PPP: Glass Fusing in a Microwave Oven

March 1, 2009

A pendant made in a microwave kiln.
Blue glass sprinkled with dichroic glass.

It is difficult to believe that the microwave ovens we use to heat soup and bake potatoes can get hot enough to fuse or melt glass. In reality, not only can microwave ovens fuse glass, but most ovens can do it in less than 10 minutes.

I first began experimenting with microwave firing last year to make glass-fused pendants. I used the microwave oven in the Paragon employee kitchen for most of the firings, and I also used two smaller and inexpensive microwaves to compare the results I was getting with the larger microwave.

Microwave Basics

Making glass jewelry is fun and will teach you the principles of glass fusing. With practice, you should be able to achieve consistent results in microwave firing. You will need a microwave kiln, which is a small ceramic fiber container that fits inside the microwave oven. The microwave kiln captures microwave energy and concentrates it in the firing chamber. The concept is similar to using a magnifying glass to focus sunlight.

The biggest challenge to microwave firing is the fast heating rate. A microwave kiln reaches 1650°F (899°C) in only 5-10 minutes. Firing times depend on the wattage of the microwave oven; the lower the wattage, the longer the firing time. However, firings can be controlled to achieve consistent results.

Only small glass projects can withstand rapid heating. The glass should be no larger than 1 x 1½ in. and no thicker than two layers of 1/8-in. stained glass. The glass must be clean; even a greasy fingerprint can cause the glass to break apart during the initial heating.

Figure 1a. Place the bottom of the kiln on top of three ½-in. ceramic posts.

Getting Started

Though these instructions are basic, more detailed information will be included in your microwave kiln manual. Fire only fusing-compatible stained glass, or the pieces will break due to differences in their coefficients of expansion. Your stained glass supplier can show you how to cut the glass. Required supplies include a microwave oven and kiln, kiln wash and a small paintbrush, clear firing safety glasses, heat-protective gloves, six ½-in. kiln posts, a 12 x 12-in. kiln shelf or other heat-proof surface, and a glass cutter.

First, protect the base of the kiln by applying a coating of kiln wash or glass separator with a small paintbrush. The coating prevents the glass from sticking to the kiln base. After the coating is completely dry, place the glass pieces on the base.

Lay the 12 x 12-in. ceramic shelf or other heat-proof surface near the microwave oven. Arrange three ½-in. ceramic posts on the shelf. When the glass has fused, you will remove the kiln from the microwave and place the kiln on the three posts.

Figure 1b. Position the kiln gently over the kiln base to avoid jarring the glass.

Most microwaves have a rotator tray. If the rotator tray in your microwave is stable, it will provide better heat distribution within the kiln. The tray in some models vibrates or jerks, however, which will cause stacked glass pieces to fall apart as the tray moves. If that is the case with your microwave, remove the tray. Alternatively, a tiny drop of Elmer’s white glue will hold glass pieces together, though the glue does increase the chance of glass breakage.

Position the glass pieces on the kiln base. For your first firing, load only one test piece. (Later, you can try firing three or more pieces per load.) Place the kiln bottom and glass on top of three ½-in. posts inside the microwave. You can place the posts on the rotator tray or directly on the bottom of the oven if you’ve removed the rotator tray. Gently position the kiln over the kiln base and close the microwave oven door. The loading process is shown in Figure 1. You are ready to fire the glass.

While it is okay to fire small pieces at full power, starting with 50% power for five minutes increases the success rate, especially with larger pieces. The glass is less likely to break with a pre-heat. See your microwave manual for instructions on reduced power settings. (You can probably find the instructions on the Internet if you don’t have the manual.)

Figure 2. Lift the kiln quickly to check the progress of the glass.

After a five minute pre-heat, set the microwave oven to 10 minutes at full power and press the start button. Wear heat-protective gloves and clear safety glasses. After four minutes, open the door. Working at arm’s length, lift the kiln top about ½ in. and quickly look at the glass (see Figure 2). (Do not get close to the hot glass as you check it visually.) The glass will probably not be fused yet. If that is the case, close the door and press start. The microwave will continue to heat.

Figure 3. Remove the kiln immediately after the glass has fused to completion.

Keep checking the glass every minute. When it has fused just the way you want it to, remove the kiln and base from the microwave. Gently set the kiln on the three ½-in. posts that you placed on the ceramic shelf (see Figure 3). Do not disturb the kiln. Allow it to cool for around 45 minutes.

As you gain experience, you will sense just when to start visually checking the glass. I check every 20 seconds toward the end of a firing because I want to stop the firing at a precise stage of fusing, usually between tack fuse and medium fuse.

The stages of glass fusing (from left to right): tack fuse, medium fuse and full fuse.

Fusing Fun

Once you learn how to achieve good results consistently, you will be amazed at what you can make. I gave a glass-fused pendant to my 13-year-old niece and she liked it so much that she even wore it while she slept.

I learn something new every time I fire a microwave kiln. You can experiment endlessly with designs and colors. I used hairspray as an adhesive once, and the spray produced a beautiful silvery sheen over dichroic glass. You will make many such serendipitous discoveries with microwave glass fusing.

For additional information regarding glass fusing, contact Paragon Industries, L.P. at 2011 South Town East Blvd., Mesquite, TX 75149; (972) 288-7557; fax (972) 222-0646; e-mail info@paragonweb.com; or visit www.paragonweb.com.

Author's Note

Please follow the safety rules in your kiln manual, and always wear clear safety glasses when looking at the hot glass.

SIDEBAR: Microwave Kiln Safety