capping with coarse frit to reduce bubbles

[seachange]

Hi,

Have been making some tests with really slow fusings, long bubble squeezes and top firing, to try and minimize bubbles.

Test pieces are small, square 4 1/4", round 9cm diameter (slightly less than 4"), this is in preparation for larger pieces.

Tests 1 & 2: 2 layers of Tekta 1/8", 1x smooth side in, 1x bumpy side in. Still more bubbles than I'd like.

Test 3: 1x Tekta 1/8" covered with one single layer of coarse frit (number 3 size).

Arranged the frit carefully, tightly close to each other but in a single layer - took a while to do this - to make sure that absolutely all air could escape. It made a huge bubble in the middle, which then collapsed and left a big hole. However, the remaining border around the hole had no bubbles , so there is some potential in this.

Thinking the hole was due either to a gap between the kiln shelf and the glass, or to total volume of glass not adding to 1/4", tried again same setting on a perfectly straight shelf. Same result, same big hole...again no bubbles on the remaining border.

So, it seems to me that I need more frit (would this be the equivalent in weight to the tekta piece?) to achieve a 1/4" thickness at full fuse. Tekta sheet does come in 4mm and 6mm thickness, but the intention is to use colored glass capped with clear once tests are done.

Since I have to pile the frit higher, could someone kindly advise me if is it better to use size 3 for this, or the extra large size 5 to achieve the most bubble free result?

BE catalog says:
03 - coarse ranges from 2.7 to 5.2 mm (6mm is just under 1/4")
05 - extra large ranges from 5.2 to 30 mm (25.4mm = 1").

From this, 05 seems pretty large, at least some of the pieces, but I have never seen it.

Hoping for your help,

Many thanks as always, seachange

[Stephen Richard]

Even the 30mm frit size is only 3mm thick.
I am not positive what your aims are, but to get a coloured sheet, I use powder sifted on to about 1-2 mm thick. Tack fuse to about 20C below your rounded fuse temperature. When finished, I cut off the rounded edges (so make the coloured layer bigger than the final size) and add other layers if needed.
If the tack fuse does not give you the flatness you need, take to full fuse on a larger sheet than for tack fuse and again cut off the excess.
This may be way off what you are attempting though.

[seachange]

Hi Stephen

Thank you for replying. Sorry I haven't explained myself properly . Have changed the subject of the thread hoping to correct this.

Basically what I am trying to do is to fuse two 1/8" layers together, or close equivalent in thickness, and minimize the bubbles. My tests showed a significant reduction in bubbles with a slow schedule and a good bubble squeeze, as taught and recommended here by so many people (many thanks for that! ). I did try to cheat a bit with the schedules to save some time, but in the end, the slow one is really and truly the best.

The bottom layer of my plate has decoration, fully fused in and smooth. The decoration faces down, towards the shelf. Therefore when I cap, on a second firing, the cap sits on the totally smooth, facing up, side of the bottom layer.

Because the bottom layer, due to the decoration, is already about 4 or 4+ mm thick, instead of the standard 3mm, I thought I'd try to cap with coarse frit.

This - my theory went - should bring me close to about 5mm, good enough for my use, and further minimize, or completely eliminate, the bubbles. In two tests, the remaining glass around the big hole that formed in the middle has no bubbles, so that part of my theory seems to work.

However, with a single layer of coarse number 3 frit, there does not seem to be enough thickness for a full fuse. It tries to pull to 1/4" (6mm) and ends up with a crater in the center. At least this is what i think is happening. If I don't fully fuse, the frit remains a bit bumpy and visible.

Therefore thinking I either have to pile number 3 frit higher, therefore risking some trapped air and bubbles, or perhaps the number 5 frit, being bigger, will provide enough thickness in a single layer.

Learned from Brock Craig about placing chads around the edge to allow air to escape. At present can only use my small kiln, therefore can't make the plate big enough to cut the edge off when finished.

Have read about making frit by dropping the hot glass in a metal bucket of cold water, but have to build up some courage to do this in my home studio...not quite there yet.

Have also tried to cut small pieces of 3mm tekta with one of those wheel pliers, and use this as large frit. Problem is that some (too many) pieces leave a faint but visible halo when fused. This didn't happen with the number 3 frit.

Would take about a week for number 5 frit to arrive here for a test, and can only get it in a large, not cheap, bottle. Was hoping someone with experience in capping with frit could share with me how they do it, and advise me if size 5 does work better than the size 3 I already have, or if I will be wasting both time and money.

So, at the moment this is where I am

Best regards, seachange

[Valerie Adams]

Try the technique I heard about from Steve Klein: put your first layer down, then sift a very fine layer of clear powder evenly over it, then cap and fire with your bubble squeeze program.

[edited to reflect Steve's method, which uses powder, not fine frit as I originally stated.]

[S. Klein]

I sift a minimal firly even layer of clear powder between the sheets. SLIGHTLY heavier on the corners.

[seachange]

Thanks Valerie for this new (I mean new to me) idea.

Best regards, seachange.

[seachange]

I sift a minimal firly even layer of clear powder between the sheets. SLIGHTLY heavier on the corners.

Hi Steve

The "SLIGTHLY heavier on the corners" part, is this to keep the top plate minimally elevated at the corners to aid the elimination of air?

Asking this in detail because all the work I am doing are circles. If this is the reason I'll test with four slightly heavier areas at oppossite points on the circle to achieve a similar effect.

Many thanks for your help, seachange

[S. Klein]

I would go SLIGHTLY heavier around the entire edge of a circle. If you can't see what is below the poeder, it's too heavy. You might want to do some tests to get the right amt of powder. I really use a very minimal amount.

[seachange]

I would go SLIGHTLY heavier around the entire edge of a circle. If you can't see what is below the poeder, it's too heavy. You might want to do some tests to get the right amt of powder. I really use a very minimal amount.

Many thanks Steve, I will surely test...very interested to see if I can master this

Best regards, seachange

[seachange]

Reporting back, test 1

Have just made a 7" round blank. The tekta is on the bottom, capped with egyptian blue opal. Sifted the clear powder over the tekta before capping, and there isn't a single bubble visible.

This is not the very best test, as this combination is probably less inclined to show bubbles than if one was fusing two layers of clear. Will report with this set up when I make one.

Had three goes at sifting such a thin layer, but finally managed, by increasing the distance between the sifter and the plate, and moving in a very regular way from left to right and back, instead of trying to follow the round contours.

Didn't manage to make a sightly thicker layer all around the edge. Tried a line sifter, didn't work well. Then tried sifting from up high, a little on to the plate, the rest on the paper, but it was still too much powder. I imagine the line sifter might be good to add just a tiny bit on each corner of a square plate.

Therefore fired with only the single even layer...have to practice more

Cheers, seachange