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What Is Glass Fusing? Part 1 Of How To Fuse Glass

Here at Camp Copeland Studio, we are passionate about kilnformed glass. We make it, teach it to others, promote it, and constantly try to advance the art form in new and exciting ways. I have been a glass artist for twelve years, starting with a job at Bullseye Glass Company in Portland, Oregon. I fell in love with kilnforming and glass fusing from the first sheet of glass I cut down. Now I stay busy making fused glass art at Camp Copeland Studio and teaching glass fusing classes at The Pittsburgh Glass Center. Because I am so passionate about kilnformed glass, I thought it would be fun to write a series of articles about how to fuse glass to help those who are just getting started or are ready to take their glass art to the next level. Soon I will have instructional videos and ebooks with specific projects, but all in due time. Since this is the first article in the series, I should start at the beginning, explain what glass fusing is and give you some valuable information so you can get started creating your projects.


What Is Glass Fusing?


People often use the terms kilnformed glass, kiln glass, and fused glass art interchangeably, but this is incorrect. Glass fusing is a specific technique in the artform that is kilnformed glass. Think of it like printmaking. Printmaking is an art form that includes screen printing, block printing, letterpress, etc. Each of these specific types of printmaking is a unique technique in the overall art form of making prints. Kilnformed glass is an art form that includes glass fusing, casting, vitrigraph, and kiln carving, to name a few. I will write more about other kiln glass techniques later, but this article will specifically discuss an introduction to fused glass art.


Glass fusing combines two or more layers of glass in an electric kiln, which are then fired to varying degrees to achieve different results. Examples of projects you can make through glass fusing are bowls, plates, coasters, and platters. Kiln temperature plays a huge role and is one of the things I find most interesting. To fully fuse two or more layers of glass to form one object, the pieces must fire in a kiln to approximately 1500 degrees Fahrenheit or 815 Celsius. I only work with Bullseye Glass, so everything I discuss is based on my extensive knowledge of that particular material. Firing glass to higher or lower temperatures will give you different results. Read on to learn more about the different temperature ranges in fused glass art.


How To Fuse Glass (Heat And Glass)


Kilnformed Glass Above 1500 Degrees Fahrenheit (815 Celsius)


It is possible to fire glass in a kiln at temperatures above 1500 degrees to fuse it fully. The hotter your glass fires, the less crystalline it becomes. When glass loses its crystalline structure, it melts. Bullseye Glass is a soda lime glass. Soda-lime glass is softer glass that melts at lower temperatures than the glass you may see in a hot shop. Typically, artists fire Bullseye Glass above 1500 degrees for casting purposes, which occurs around 1550 (843 C). Firing glass at this temperature allows it to melt and fill in the details of plaster molds. Below is a photograph of pre-fired student work from one of my casting classes at the Pittsburgh Glass Center. At 1550 degrees Fahrenheit, the cold glass will melt and fill the plaster leaf mold, picking up even the slightest bit of leaf texture.


plaster glass casting molds in the shape of leaves on a kiln shelf waiting to be fired.
Plaster Leaf Molds in a kiln. Crushed up glass can be cast at 1550 degrees (F) to melt and fill plaster molds

Here is another example. I create cast vessels from remnant glass using this temperature range. Sheet glass is assembled in a handmade mold and fired to the point of melting. The glass is held at that temperature until it fills the void of the mold and forms the vessel. In glass fusing, we will mostly stay below these hotter temperatures to get the results we want to achieve.


cast glass square bowl sustainably created from recycled glass
A cast glass square bowl sustainably created from recycled glass

Glass Slumping From 1160 - 1265 Degrees (627 - 685 C)


As I mentioned above, a full fuse of two or more layers of glass occurs at approximately 1500 degrees Fahrenheit. The fired result is a flat object. If you want to shape the flat piece into a plate or bowl, you will need to re-fire it on a ceramic or stainless steel mold at a lower temperature to slump it. Glass slumping occurs anywhere from 1160 - 1265 degrees, depending on the mold shape and size. During the slumping process, the flat piece of glass gets warm enough to bend but not warm enough to fuse or melt. It essentially sits down in the mold. Your glass object will slump into a gentle curved ceramic mold on the lower end of this temperature range, but a form with a flat bottom or hard edge will need more heat and most likely more time. This temperature range from 1160 - 1265 degrees is the lowest range where you will see a visible result.


