips into the mold.
Pate De Verre
Pate de verre involves making a paste of glass that is applied to the surface of the mold, then fired. The big advantage to pate de verre is that it allows for precise placement of particular glass colors in the mold. Other ways of filling the mold often result in some shifting of glass from where it has been placed prior to firing, but the pate de verre process helps to control this shifting.
Pate de verre dates back to the ancient Egyptians, but it really came into its own about a century ago when it was revived by a group of French artists who gave this warm glass technique its current name.
In traditional French pate de verre, the artist mixed crushed glass with enamels or paint to form a paste that was carefully placed in a mold and then fired. Many of the pieces that were made using this technique were relatively small, elaborately decorated, and required more than one firing before they were complete.
The modern equivalent builds on this traditional foundation. Generally the pate de verre process involves creating a paste from frit (small particles of glass). Frit of any size may be used, but most good glass pastes require smaller sizes (even powders) to be used. For this reason (and because the smaller the pieces of frit the more opaque the casting), pate de verre castings tend to be translucent (or even opaque).
Once the mold is thoroughly dry and the frit has been secured, the next step is to make the glass paste. In some cases, where the mold has gently sloping sides, the glass can simply be mixed with distilled water to form the paste. Most molds, however, will require that glue be mixed with the glass to form the paste. Special glues can be purchased, but white glue or gelatin diluted with distilled water will generally work well. It's a good idea to wear a mask or respirator while mixing the paste to prevent inhaling small glass particles.
Use a brush or thin palette knife to apply the glass paste to the sides of the mold. Start with a relatively thin coating (about 1/16"). Some artists fire this initial coating to tack fuse (about 1400 F), others let it air dry or use a hair dryer to speed up the process. After the first layer dries, a second layer of paste should be added to bring the total thickness to around 1/8" (3 mm). Gently pack the layer down as much as possible.
If your mold is hollow or slopes significantly, you will need to pack the inner surface of the mold to prevent glass movement during firing. Sometimes a second mold is created to fit inside the first mold. Alternatively, you may pack the mold with fiber paper to prevent the glass from slipping out of place.
Firing the Kiln Casting
The process of firing the casting is basically the same, regardless of whether you fill the mold using loose frit, pate de verre or a crucible drip. In general, you follow the same basic procedures and steps as heating for fusing and slumping, except that the thickness of the glass and mold generally dictate slower heating and cooling times. It's also very important to recognize that the mold can crack during firing, so extra care should be taken to ensure that everything stays intact.
A good rule of thumb for how fast to heat is to increase the temperature around 300 degrees per hour for each inch of the casting. It never hurts to go slower than this, and you can sometimes go faster, but faster heating schedules run the risk of thermal shocking the glass or mold.
It's generally a good practice to soak for half an hour or so at around 1200 degrees F, then continue more rapidly until full fuse temperature is reached. For ordinary soda-lime glass (such as Bullseye and Uroboros) this is around 1500 degrees F. Some artists prefer to soak at a temperature as much as 100-200 degrees F above full fuse; there are literally hundreds of different ways to kiln cast - the right one is the one that works for you.
Once the casting temperature is reached, the casting needs to soak for a while. How long depends on the thickness and shape of the mold and the size and complexity of the item being cast. You'll need to experiment to find the right length of time (start with half an hour to an hour for a simple casting). Take comfort in knowing that you can re-fire if necessary. In addition, you can always soak a bit longer than you think you need to just to make sure.
If you need to add more frit due to settling, just turn off the kiln, add the frit, reheat to soaking temperature, then soak some more. This process can be repeated several times if necessary.
Annealing and Cooling (Kiln Casting)
Once the casting has been formed, the mold and glass should be crash cooled (just as in fusing and slumping) to around 1100 degrees F. From this point on, however, annealing and cooling take much longer than do the same processes during fusing and slumping.
This is for three main reasons:
- Kiln castings tend to be thicker than fused and slumped items.
- Kiln castings tend to be mostly surrounded by the mold, rather than mostly open to the air (as in a fused or slumped item). This means that heat can't escape as easily.
