Low E (emissivity) Glass is something of an interesting animal. I discovered by accident that it has some special properties.
Low E is the name for glass that is coated with thin metal films to reflect heat/light. There are two different types of coating, the “sputtered” coat, which uses up to three coats of silver sprayed on glass. This is called a “soft coat” and has to be sealed inside of a unit, typically filled with argon. The other type of coating is a pyrolitic coating, which has multiple layers of tin oxide and silver baked on the glass, i.e. a “hard coating”.
If you are interested in reading more about the technical side of low E, I recommend Efficient Windows as a resource.
The first step in using Low E glass in recycled glass art is to find it. Often the coating is difficult to see, particularly if you have a broken piece of window, rather than the entire unit.
Here is a photo of plain glass and Low E side by side. You can see the Low E looks slightly darker or grayish.
If you are still unsure, you can drop a piece in water, the refractive characteristics of water make Low E glass easy to spot.
When you fire Low E glass, a funny thing happens:
These pieces of Low E were fired at a full fuse in a side fire kiln. I also included our friend Plain Glass for contrast. These pieces were fired one with the film side up, one with the film side down. It doesn’t appear to make a difference. Here is the gotcha, I haven’t figured out how to tell if a Low E coating is “hardcoat” or “softcoat.” It makes a difference. In one case, a full fuse will burn off all of the coating, in the other (this case) the silver coating stains the glass a bright yellow.
What does this mean to you? If you don’t know, either test, or just slump it.
As I’m never exactly sure what I’m cooking, I always fire with the film on the outside and vent the kiln to the outdoors.
The color is amazing, and you get some really cool effects from where the film has degraded.