|Stained Glass • Fusing • Mosaics • Jewelry Supplies|
The Coefficient of Expansion (COE) refers to the rate at which glass expands and contracts when heated and cooled. All glass has a COE but we don't always know what it is. It's important that all of the glass you are fusing together expands and contracts at the same rate or stress and cracking can occur. Glass manufactured specifically for fusing is tested for compatibility. Always fuse glass with the same COE or tested compatible glass.
When heating glass to a full fuse (1450° F) it wants to become ¼" thick and round. A standard sheet of glass is approximately 1/8" thick.
In the picture below, notice how the following stacks of glass appear before and after firing.
In the next picture, notice how each cabochon is approximately the same thickness
Viewing the cabochons from the top you can see how the stacks spread when they were fired.
Keeping the ¼" rule in mind with help you achieve the desired effect without growing or shrinking your project.
Glass cannot be heated or cooled too slowly. Heating and cooling too fast can result in cracked glass or thermal shock. Ideally you want your glass to stay approximately the same temperature as the kiln. Because the glass heats more slowly than the kiln, you should consider your project size and how long it will take the piece to "catch up" to the kiln temperature.
Slow, controlled cooling, commonly referred to as annealing, is the process in which you allow the glass to gradually cool down in order to remove internal stresses that may have occurred during the firing process.
Annealing allows the glass to stabilize and adds strength to your finished project.
If your glass breaks in the kiln, your project didn't turn out quite as expected, or you feel the need to add further embellishments to your project, you can re-fire your project until you get the results you desire. For cracked glass some artists add additional pieces of glass to the top of the glass to reinforce and cover the "seam" where the crack occurred. Do not try to re-fire projects with incompatible COEs. Even if the project survives the heating and cooling process the stress remains and the piece can crack or break later.
Key Temperatures in the Firing Process
The firing process you select will determine the final appearance of your fired project. Many projects will require multiple firings using different processes before they are complete
The photographs below show glass projects with similar embellishments, fused at various temperatures.
Common Firing Schedules
The schedule that works for you will depend largely on the size, scope and desired look of your finished piece. The firing schedules below will provide a great place to start.
All Kilns are Not Alike
Project Firing Logs
Using a project and firing log to keep track of information about your projects and firing schedules can be an extremely useful tool for every artist. Once you've achieved a good result you can reference the logs to re-create the project or simply re-use a firing schedule for a similar project. Click on the buttons at the top of the page for a copy of this document and a project firing log to get you started.
I'm curious on how to get a "domed" or r"ounded top" (looks like half a marble) like You see on earrings, on glass cabochons?
Kayleigh ClarkThursday, June 13, 2019
Based on your second description it sounds like it is too hot, single layers shrinking trying to get to a 1/4" thick. The temperature doesn t sound too hot though. The first one may have worked out ok because it was larger but really if the smaller pieces shrunk, the first one should have also. I don t know what the wrinkling on the surface is. Maybe the paint was thicker on the second firing? If the second fire was too hot maybe the shrinking caused the wrinkling around the paint?
Kayleigh ClarkThursday, June 13, 2019
@krisd This question is subjective and depends on the size of the project, the type of glass being used, what kiln is being used, etc. When I fuse I typically ramp up all projects except my jewelry cabochons at 200* F an hour. Whichout knowing what your kiln settings are for Fast, Medium and Slow that is harder to answer. The other part of this is that the end temperature is what really helps define the end results. A basic tack fuse is around 1340* F again, depending on your kiln it might be a bit lower or higher, it will take some trial and error to find the perfect temperature for each kiln.
I'm tack fusing small panels, (4x7 90 coe) the bottom clear glass is 3mm, and I'm stacking a few pieces 1 to 3mm high. So in some parts, it is over 6 mm. I have it at a tack fuse 90 medium speed. (which is what is says on my Evenheat kiln preprogrammed) I usually do a tack fuse 90 fast speed, but since these are a bit thicker, I am using a medium speed. However, it seems it reached a high temp, faster than the "fast" speed would have. I'm hoping this will still be a tack fuse by the time its done. So, my question is, if I use more than 6 mm thickness, how should i tack fuse this? Fast, medium or slow?
@ Creating with a mold that has some texture and depth like this one can be a bit challenging. Our instructors would recommend that cutting 2 layers of glass and fusing them together in an initial firing, then placing the fused glass on the mold and slumping is going to be the most likely way to achieve good results. For slumping, you can use a fairly common schedule with a ramp rate of 200 degrees an hour, the key to getting depth and detail is going to be going to a slightly higher temperature than usual, around 1300 degrees F, but only holding at the top temperature for 5 to 10 minutes before cooling as fast as possible to your annealing soak. Happy fusing, and we sure hope you get a perfect pumpkin plate to enjoy this fall season!
@curly6 The slightly matte or textured surface is the result of the glass touching the mold. Unfortunately, at the temperature needed to "fire polish" something, the glass is soft enough that it will not retain the slumped shape. For future projects, you can attempt to avoid this finish by reducing the time that the glass is exposed to slumping temperatures. You may wish to set an alarm on your controller for when the kiln reaches the target process temperature so you can check the slump progress and skip to the next (cooling) segment of your cycle. This may help prevent the glass from fully settling into the mold and imprinting the texture of the ceramic mold surface.
I just slumped a very nice 12 inch bowl. The inner surface and edges are smooth and glossy. The outer surface (that went down into the mold) is not looking good. Can I refire so the outer surface is "polished" without disrupting the rest of the bowl?