FAQs for Glass Fusers
I’ve been teaching how to work with glass for more than 30 years. I’ve helped thousands of students create beautiful, unique pieces of art. With my lengthy teaching experience, I get a lot of questions from students. I thought I’d share some with you.
Students try to get absolute, it’s only done this way, fusing advice from me. My advice to them is there is no always or never in glass fusing. I approach every project with the intention of getting the best results possible. I use any and all means and combination of materials I have available to achieve my artistic goals. There are many ways to approach glass fusing. The following list is intended to be a guide to help you understand different aspects of the craft and improve your results.
Here we go:
Q. Do you have to re-coat the ceramic kiln shelf with primer every time you use it?
A. I do re-coat my ceramic shelf with primer every time I use it. I apply three coats of shelf primer on top of the used primer. However, if the shelf shows signs of wear, I scrape it down to the bare ceramic with a single edge razor blade. I then re-coat the shelf with 3 fresh coats of primer. Visible signs of wear would include the primer looks thin, the coating has cracks in it or it’s flaking off the shelf.
Q. Why does the kiln wash stick to the back of the project?
A. This is usually the result of exhausted kiln wash; the shelf should have been reprimed prior to use. It may also occur when the glass has been fired to too hot a temperature or kept at full fuse too long. Kiln wash can be removed with fine steel wool or a scraping tool. Stubborn spots can be soaked off with a bathroom cleaning product used to remove hard water stains.
Q. After the project is fused, how close to room temperature does the kiln have to be before opening the kiln lid?
A. How badly do you want the project inside? Room temperature is usually well below 100 degrees Fahrenheit. If you open the lid prematurely thermal shot can occur; breakage caused by changing temperatures too fast. Just a quick peek in the kiln, done too early can ruin your day. Wait, it will be worth it!
Q. What causes tiny bubbles and how do I get rid of them?
A. Air becomes trapped between the layers of glass when it is heated. The number and size of the air bubbles can be reduced by heating the glass more slowly between the temperatures of 1000 to 1465 degrees Fahrenheit. This gives the air more time to escape before the glass edges become sealed.
Q. How do I avoid big bubbles?
A. Large dome shaped bubbles sometimes form in the middle of medium to large size projects. This is usually the result of firing too fast on a smooth surface like a ceramic kiln shelf. The outer edges of the project become soft first and form a “seal” like a suction cup on the shelf. Air becomes trapped under the glass. When it expands it lifts the glass and forms an unattractive bubble. This is less likely to happen when using shelf paper or a fiberboard kiln shelf, as the air can escape through the porous material. The best advice is to slow down the firing process.
Q. Can I stack kiln shelves?
A. Not recommended. Unlike ceramics, glass is thin and reacts to heat quickly. Stacking shelves causes uneven heating. The outer edges of the project become molten before the center has a chance to react; your firing results may be inconsistent.
Q. How does the thickness of the glass relate to my target temperature?
A. A project made with two layers of thin fusible 2mm glass will usually reach the desired “look” before a project made with two layers of 3mm glass. When you change project sizes or materials (2mm vs. 3mm) you will want to monitor the first few firings and make any necessary adjustments to your firing schedule.
Q. Can more than one project be fired in the kiln at one time?
A. Yes, if the projects are similar. They should be made from the same thickness of glass, have the same number of layers and be close in size. Use a firing speed for the largest project. Small projects will not be harmed by firing slowly but the large project will suffer if rushed.
Q. What causes sharp edges on the project?
A. Dragging is usually caused by over firing a project that is resting on fiber paper. As the glass contracts and draws in on itself the paper resists, sharp edges are the result. This edge can be made safe with a file or grinder. After grinding the project, it can be put back into the kiln and heated until the ground edge is polished.
Q. Why is there debris between the glass layers?
A. Too much glue. Small amounts of glue will burn off leaving no trace. I avoid using glue if possible. When I do use glue, I apply a few pin head size dots on the backside of the glass. If you use an excessive amount of glue, it will burn, becoming sealed between the layers, leaving dark blotches. Excess glue can also cause small eruptions that will blow a hole through the glass or cause pieces to jump and move in the kiln.
Q. What is the difference between using a ceramic kiln shelf or fiber paper?
A. Ceramic Shelf:
- Fused glass will take on the texture/pattern of whatever it is fired on. If you fire on a ceramic kiln shelf your project will have a smooth back surface. (Great for bowls, plates, fine art pieces; projects where a sleek underside is desired.)
- A Ceramic shelf can be used repeatedly.
- A ceramic shelf is dense; it retains heat which allows the glass to pass through the critical stages slowly.
- Ceramic shelves must be primed frequently; this can be time consuming.
Fiber Paper:
- Shelf paper comes in a variety of thicknesses and finishes. Generally, the thinner the material the smoother the fusing surface. (Great for coasters, pins, wall art; projects where a textured back side will make it easy to glue accessories on like rubber feet, pin back, etc.)
- The thin material can only be used once. The medium material can sometimes be reused; it leaves a matte finish on the back of the project. The thick material can be used multiple times but the texture on the back of the project will be course.
- The fiber paper is porous. Air can circulate, therefore large air bubbles are less likely to form between the glass and the shelf.
- Shelf paper must be cut to size and can be costly if you are using it every time you fire.
Q. What are the differences between ceramic molds and stainless-steel molds?
A. Ceramic Molds:
- Ceramic molds come in a wide variety of sizes and shapes. They must be primed like the ceramic kiln shelf. The primer tends to last longer on a form than on a shelf because slumping temperatures are lower than fusing temperatures. Slumping occurs at approximately between 1200-1300 degrees.
- Ceramic molds are inexpensive and durable, but they will break if dropped or used unprimed.
- Ceramic molds should be sanded and re-primed if there are pits or cracks in the primer coating.
- Ceramic molds cool more slowly than glass. Therefore, most molds are shaped so the glass slumps down into the mold, allowing the glass to contract first.
Stainless-Steel Molds:
- Stainless steel molds come in a variety of shapes and sizes as well. They must be primed also. The slick surface makes priming more difficult. The mold can be sanded, sandblasted, or heated to make the priming process easier.
- Stainless steel molds are extremely durable “forever” molds. But they tend to be several times more expensive than ceramic molds.
- Stainless steel molds should also be sanded and re-primed if there are pits or cracks in the coating,
- Stainless steel cools more quickly than glass. The metal contracts underneath the glass which gives the room needed to slide the draped glass off a floral former style mold.
Some advice:
Enjoy the creative process. Don’t rush it. Take your time. Make small test pieces to establish a relationship with your kiln and with fusing materials. Take before and after pictures. Take accurate notes so you can repeat the successful projects and learn how to avoid failures. Don’t worry that the test pieces won’t be fun. Some of my best work originated as a small sample. Trust that as your confidence and skills grow, so will the size and complexity of your projects.
This FAQ’s is from the Free Firing Guides and More page of my website. Find more helpful there.
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Lisa & Niki
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