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  • Writer's pictureDrew

What Is Fused Glass Art? Firing Schedules For COE 90 + COE 96


yellow fused glass art plate with colorful stringer design
Fused glass art is a great way to start working in kiln glass

I have been a kilnformed glass artist for over twelve years. I have worked at Bullseye Glass Company and founded Camp Copeland Studio in 2015. I also teach at the Pittsburgh Glass Center and am the Kilnforming Consultant for Wissmach Glass Company. I have plenty of experience as a glass artist and educator using COE 90 and COE 96 glass types. Many of the questions I get from glass artists are about firing schedules and temperatures when creating fused glass projects. This article will discuss my recommended firing schedules for fusing and slumping a 6mm glass plate using COE 90 and COE 96 glass. I will also explain the segments of the firing schedule and why each is so important to the success of your project. These schedules are a guideline based on firings in my Paragon GL24 kilns and may need adjusting for other kiln styles. With that said, let's get started.


Glass Fusing And Slumping Are The Foundation Of Kiln Glass Art


Understanding the principles of glass fusing and slumping is the foundation of kiln glass. Both processes come to mind when answering the question, what is kilnformed glass art? Creating fused and slumped glass plates, bowls, and trays is always a fun project to undertake in your studio. Learning to fire the material in your kiln is essential to the success of your project. There is no one size fits all firing schedule for every glass fusing project.


My Firing Schedules For Glass Fusing With COE 90 + COE 96


Below are my recommended full fuse firing schedules to create a 6mm project using two 3mm layers of glass. The first schedule is for glass fusing with Bullseye Glass (or other COE 90). The second schedule is for Wissmach Glass (or other COE 96). The firing schedule lists Fahrenheit and Celsius.


Full Fuse Firing Schedule For Bullseye Glass (COE 90)

​Initial Heat To Pre-Rapid Heat Soak

400° F (205° C) per/hr

1225° F (663° C)

:30

Rapid Heat To Process Soak

600° F (315° C) per/hr

1490° F (810° C)

:10

Rapid Cool To Anneal Soak

AFAP

900° F (482° C)

1:00

Anneal Cool

100° F (38° C) per/hr

700° F (371° C)

:01

Cool To Room Temperature

AFAP

70° F (21° C)

:00

Full Fuse Firing Schedule For Wissmach Glass (COE 96)

​Initial Heat To Pre-Rapid Heat Soak

400° F (205° C)

1225° F (663° C)

:30

​Rapid Heat To Process Soak

600° F (315° C)

1460° F (793° C)

:10

Rapid Cool To Anneal Soak

AFAP

960° F (516° C)

1:00

Anneal Cool

100° F (38° C)

700° F (371° C)

:01

Cool To Room Temperature

AFAP

70° F (21° C)

:00

Dissecting this firing schedule gives a better understanding of what happens during the glass fusing process. Understanding how heat affects the material in a kiln is essential to all types of kilnformed glass art. Heat work in kiln glass is most interesting, in my opinion, and I love talking about it.


Segment #1). Initial Heat


fused glass art project on a primed kiln shelf
The initial heat segment of a fused glass firing schedule takes the glass from room temperature to the bubble soak

This first segment of the firing schedule brings the fused glass project from room temperature to the first hold in the program. The initial temperature ramp to heat individual 3 mm layers of sheet glass can be aggressive, though heating the project too aggressively can cause thermal shock. Thermal shock occurs from rapid but uneven heating of the pieces of glass in a kiln. The fused glass project can experience thermal shock anywhere from room temperature to 1000° F (538° C). I recommend an initial heat of 400° F (205° C) per hour to 1225° F (663° C) for a project using two 3mm layers of sheet glass. The ramp will work with either Bullseye Glass or Wissmach Glass. The initial heating of a glass project that is thicker than 3mm should be slower. For example, I recommend an initial heat ramp temperature of 300° F (149° C) per hour for a 6mm fused glass project.


Segment #2). Pre-Rapid Heat Soak, or Bubble Soak


The initial heat segment in the firing schedule brings the fused glass project to the first soak segment in the program. The first soak in the program is the pre-rapid heat soak. It is also known as the bubble soak. This soak (or hold) performs two jobs helping the fused glass project be successful. The first job is to allow the entire project to reach a safe temperature above the thermal shock zone at 1000° F (538° C). Free of any temperature variances.


The second job is to reduce the number and size of bubbles in the finished project. The glass at this temperature loses its brittleness and behaves like a stiff liquid. When this occurs, both layers of glass begin to make closer contact and push any air out from between them. The reduction of air pockets is why this hold segment is called a bubble soak. It is important to note that larger objects require a longer hold time at this soak to reduce bubbles.


Segment #3). Rapid-Heat


Now that the fused glass project is no longer in danger of thermal shock and has completed its bubble soak, it is time to proceed with the kiln program. Segment #3 of the firing schedule is the rapid-heat segment. The rapid-heat portion of the program takes the glass project from the bubble soak to the process soak. Because the glass has passed the thermal shock zone, we can heat it more aggressively than during segment #1 (initial heat). Heating the glass quicker during this segment helps to prevent devitrification. Devitrification is a hazy or scummy crystalline formation that can develop on the glass surface anywhere from approximately 1300° F (704° C) up. Firing the project hotter and faster during the rapid heat segment speeds it through the devitrification zone and reduces the chance of its occurrence. I recommend firing your fused glass projects at 600° F (315° C) per hour during the rapid heat segment. As I mentioned, The rapid-heat ramp of the firing schedule takes the project to the process soak segment, which is a full fuse. Use this ramp for Bullseye Glass and Wissmach Glass.


