Thank you Paul. What you
Thank you Paul. What you have depicted is EXACTLY the method I use in all my firings for thicknesses greater than 1/4″. This method worked very well to fuse the crystal frit into a 1/2″ slab at 1500 F. I used 1/8″ fiber blanket and first cut a piece to cover the kiln shelf. I used pieces of 1″ thick kiln shelf cut into 1″ wide strips to form a box on top of the underlying fiber blanket. Then I cut 1″ wide strips of the 1/8″ fiber blanket to line the inside edges of the box. I cut and fitted the peices carefully so there were no gaps or holes at the seams. I have found that this method ALWAYS works for full fuse firings.
But when I fired the crystal to 2100 F, it did not contain the glass. Glass ran through the corner seams and glued the ends of the shelf strips together. Glass ran under the bottom seam, across the edge of the underlying fiber blanket, onto the kiln shelf, and over the edge onto the floor of the kiln. It was as if you had poured a cup of water into the box. Well, not quite as dramatic, but there were multiple leaks. This was in spite of the fact that the whole containment system was carefully constructed so there would be no gaps in it.
At full fuse temperatures glass has a fairly high viscosity. It will move, but it moves slowly. It has enough surface tension to generally hold it in place within the dams. If there is a gap in the fiber blanket lining, it will enter that gap and form a ridge on the edge of the glass, but generally it will not flow through a 1/16″ gap in the 15 to 30 minutes time at full fuse. Even at raking temperature, 1650 F, the viscosity of the glass is fairly stiff. If you have ever pulled a rake through it, you know just how stiff the glass is at even that temperature.
We know that different compositions of glass have widely varying viscosities at different temperatures and generally the higher the temperature, the lower the viscosity. Your assessment that the copper diffusion may be changing the composition of the lead crystal is probably correct. My guess from observation is that it dramatically lowers the viscosity. I have also observed that the copper tends to sink to the bottom. Partially diffused copper on the bottom is EXTREMELY caustic. It eats right through the fiber blanket and into the mullite kiln shelf. The effect is like dropping a drop of vinegar from an eye dropper onto a layer of baking soda.
Through a series of disastrous experiments I have learned to control the thickness of glass and copper penetration (sinking) based on particle size and distribution so that each particle is completely diffused into the glass before it reaches the bottom and starts eating the underlying fiber blanket. I have also learned to keep copper particles at least 1/4″ from the edge of the crystal blank for the same reason.
When the copper particles sink into the glass, it is not a straight descent. From the diffusion traces they appear to swirl in random paths. This is what creates the beautiful color effects I am seeking. But it is quite possible that some of these particles do get to the edge where they would eat holes in the fiber blanket. That may be a contributing factor to some of the leaks, but I don’t think that is what happened in the last experiment. I did not have the characteristic “knots” on the edge or bottom of the tile where this has happened in earlier experiments. The “happy” factor of the last experiment is that I think I have finally developed a good recipe for glass thickness, particle size and particle distribution that will create the exact effects I want.
The “unhappy” factor is containment. I had 4 leaks that each dribbled out about one to two teaspoons of molten glass. There were another 3 leaks of about 1/4 teaspoon that ran under the mullite dam strips, but did not make it to the kiln shelf. My best guess is that the viscosity of the glass with the added copper at 2100 F is about that of hot honey. That is pretty runny, and acting more like water than molasses (which is how glass acts at fusing temps).
That, Paul, is why I am “complicating” things and going to a fully contained mold. If anyone has ever tried this, they are not admitting it. And they probably gave up on the idea following 2 or 3 disasters, destroying a couple of kiln shelfs, assorted kiln furniture, and suffering a week of down time to repair the kiln. I’m not bright enough to accept the fact that “it cannot be done”. I have suceedded in achieving the color effects I envisioned when I started this with the last disasterous firing. I am so close to ultimate success that it has heightened my excitement and I just can’t abandon the quest. I have to go on.
Thanks for all your help.
–Joseph 2bears, Lomita, CA