Iridescent Glass - A Lesson Learned

Kiln carving using red irid glass
Iridescent glass (or "irid" for short)  is a special glass with a metallic surface coating. The coating results in glass with a rainbow shimmer when light reflects off its surface. It's lovely in fused glass pieces and is often used for kiln carving with fiber paper.

I recently purchased a stainless steel "s" curve mold and began thinking about a design for my first piece on this mold. I decided on a strip construction of mostly clear, with translucent blue and teal strips placed randomly throughout. I wanted to build the strip construction on a piece of clear irid glass, firing with the irid side down. I've fired irid side down on fiber paper before. I like the look that the texture from the fiber paper leaves on the surface and how "sparkly" the glass is when viewed from the front.

I cut a 10" x 13" base of irid glass and placed it face down. Next, a border was made for the rectangle with 3/8" wide teal strips placed on edge. I then cut and placed the clear, blue, and teal strips on edge within the frame. If you've done strip construction before, you know this was time consuming. Also, it didn't help that I got half way through, decided my random pattern was not random enough, and started over!

I placed the constructed piece on Spectrum's Papyros, dammed the piece with fiber paper and ceramic dams, and fired it to a full fuse.

As always, I was excited to get the piece out of the kiln. When I removed it I noticed a slight haze across the back, but I thought it was just some of the Papyros stuck to the back that would come off with a good scrubbing. So I scrubbed. And scrubbed. Bought a new scrub brush, soaked in CLR, and scrubbed some more. The haze would  not come off!

Strip construction after first fire

Close-up of haze
I posted a question to the warm glass forum. The forum is full of knowledgeable and talented glass artists who are always willing to answer a question or two. One of the members pointed me to the Bullseye Glass ThinFire technical note which reads:

ThinFire has been used with excellent results in Bullseye’s factory studio for many types of fusing applications. However, it does not work in every application. The one example we have noted to date is this: used in direct contact with iridized glass, ThinFire may cause a reaction resulting in surface pitting.

Surface pitting! Note that I was using Spectrum's Papyros and not Bullseye's Thinfire, though I assume the two are similar in composition. So apparently, while it is safe to fire irid glass coating-side-down on fiber paper (sometimes called fiber blanket) firing coating-side-down on shelf paper can lead to undesirable (and unfortunate!) results.

I'm still trying to decide how to rescue this piece. It could be sanded and refired, but then the irid would be removed. I've also considered etching a pattern over the haze, coating the entire back with coarse clear frit & refiring, painting, etc. It may be that I just slump over the mold "as is" and call it a Keeper!

At any rate, I learned a new lesson about firing on shelf paper. Hopefully, others can benefit from my experience!


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