Segment #4). The Process Soak


The process soak is the reason for the firing. It is the hottest temperature the glass will experience throughout the entire kiln program. In this case, the reason for the firing is a full fuse. When firing a glass fusing project using Wissmach Glass to a full fuse, I recommend a process soak temperature of 1460° F (793° C) at a 10-minute hold. For Bullseye Glass, I recommend a process soak of 1490° F (810° C) at a 10-minute hold time. Wissmach Glass requires less heat work because COE 96 is softer than COE 90 glass. Only hold the soak for the necessary time to complete the intended result. The temperature to fully fuse a glass project is well within the devitrification zone. Because of this, we must limit the time the material sits at this temperature to reduce the chance of devitrification forming.


Segment # 5). Rapid Cool


When the full fuse is complete, the glass must pass through the devitrification zone again as it cools. This cool-down occurs in the rapid-cool segment of the firing program. The rapid-cool segment cools the project as fast as the kiln allows to prevent the formation of devitrification on the surface of the glass. The schedule above shows the rate "as fast as possible" for this segment. This quickness of the rate will help ensure the glass project does not get devitrification cooling down to its anneal soak temperature. The glass can cool as fast as possible at this point because it is still in its liquid-like physical state.


Segment #6). Anneal Soak


The purpose of the anneal soak is for the fused glass project to reach an even temperature before beginning the final cool-down. When firing glass in a kiln, it is just as essential for the object to cool down evenly as it is to heat up evenly. There cannot be more than a 10° F temperature difference between any portion of the glass project as it cools. If a difference of more than 10° F occurs, the glass can crack due to annealing strain. I recommend holding Wissmach Glass during the annealing soak for 1 hour at 960° F (516° C) for a finished object that is 6mm in thickness. For Bullseye Glass, a anneal hold at 900° F (482° C) for one hour will suffice. Hold time must increase if annealing thicker objects or objects not of uniform thickness.


Segment #7). Anneal Cool


When the anneal soak segment is complete, the glass can begin the remainder of the annealing process. The anneal cooling segment allows the fused glass object to cool from 960° F (516° C) or 900° F (482° C)through the anneal strain zone. The glass must be cooled very slowly during this segment to reduce the chance of cracking. I recommend cooling Wissmach and Bullseye at 100° F (38° C) per hour to 700° F (371° C). Thicker projects, or fuse glass projects not uniform in thickness, must be cooled through the anneal strain zone even slower than this rate.


Segment #8). Cool To Room Temperature


After the glass is safely through the annealing strain zone, it is best left in the kiln to cool from 700° F (371° C) to room temperature. Do not peek before the glass is completely cool. You have worked too hard on your project. If the glass project is too hot to touch, it can crack if removed from the kiln. I recommend allowing your glass to cool to 70° F (21° C) before removing the piece.


Wissmach Glass fused glass coaster set
Flat fused glass art pieces before slumping

When the kiln is at room temperature, remove the glass project. Now you have a beautiful piece of fused glass art! But why stop there? Let us dive into a slumping schedule.


Slumping Firing Schedule For Sushi Plate Mold


two fused and slumped glass plates in sushi slumping molds
The gentle shape of a sushi mold requires little heat and hold time to slump a fused glass plate

Below is a firing schedule to slump using a standard square sushi plate. This slumping schedule works for Wissmach and Bullseye Glass. There are, however, no universal slumping schedules usable for every mold of every size and shape. Flat bottom molds need more heat and hold time during the process soak than a gently curved sushi plate mold like the example. Slumping temperatures range from 1180° F -1250° F (638° C -676° C). It is possible to over-slump your glass using too much heat or hold time. Clear signs you over-slumped your fused glass plate will show on the bottom of your glass. You may notice the glass has seeped into the air holes of the ceramic mold, causing points on the bottom. This evidence is a definite sign you have slumped too hot.

300° F (149° C) per/hr

1200° F (649° C)

:10

AFAP

900° F (482° C) for COE 90 960° F (516° C) for COE 96

1:00

100° F (38° C) per/hr

700° F (371° C)

:01

AFAP

70° F (21° C)

:00

It is easy to see the differences between this slumping schedule and the full fuse schedule I wrote about above. There are also several similarities. Let us break down the differences starting with the initial heat segment.


Segment #1). Initial Heat


The initial heat drops from 400° F (205° C) to 300° F (149° C). This slumping schedule is for a 6mm fused glass plate created using the glass fusing schedules above. In the full fuse firing, the pieces of glass were 3mm thick before the fusing was complete. Now the whole object is 6mm. Because the fused glass plate is now twice the thickness, the initial heat must be slowed from 400° F (205° C) per hour to 300° F (149° C)per hour to reduce the risk of thermal shock.


Segment #2). Process Soak


The second difference between the two schedules is the project has no need for a bubble soak during slumping. Since the fused glass plate is one solid piece, no bubbles will form between layers. Instead of a bubble soak, the initial heat segment brings the glass plate directly to the process soak hold, which is the slump. A simple shape such as a sushi plate only requires a 10-minute hold time at 1200° F (649° C) to complete the process.


After the slump is complete, the slumping schedule follows the same annealing pattern as the fusing schedules above for a 6mm thick object. Reference the Wissmach Glass schedule for COE 96 glass or the Bullseye Glass schedule for COE 90.


I hope you find this information informative and inspiring. If there are any topics that you would like me to write about, let me know in the comments. I would also love to hear your thoughts on this topic. I am in the process of creating a series of kiln glass and fused glass art e-books and project tutorials that will be ready in the fall. Make sure to subscribe if you are interested, share it with your friends, pin it on Pinterest, or follow us on Instagram.


Happy Kilnforming!


Drew

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