Preparing Mica Minerals for Fusing

If you’ve been around here for any amount of time, you know I LOVE mica, but there is a story there that the mad scientists among you may find interesting. Way back in 2016, a lovely reader from Idaho sent me some raw mica chunks.

Raw mica rock

Not a very impressive item all by itself. Mica is a mineral (a group of minerals) with the major identifying characteristic of perfect basal cleavage, i.e. it separates into individual crystals (layers). Of course the first thing I did was chuck a bunch of little bits into the kiln and fused them in float glass.

raw mica rock bits fused into glass
Lots o’bubbles going on there

The results were interesting to me for two reasons. First, those bubbles are impressive, there is obviously quite a bit of off-gassing going on in this process.

Second, and the far more fascinating one in my opinion, is this:

Close up of mica rocks
Close up of fused raw mica test

If you look closely, you can see that the mica has actually changed color, from a translucent gray/brown to a metallic gold. . This color change led to some new tests, mainly firing the raw mica by itself to see if I could trigger color change and perhaps accomplish most of the off gassing before fusing. These tests were fired in my ceramics kiln at higher than fusing temperatures, the rational being that the higher temperature would take care of any off-gassing that would happen at lower fusing temperatures later. There was an interesting wrinkle, the samples fired at different temperatures came out in different colors.

Fired raw mica tests
Fused raw mica
Fired Mica Samples

But what about the bubbles??

I tried a second fusing test with some of the mica on the right of this photo, which is copper colored. Here are the results:

Fired mica test fused in float glass.

THREE interesting things here, 1. the bubbles situation is much less dire, 2. The mica appears to be completely inert from the glass, sandwiched in the glass but not structurally fused in, and 3. The mica has continued to change color, becoming more copper than before fusing. Well, this is just the type of situation that makes my heart sing, because it means that there is TESTING in my future! Testing is my favorite fusing aspect.

All the tests

At this point though I was running out of raw mica. So, after finding a rock shop I could buy quantities of mixed mica, I began planning tests. I also noticed that not all raw mica was the same color, so the first step was sorting the raw mica into ‘types’ by color, and then running firing tests by 50 degree f increments from 1600f to 2000f. This is the basic results chart:

Scan of firing tests mica types and temperatures

This gives us options for targeted firings to achieve the color of mica that we want, rather than a random shotgun approach. Of course, there are still surprises, but the end result is much more predictable.

The other advantage to bulk mica acquisition is the variety of shapes and sizes. Many of the pieces are large enough to fire and use as sheets. Mica is often used as an insulator, these odd shapes are left overs from industrial production processes.

Detail of many layers of mica
Mica layers separated after firing

The fired mica is sorted into colors, and then packaged as sheets, or processed into flakes by grinding and filtering (using extensive safety gear). The end results of all my experimenting:

Mica lessons I have learned:

  • Fire the raw mica at higher than fusing temperatures 1600f – 2000f
  • Mica will continue to change colors slightly during continued heat processes such as fusing
  • some bubbling will still occur as the mica stays independent of the glass
  • mica can be cut, ground, stenciled, etc.
  • mica can act as a separator between layers of glass, to get a solid fuse, there must be a clean glass perimeter around the mica.
  • mica is AMAZING!! But I already knew that. 🙂

Other mica tutorials

Avoiding bubbles with mica sheets and shapes

Working with mica sheets

Mica Flake Stencil Lantern

Sparkly Fused Bottles

Mica Shop

Mica is a silicate, use a respirator!