Detail of a slumped glass tray with white and clear glass
The glass slumping process shapes the glass after it is fused.

Tak Fusing Glass From 1265 - 1465 (685 - 796 C)


Firing glass above the slumping temperature range but below a full fuse is a great way to explore different design options. It is also a great way to learn how different temperatures affect the glass in the kiln. The glass begins to melt slightly and will lightly stick to other glass it touches without fully fusing at approximately 1265 degrees. This process is called a tak fuse. If you have fully fused two layers of glass to create a flat piece, you can tak smaller pieces of glass on top to create dimension and fun designs. Tak fusing glass occurs from this lower temperature to just below a full fuse. The hotter the temperature you tak fuse with, the more the glass on top of your flat base piece will melt. The more the tak glass melts, the more the edges begin to soften and melt into the flat base piece.


Below is a photograph of a round green plate I made last year. It is made by fully fusing a clear circle of glass and a green glass circle in a kiln and firing to 1500 degrees. When the flat, fused, green plate is out of the kiln, I create the white pattern on top using ground-up glass powder called frit. I then re-fired the circle with the glass powder on top to a low temperature of 1265 degrees to tak it and keep the grainy texture of the glass powder. I fire the piece with the tak fused pattern a third time to slump it into the plate you see in the photo. If I did not want to keep the grainy texture of the glass powder but still wanted to tak the pattern, I would increase the temperature a bit. The same logic goes if I want to tak fuse bits of sheet glass or frit to a base plate. The higher the temperature, the stronger the tak and the more rounded the edges of the tak'd material.



Now that we have discussed what different temperatures can do in the kiln during the glass fusing, let us go over the fundamentals of fused glass art.


Basic Rules for How To Fuse Glass


Compatability in Fused Glass Art


As I mentioned earlier, I have worked at Bullseye Glass and continue to make fused glass art and teach glass fusing classes using the material. I do not let any other manufactured glass into my studio for two reasons. One, I believe Bullseye makes the best products for creating kilnformed glass. Two, I do not want to risk having any material in my studio incompatible with Bullseye. What is compatibility in glass fusing, you may be thinking? Read on to find out.


To successfully create a fused glass project, you must understand a few things about the material. One of the main things you need to understand is how temperature affects glass in the kiln, which we have already covered. The next topic that needs to addressing is compatibility. When glass heats in a kiln, its crystalline structure alters. The heated glass expands and melts. When combing two or more layers by fully fusing or tak fusing, expansion, and contraction, along with the viscosity of the materials, will make or literally break your project. You will hear a lot about COE in glass fusing. COE stands for the coefficient of expansion for the glass you are using. For example, Bullseye Glass has a COE of 90. Often you will see glass materials marketed as "Bullseye Compatible" because they are also COE 90. Although this may be true, there is also a chance it is not. COE and viscosity of all the materials you are using both play vital roles in the success of your fused glass art project. If you buy a third-party glass supply and use it with Bullseye or another manufacturer, always start with a small test to see if they are compatible.


A sure sign that you are working with incompatible materials is your piece shows cracks or breaks where the materials fuse, although it may not always be so easy to tell. The best way to check for incompatibility is by fully fusing the third-party supply with a piece of clear Bullseye glass, then checking for stress with polarizing film. You can buy polarizing film at a glass supply shop. Place one sheet of film on either side of the glass, and hold the object to a light source. Turn one of the sheets of film 90 degrees. If there is stress in the test, you will most likely see white halos or brightly colored rainbows appear in the finished piece. The more colorful the rainbows are, the more stress there is. This piece can eventually crack and break. I recommend throwing it away.


