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How to Fire Glass and Identify When You’ve Achieved the Best Artistic Results

 Firing your own projects is exciting. It empowers you to make your own decisions and gives you the freedom to try new advanced techniques. The best way to gain confidence in your fusing is to build consistency into your firings. You want to get to know how your particular kiln works and define the results you want to achieve. At first, you’ll make mistakes and have varying results. That’s okay. You’ll learn something valuable from every firing. Plus, you’ll gain experience and learn to trust your process.    

Start with project assembly.  

How projects are assembled has a direct effect on the durability and, “look” of the finished piece. Most of my projects are assembled using two full layers of glass.

Glass settles at a thickness of ¼ inch when it’s heated to the full fuse temperature. By assembling with two layers of glass, I maintain control over the size and shape of my finished piece.

Fusing one layer of glass will cause the project to shrink. And it will have a fragile, sharp, irregular edge. While fusing three layers of glass will result in the project growing in size, which changes the original shape considerably.

Since two layers is my go-to type of assembly, the projects I made for these comparisons are made with the 2 layers of glass. If your go-to is a different type of assembly, you’ll want to duplicate that for the following firing comparisons.

I fired several 6” x 6” square projects made with 2 layers of glass for these comparisons.

Characteristics of a terrific full fuse firing.

The full fused glass retains its shape and original size. It also has a uniform thickness, smooth surface, rounded edges, and a clean back side. The design accent details melt into the top layer giving the project a shiny, smooth surface. 

Characteristics of an under-fired full fuse firing.  

An under fired project will retain its rigid shape and its original size. The edges are hard, angular, and sometimes sharp. Since the two layers are not completely fused together, a seam may appear on the edge showing how the layers were stacked. The back side of the project is clean and smooth. Design details, like frit have texture and are not fully melted into the top layer.

The solution to an under fired project is to re-fire it a little hotter. To get more heat results, I usually add ten degrees to the target temperature. The target temperature is the temperature that gives me the desired results.

Firing Target Temperatures for 96 COE & 90 COE.

Full Fuse: 96 COE 1465°F (796°C), 90 COE 1480°F (804°C), hold 10 minutes

Tack Fuse: 96 COE 1365°F (741°C), 90 COE 1385°F (752°C), hole 10 minutes

Slump: 96 COE 1265°F (685°C), 90 COE 1265°F (685°C), hold 10 minutes

Characteristics of an over-fired full fuse firing.

An over fired project will grow in size. The outside edges will round out. The glass will lose its square shape and be larger than the original 6 inch by 6 inch size. Kiln wash may stick to the backside and may be difficult to remove. The design details, like frit will be completely fired in giving the project a smooth shiny surface.

The solution for an over fired project might be to grind the edges to regain the square shape. Or you can remake and re-fire the project at a lower temperature.

Characteristics of a successful slump.

A successfully slumped glass project conforms to the shape of the mold. It also retains a clean shape and the sides of the project retain their height.

Characteristics of an under-slumped project.

An under-slumped project does not conform to the mold. The glass project doesn’t have enough drop and so the piece is shallow. The piece may also wobble on the table because it didn’t come in contact with the flat bottom of the mold like it should have.

The solution is to re-fire the glass in the mold and hold it at the target temperature a little longer than the first time. I usually add 10 minutes hold to the original program.   

Characteristics of an over-slumped project.

The backside of an over-slumped project will pick up unwanted texture from the mold. It also loses some of its shape because the sides slide down into the mold. The result is an uneven top edge or rim. The solution is to call it something new or remake the project.

Tack fusing test.

I made three different size nuggets to make a bowl that would look like it was made out of rocks. I cut a bunch of round shapes out a grey glass. Then I cut matching pieces out of clear glass. I capped the grey with the clear and fired the stacked glass nuggets to a full fuse temperature. The pieces came out great.

Don’t hesitate to customize your programs.

Then I arranged the fused glass nuggets on a kiln shelf to make a circular design. I tried using the Auto Mode on my kiln and set the controller to tack fuse the glass together. The finished project looked great, but the pieces didn’t stick together. In Auto Mode the kiln didn’t fire hot enough to tack the glass pieces together while they were resting next to each other.  The lower temperature was intended to tack fuse glass pieces stacked on top of each other.

The solution was to enter my own program in the kiln controller and re-fire the glass to a hotter temperature. It worked! I then slumped the tacked piece to make a shallow bowl. The finished art was a beautiful success!

Take notes.

I recommend taking notes outlining how you assemble your pieces and on your firing steps. This way you can easily reproduce your successes and learn from the pieces that didn’t come out as you expected.

You can do it. I believe in you!

It takes a time to get comfortable firing a kiln on your own. The upside is you’ll learn new and exciting things every step of the way. Relax and enjoy your process. No matter what your results are be grateful for your creative development. And take advantage of the opportunity to make your own personalized pieces of art. 

Happy Fusing!


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Fire & Ice by Lisa Vogt

REPEAT! NEW Fire & Ice Webinar August 4, 2022

Hot techniques for cool projects!

Get fired up for advanced design techniques that invigorate your artistic spirit. You’ve loved making all the simple projects. Now experience the thrill of breaking away from familiar shapes to create striking free-form fused glass designs. 

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Sculptural Fused Glass, 4-Day, Hands-on Workshop, October 18-21, 2022

Register now. Class is limited to 3 students and held in my private studio in Wesley Chapel, Florida. All class materials are included.

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