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Devitrification and other problems, such as staining and foreign bodies, can render your beautiful fused glass cabochon or feature a rather ugly oddity. This has happened to me many times.
My usual approach was to grind or sand away the offending area and then fire polish the pieces. The grinding and sanding can be very tedious and is always time consuming.
After spending a lot of time doing this I became determined to find another option.
I had read a lot about sand blasting items to remove the problem area, but I don’t have a sand blaster. The cost to purchase and the problem with an area to put the equipment meant this wasn’t really an option for me.
Another option I had read about was tumbling the pieces in a small tumbler. The type commonly used for polishing gemstones and pebbles. Guess what? I have one of those. Ann posted about it on July 3.
So I looked around to find out just how to do this. As usual, a lot of talk, but not a lot of good information about the actual process.
We have a local lapidary group and a materials supplier here so I asked around and again they had never used a tumbler for working glass so were not a lot of help.
Not to be deterred I decided to simply give it a go.
In my first attempt I loaded the tumbler with about a quarter plastic pellets, then glass pieces to half, water to just cover and a couple of tablespoons of 120 silicon carbide grit. The same type of grit used to polish stones.
Well, after about 6 hours the glass came out almost the same as when it went in. The shine had been dulled slightly, but I would have needed to tumble the glass for days before the problem areas were sufficiently removed. That wasn’t quite the solution I was looking for.
The abrasive nature of the tumbler had to be more extreme, so my next attempt was the same.
I purchased some 80 grit, omitted the plastic pellets, loaded the tumbler to about half with a mix of small and larger size pieces, added 2 tablespoons of the grit and filled with water to just below the level of glass cabochons.
With just a little nervousness, mainly concern that the pieces would be totally chipped and destroyed, I put the tumbler on and left it for 6 hours.
I opened the tumbler and was surprised to not see a disaster. In fact, things looked great. Actually, after some cleaning the pieces looked fantastic. Nicely frosted and smooth, a bit like sea glass. Yes, I know this how some people make sea glass using a tumbler, so I suppose that isn’t too surprising. My surprise was that the tumbler hadn’t been too aggressive.
On inspection all the pieces looked great, so I eagerly worked towards doing a fire polish.
All pieces were thoroughly cleaned then fire polished on a prepared kiln shelf, not kiln paper. The firing schedule is below.
As you can see in the photograph the pieces all appeared to polish up nicely. However, there were a few failures.
Some of the pieces were badly devitrified and a couple had rough imperfections. The problem area had not cleaned off completely and these didn’t polish as well.
In hindsight, I think it would have been better to separate the cabs into those with a fine devitrified surface and those with a more sever problem. You could then tumble each group for a different period.
This whole process seems to be a solution for small fused glass pieces like cabochons, but would not be suitable for large items.
Lessens Leaned
- Tumble glass cabochons without plastic pellets. They won’t break or chip.
- Use 80 grit silicon carbide
- 6 hours should be okay for light devitrification, but longer may be necessary for deeper issues.
- Only suitable for cleaning small pieces.
Firing Schedule
Fire Polish – Cabochons
Please be aware that all kilns fire differently and this schedule may not produce the same results in your kiln.
| Segment | Rate (C/hr) | Target (C) | Hold (mns) |
| 1. Strain Release | 222 | 537 | 10 |
| 2. Process | FULL | 732 | 10 |
| Crash Cool | Open Door | 550 | 0 |
| 3. Anneal | Full | 516 | 30 |
| 4. Cool | 83 | 371 | 5 |