- Kiln castings are surrounded by a thick, often fragile mold. As a result, the temperature variations within the casting are greater than in fusing and slumping. Extra time is needed to allow for temperature to stabilize.
How long do you anneal and cool? It is not unusual for annealing of large items to take several days, or even weeks. Even small cast items will generally require several hours to anneal. When figuring the length of time, you must consider both the thickness of the casting and the thickness of the mold. You should calculate conservatively; there's nothing worse than losing a casting during the annealing and cooling phase.
Once the glass has been annealed, cooling to room temperature follows. Here caution is also in order. The thickness of the glass, combined with the thickness of the mold (which acts to shield the glass from temperature changes), means that cooling should also take (no surprise here) longer than with simple fused or slumped items. Even when the item has apparently cooled to room temperature, it should be left to sit another day or so. This is because the inside of the mold can still be hot even though the mold itself feels cool to the touch.
Removing and Cleaning The Casting
Once you are certain that the mold and glass inside are cool, carefully break the mold away from the glass. Proceed slowly, taking care not to break the glass. This can be easier said than done, but if you are careful you will eventually be able to remove the entire mold and free the glass.
The surface of the casting will probably look ugly, not at all like the shimmering piece of glass you may have imagined. Let the casting sit for a few hours just to make certain it has completely cooled, then you can begin the cleaning process.
Wash the casting in water. You may want to use a stainless steel or stiff nylon brush to clean off the mold material residue. Sometimes you will need to use a wet/dry sandpaper or Dremel-type tool to help with the cleaning. It's important to keep the casting wet during this cleaning and to wear a respirator to keep from breathing in any silica dust particles.
Once the casting is clean, you may want to polish it (see Polishing equipment section) or shine it up using an oil or acrylic enamel. Clear spray oil, such as Varathane Natural Oil Finish #66 Clear, will permanently give the casting a shiny finish. Similarly, you can use a clear acrylic or enamel spray to give the glass a permanent wet look. For all of these products, make certain the casting is completely clean and clear of fingerprints and other blemishes prior to using.
If you have a soda-lime glass such as Bullseye or Uroboros, you can use a commercial product such as Back Magic to impart sheen to the casting. These products are painted or brushed on the casting, which is heated in a kiln to around 1100 degrees F. If you use this approach, make sure you heat, anneal, and cool the item slowly so as to prevent cracking. Unfortunately, these products are lead-based, so they can not be used on surfaces that will come into contact with food.
Finally, although kiln casting can be more rewarding and interesting that fusing and slumping, it is also more complicated. There are many more tricks and techniques, and most artists have developed their own approaches to kiln casting. This variety can be a source of frustration (there is, after all, no single "right way"), but it is also part of what gives kiln casting its charm and excitement.
Troubleshooting Fused Glass Problems
Devitrification
Devitrification, a scummy white crystallization on the glass surface, occurs when glass lingers too long in the temperature range just before it becomes molten. This is about 1400 degrees Fahrenheit for most glasses.
The cure starts with minimizing the time spent in that temperature range. If you're fusing, flash vent to quickly "crash cool" from the fusing temperature to just above the annealing zone. If you're slumping, work at lower temperatures if at all possible.
The textbook solution to this is to apply a devitrification spray ("devit" spray) prior to firing. This process involves spraying or brushing a thin, even layer of spray on the top surface of the item. Be sure to allow it to dry thoroughly before firing.
Although devit sprays are widely used, you should be aware that some people believe that it is better not to use them. This position rests on the belief that devit sprays create an "artificial layer" on the surface of the glass that will deteriorate in time. Instead, argue proponents of this view, you should control devitrification by improving your kiln procedures and using glass that is less likely to devitrify.
In addition, some glass artists believe that devitrification imparts a unique beauty to the surface of the glass. They actually encourage its formation and use it as an element of their artwork. Even if you normally work to prevent devitrification, experimenting with "controlled devitrification" can lead to some interesting pieces.
But if all else fails and you do end up with unwanted devitrification, you can attempt to remove it. You can lightly sandblast the piece or use an acid bath to remove it (take proper safety precautions, please). You can scrub the piece with pumice or and a rotary brush (or even an old toothbrush). Sometimes it even helps to apply a devit spray and re-fuse. Many of these "treatments" will change the outward appearance of the piece, so proceed with caution.
