Frit Tinting and Bubble Elimination

[Jeff Wright]

I'm trying to do some frit tinting by using frit and powder. I've run a number of tests and really like what I'm getting in terms of the color range. What I don't like, is the bubbles I'm still dealing with. I use Bullseye medium clear frit and various powders. The mixing of the powder with the clear is working. I spritz the clear with a little water, sprinkle the powder in, and shake well until well coated and then fill the mold. My tests are little 1.5" x 1.5" x 0.5" squares and are a perfect size for what I want to do. Each square takes 25g of color. I'm doing 8 at a time with tint percentages from 20% down to 3%.

When I fire, I end up with nice little squares of great color, except for the number of bubbles. I tried coarse frit and didn't get any better results. With the medium frit I get lots of little bubbles. I'm sure my schedule is wrong. I know I can't get rid of every single bubble, but the stuff I've seen others do has very few bubbles. Here is my schedule:

1. 500/hr to 1100F, hold 20 min (ramp up without breaking the mold and even out the heat in the frit)
2. 375/hr to 1240F, hold 120 min (a little slower to bubble squeeze temp, hold a long time)
3. 375/hr to 1460F, hold 120 min (ramp to process temp and hold a long time)
4. AFAP to 900F, hold 120 min (no venting, hold for 0.5" anneal)
5. 150/hr to room temp (about as fast as my small kiln loses heat)

What I need is advice on the bubble squeeze/process temps and times. What do you do?

[B.J.]

The larger the frit size the less bubbles it will produce but you also end up with glass that has more clarity so you're going to see whatever bubbles are there. Colour de Verre has a worksheet for color mixing and tinting frit that may be of help.
http://www.colourdeverre.com/docs/19.pdf

[Laurie Spray]

Step 2 - I would go 50/ hr

[Geo]

I feel like tinting frit ends up the opposite of what one thinks in terms of bubbles. Whenever I make samples, I tint powder (08) and fine frit (01) using Bullseye glass. The samples made with 08 come out more homogeneous, albeit opaque. I almost always have bubbles in the fine frit samples. The times I tried coloring medium and coarse frits, it seems I end up seeing the individual pieces of the frit in my samples.