Overcoming the 6mm rule in glass fusion

Question

I am trying to pull off glass magic. I am trying to take fine glass frit (System 96) and turn it into thin ~1-2mm plates without bubbles. I have been reading about the 6mm rule and how the glass will move to accomedate the surface tension. Since I am going for thinner plates my guess is it will run to the edges.

I wish to prevent this some how. Are there techniques to force the glass to stay in the thin plate? My best guess would be to put a top on the mold that restricts the flow of the frit.

I would welcome any assistance in this matter.

Thanks in advance.

Answer 1

The results may depend a lot on luck, practice, and trial and error. You might need access to a machine shop to prepare whatever will serve as the "mold".

I wouldn't count on using a process that requires trimming after the glass is cool. The glass can build high internal stress as it cools, and it may have a tendency to shatter if you try to trim it.

To some degree, you can work molten glass at a workbench; press it flat, etc. But your goal is to get it thinner than it wants to be on its own, which means flattening or molding it, and keeping it that way until it cools.

The thinner it is, the faster it loses heat through the surface. You won't be able to put a blob of molten glass on a room temperature metal surface, and press it with another room temperature piece of metal down to 1 or 2 mm thickness. It will lose fluidity or break before it gets there. If you want to press molten glass to your thickness outside the kiln, you would need some high temperature insulating material as the mold. The "practical" approach is to heat the mold along with the glass.

You also need to deal with the air between the frit particles. As the glass melts, the air needs a place to go or it will be trapped in the glass. So you probably can't start with the frit already contained top and bottom. That suggests melting the frit, then flattening it to the desired thickness.

So everything would go into the kiln. When the glass is melted, the top of the mold would be moved and pressed into place.

I can think of a couple of possible methods using this approach:

• Create a thin irregular shape, whatever that turns out to be. You might be able to do that with two large, polished metal plates ("large" being something larger than the finished object). The top plate would need to have some weight to it.

  Mound the glass frit on the bottom plate, and have spacers of the desired glass thickness in the corners. You might want some external guides to keep the top plate aligned once in place. After the glass melts, move the top plate into place on the glass, and press it down to the spacers. Maybe leave the assembly at full heat for a bit before starting the cool down process. Whatever "organic" shape the glass ends up is the result.

• Contain the shape in a polished metal mold with a heavy top plate that fits tightly within the sides when cool (when it's hot, the expansion will leave a gap for air to escape when you insert the top). Play with the amount of material to determine the thickness. The process would be similar to the first case, but the mold would control the shape when the glass is flattened to a uniform thickness by the top.

• I suspect you would need this to be somewhat of a hybrid of the two methods. The bottom portion of the mold may need to be a bottom plate with removable sides. This would be not just to get the glass out. A metal mold would contract more than the glass when it cools, which would mean that the mold would become smaller than the material it contains (see where I'm going here?). Unless the mold is a TARDIS (bigger inside than outside), I'm not sure the glass would survive the necessary change in shape to convert cross section to height. The process might require flattening the molten glass inside the mold, then removing the sides of the mold and using some form of spacer to maintain the height of the top. Hopefully, the glass would maintain its shape from the mold.

  Perhaps an "easier" way to accomplish this would be to start with the first approach, where you set the height between the top and bottom plates. Once the top is in place on the molten glass, use adjustable sides (which will be a known thickness), to push the molten glass into the desired outline, then remove the sides.

I've never done this, and don't know if, or how well, it would work. But the processes at least deal with the obvious issues, and provide a starting point for experimentation.

Answer 2

The simple way to keep the form thin is to under fire it. If you fire to less than a full fuse the tension won’t pull the glass up to the 6mm height. If you have fine fruit or powder you can use a binder to make a paste before forming and get quite a bit of control (see pate de verre). This will leave some texture that you may not find desirable.

To create a fully fused but thin plate I’d do this in two firings: one to create a solid blank out of the frit and another to squeeze it flat.

The first firing should be straight forward: I’d do the math to see how large a sheet I’d get at 7mm with the weight of frit I wanted to use, Dam off a square that size, then pile in the frit and full fuse. I’d probably use a sheet of clear as a base rather than using all frit, but it depends on your specific design goal.

For the second firing use the “kiln pressed glass” technique to flatten out the blank. Put your blank in the middle of the kiln, put 1-2mm fiber spacers around the edges, and put a kiln-washed shelf on top of the blank. Weight that shelf with fire brick. Fire to a full fuse with enough of a hold (probably several hours) for the shelf to squish the blank flat.

There is a tutorial on the kiln pressed glass technique at https://fusedglass.org/learn/project-tutorials/kiln_pressed_glass