Removing Kiln Wash
There are several reasons for kiln wash to stick to the underside of a piece. Firing with wet kiln shelves or with shelves that have picked up moisture from the air can result in the glass picking up kiln wash. Using shelves where the kiln wash has started to deteriorate can also lead to problems with kiln wash.
These kinds of problems are easily eliminated by a fresh application of kiln wash and by pre-firing the shelf to around 400-500 degrees F to make sure all moisture has been driven off.
Kiln wash can also stick if you overfire or soak for too long. It frequently happens at high temperatures, such as during combing or long soaking periods. Some brands of kiln wash are more susceptible to sticking than others, so trying an alternative brand might help.
Removing kiln wash can be an extremely difficult process. If you have the equipment, then sandblasting is fast and effective. However, it does change the appearance of the piece. This may not be a problem for the backside of a platter or similar object, but it will dramatically change the appearance of a more three-dimensional piece.
The safest way to attempt to remove baked on kiln wash is by soaking the piece for several days in a bath of white vinegar, then scraping with a paint scraper or scrubbing with a nylon scrubbing pad or piece of steel wool. Very fine steel wool - 000 or 0000 - is available from most hardware stores and is far preferable to household steel wool.
Unfortunately, gentle methods such as white vinegar don't always work. Lysol brand toilet cleaner works on some pieces. More stubborn cases call for a commercial stain remover such as CLR or Limeaway. Preparations specifically made for glass (Wash-away is one brand) are also available. Many of these solutions can be hazardous and require the use of gloves and/or respirators, so be sure to follow the directions on the product label.
A diluted solution of muriatic acid (diluted hydrochloric acid) works when virtually everything else fails, but this is an extremely hazardous operation. It requires gloves, a respirator, plenty of ventilation, and extreme care. Do not attempt to use strong solvents such as muriatic acid unless you have the proper equipment and experience.
As with many problems, prevention is preferable to removal. Frequent reapplication of kiln wash will help, paying special attention to removing the old kiln wash thoroughly prior to applying the new. Firing the shelf to dry the kiln wash and remove all moisture also helps. And selecting the right brand can be a factor: some brands do not work as well at high temperatures.
When firing, it also helps to fire a bit slower and to a slightly lower fusing temperature than normal. Similarly, reducing soak time will often help.
Finally, you can sidestep kiln wash problems entirely by firing on fiber paper.
Rounding the Edges
One of the most common complaints voiced by beginners concerns the edges of the fused or slumped glass. This falls into two categories: edges that suffer following the fusing process and edges that remain unacceptable following slumping. Each category demands its own kind of treatment.
If the edges of your fused piece are not as smooth and well rounded as you want, then you may not have fired the the piece enough. Just heat it again, this time firing a bit higher (or soaking for a bit longer). If you can, keep an eye on the piece and let it soak until it rounds the way you'd like.
If the fused piece has needle-like projections along the edge, then you've overfired. Grind the projections away and then re-fire, but this time at a lower temperature (or soaking for a bit less). (Around 1400 degrees F will often work for work that's been fused but not slumped.)
For glass that you're unfamiliar with, it's also a good idea to peek through a peephole while the piece is firing and stop soaking when the edges look the way you'd like.
Another way to improve the edges following fusing is to round them the way you'd like using a grinder or similar piece of equipment, then fire them using a more controlled slump firing. This technique, which requires a kiln controller or greater-than-usual attention to the firing, involves firing with three soak periods rather than the one typically used in slumping.
(Please note that the temperatures and soak times indicated below would probably need to be adjusted for your kiln and for the size of the work being slumped. This technique has been adapted from one discussed in Henry Halem's Glass Notes, and is attributed to Klaus Moje.)
The first soak, which occurs at around 1100 degrees Fahrenheit, is designed to give the glass a few moments to equalize temperature. This will lead to more even slumping and more controllable glass movement. This soak, which should take place at the point where the glass is just starting to melt, should last for about half an hour (longer for thicker pieces).