If you want to ensure that your fused glass art projects will be compatible, choose a glass manufacturer, such as Bullseye, and stick to buying what they make and what they have tested with their materials. Bullseye puts its fusible glass and supplies through vigorous testing for compatibility, so you do not have to worry about that. The same goes for other glass manufacturers as well. Once you choose a supplier, stick with them to ensure smooth sailing on all your glass fusing adventures.


Now that you know about compatibility when making fused glass art let us next talk about the physics of glass in the kiln.


Getting Physical With The 6mm Rule


Throughout this article, I mention fusing two or more layers of glass in the kiln at 1500 degrees. There is a reason I have not ever said fuse one layer of glass. You can put one layer of glass in the kiln and fire it to 1500 degrees, but you will probably not be happy with the results. Soda-lime glass heated to the point of losing its crystalline structure wants to be 6mm thick (or about a quarter inch) and will deform itself trying to get there. The center and the edges will become thinner as the corners try as hard as they can to reach 6mm. The sheets of glass we use to create our projects are typically 3mm (1/8") thick. The thickness is intentional, making the sheets easier to score with a glass cutter and snap apart. You can score thicker glass with a glass cutter, but it will be harder to break the pieces apart. With 3mm sheet glass, you can cut creative designs and assemble them in a kiln to create your fused glass art project. Make sure to use at least two layers of the same dimension to create your piece. For instance, if making a 4"x4" fused glass coaster, you need both layers to equal 4"x4" to ensure a quality result. Having two uneven layers will result in nonuniform edges to your finished piece. Below is a photograph of how to create a two-layered fused glass art plate project and the finished piece. My next article will explain this further in a step-by-step tutorial on creating a fused glass art project, including how to score and cut glass. Be sure to subscribe so you know when that comes out.



You can fuse more than two layers of glass if you desire, but the 6mm rule still plays a role. Adding a third layer of glass to your project makes your piece 9mm when it goes into the kiln. When the project becomes molten at 1500 degrees, it will spread out and flow and attempt to become 6mm thick. There are ways to prevent this flow by using kiln dams, which I will discuss in a later article and lesson. At this point, I only want to make you aware.


Fused Glass Art Examples


Now that you know the fundamentals of glass fusing, here are some examples of fused glass art I have made in my studio to inspire you.


fused glass art tray created from strips of colorful glass
Strips of Bullseye Glass are fused on edge to create the pattern of this fused glass art object

Above is a photo of a fused glass art object I made titled Woven. "Woven" is an object created by cutting different sheet glass colors into 1/4" strips that are stood up on edge to make a pattern. The glass pieces are fused at 1500 degrees Fahrenheit. Then the object is coldworked and sandblasted before being slumped in a ceramic mold to shape it.


fused glass bowl with green pink and yellow
Interlocking cut pieces of glass are fused to create the design of this fused glass bowl

"Reflections" is a fused glass bowl I created. To make the object, I cut interlocking pieces of Bullseye glass in different colors. The bowl was then fully fused and slumped. As you can probably tell, I am a big fan of bright pastel colors!


The final example I want to share is a fused glass plate created by painting a calligraphy design with ground up black glass powder. I love the painterly quality you can achieve in the glass fusing process. A fused glass project similar to this one will be the subject of a later blog post and an online tutorial. Sign up for our newsletter to receive alerts for when they are released.


I hope you enjoyed my first installment on how to fuse glass. The next article will break down a glass fusing project to create a fused glass plate. As I mentioned earlier, I also plan on releasing online tutorials and ebooks in the coming months. Have a question or comment? Let us know in the comment section or email us at hi@campcopeland.com. We would love to hear from you! If you enjoyed this article, like it and sign up for our newsletter, connect with us on Instagram, and Pinterest, or subscribe to our website. That way you will be the first to get the latest updates. Also, share it with your friends if you think they would enjoy it as well.

Thank you for reading,

Drew


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