The second soak, which occurs at around 1200 degrees Fahrenheit, is where slumping begins. This step requires you to keep a close eye on the glass and be ready to override your controller if necessary. Watch the glass through a peephole (wear safety glasses) until it begins to slump, then continue watching until it is over half way slumped into the mold.
Once the object has slumped about two thirds of the way, quickly raise the temperature to around 1400 degrees Fahrenheit. This temperature increase should occur as quickly as your kiln can make it happen. The glass will slump fully and (if you've timed it right) almost instantly.
Once that occurs, allow the glass to soak for a few moments more. Ten minutes is usually sufficient. This third soak will not only complete the slumping, but also yield well rounded, smooth edges. As soon as this is achieved, crash cool the kiln and anneal and cool as normal.
Sometimes, however, despite our best efforts, the slumped pieces still have rough or distorted edges. The best way to improve this is to use coldworking equipment to grind and polish the edges. This can be tedious work, but it makes the difference between a second-rate work of art and one that is worthy of display.
Cracked Glass
There are few things more frustrating to the glass artist than opening the kiln to find that the piece you've fired has cracked. The only bright spot is in figuring out why the piece cracked so it doesn't happen again.
First, let's assume you didn't drop the piece. Let's also assume you didn't remove it while it was still warm and then "accidentally" put it on something cold or dip it in water.
Now that we've set those obvious (but they happen) reasons aside, let's try to figure out the less obvious reasons for glass to crack. You can learn a lot by examining the nature of the cracks. Do they go all the way across the glass? How are they shaped? Is the piece broken entirely in half? Are the pieces pie-shaped?
Analyze the kind of cracks you're experiencing to learn what caused them, then change your kiln processes to prevent their re-occurrence.
- Curved cracks across the middle of the piece. Improper annealing causes this kind of crack. It will show up as gentle curves (sometimes as straight lines) that break the pieces into two or three pieces. Often the crack will curve sharply as it nears the edge of the glass. This kind of crack is the piece's way of relieving stress. The solution is to raise the annealing temperature slightly and to spend longer in the Annealing phase.
- Cracks where two different glasses come together. Glass incompatibility causes these cracks. The cracks can be very small or they can cause the pieces to break apart,but they will always show up on the edges of the incompatible glass. Conduct your own compatibility tests or use "tested compatible" glass to keep this from reoccurring.
- Small, interconnected cracks (like a spider web). These cracks generally extend from a single spot on the underside of the glass. They aren't usually severe enough to cause the item to split into pieces. Sometimes shelf primer will also be stuck to the underside of the glass. Most likely, this kind of crack is caused by glass sticking to the kiln shelf. A close examination of the shelf may even reveal small pieces of glass that are stuck to the shelf. The obvious solution is to scrap the shelf clean and apply fresh kiln wash.
- Pie-shaped pieces, with smooth edges. These cracks, which usually occur with such force that they split the piece into five to ten pieces, are caused by thermal shock. The edges of the pieces are often rounded because these tend to happen early in the firing cycle (around 300 to 400 degrees Fahrenheit) and the edges round during later phases of the firing. The cure for thermal shock is to fire more slowly in the early part of the Heating phase. You might also try cutting very large pieces into smaller ones before firing. Finally, it's a good idea to peek in the kiln around 400 degrees or so just to make sure the piece is still OK.
- Cracks that occur long after firing. Sometimes a glass piece will just be sitting on a table when you hear a sharp ping. It might be quite loud, and perhaps there's a second (or even a third) ping. When you check it out, you discover that the glass piece you thought was beautifully finished has cracked. (This cracking can even be severe enough to shatter the piece, leaving the artwork in pieces on the table.)
The reason for this disaster is undoubtedly stress that has built up in the glass piece. Stress can come from many factors: improper annealing, thermal shock, incompatible glass, and even "normal" wear and tear. Improper annealing is the most likely culprit. Go back to your log - you do keep a log of your firings, don't you? - and check out the firing schedule to see if you notice anything out of the ordinary.
If you used the same schedule you've always had success with, then perhaps this piece of glass was a bit thicker or larger than normal. Perhaps it was a different glass than you usually use. Perhaps your "normal" annealing schedule needs to be adjusted to anneal just a bit longer and slower. Remember that it's impossible to anneal for too long.
