How can one leave internal void cavity in fused glass piece

I am trying to make a solid fused piece that would have a shape in the center. I suppose you could make the shape in the glass with a fuse step and then tact fuse a top and bottom on to it. I am worried about losing some features and having the glass collapse.

Is there any way to do something like this in a single step? tube running through it

• What do you mean by "fused"? – bgmCoder Dec 10 '19 at 19:12
• @bgmCoder I would start with glass frit and fuse it into the final shape. I would like to have a final thickness of 2mm. I am worried about getting around the 6mm rule with glass – TheCodeNovice Dec 10 '19 at 20:10

3 Answers

I've never tried this, so this answer will be largely speculation based on the physical properties.

You can leave a void, but I don't think you would be able to make the void a complex or delicate shape. There are a few ways to start with a well-defined void: engrave it into one or both mating faces of two pieces; start with three pieces and cut the shape out of the middle piece like a stencil; make the shape using a glass tube and seal the ends to trap the air, then embed the tube in a bed of glass dust and fuse the bed (this wouldn't be applicable for a 2 mm final thickness).

With all of these approaches, glass fuses by becoming a liquid and flowing. The molten glass will flow in a way to minimize the surface area of the void. It will fill thin areas first, forcing any trapped air into bubbles, so you will lose the shape.

You could embed a specific shape in fused glass by making the shape out of a material that melts at a higher temperature. However, that wouldn't be a void.

The only way I can think of to have a void of a specific, desired shape is to not fuse the glass. Use one of the methods, like mentioned above (e.g., engraving or stencil), to create the void, then bond the pieces together with clear adhesive that bonds glass.

If the goal is the appearance of fused glass, another approach would be to fuse the glass. Then engrave the shape in the back side.

Another idea that might come close to your objective. Create the void as in the gluing approach. Fill the void with fine dust of colored glass. Then fuse the glass. Colored glass dust would melt, and it might hold the shape, and at least be a contrasting, embedded shape.

• Thanks for the proposed approaches. Say I relax my requirement that it is completely trapped and I allow for a single access port. Is there a way I could have something that preserves the geometry and then dissolve it out after firing? – TheCodeNovice Dec 10 '19 at 21:54
• @TheCodeNovice, interesting idea. Let me check out materials that might work for that. If there is any complexity to the shape, getting solvent in and washing it out might be a challenge. Let me think on that. – fixer1234 Dec 10 '19 at 22:02
• @TheCodeNovice, I suspect identifying a material for your idea would be a good question for the chemistry site. You would need a material with a higher melting point than the glass, that's also chemically active with chemicals and equipment you might want to mess with outside a lab, and it would need to be something where neither it nor the solvent affect glass. It wouldn't necessarily require a solvent, it could use a chemical reaction that turns it into something that dissolves in something that doesn't affect glass. – fixer1234 Dec 10 '19 at 22:40
• If you could make your form from calcium carbonate (real chalk, with a melting point just over 1300C, but not most blackboard chalk which is gypsum) you could dissolve it in a household acid. Vinegar may be too slow, but domestic limescale removers should be safe on glass for long periods. Lime mortar (without sand filler) may also work and could be formed wet – Chris H Dec 12 '19 at 16:05
• @ChrisH Thanks for the suggestions. I have access to acids as I work in science lab. How would i make these into a paste to fit a mold? Do I bake them off? I am going to give this a go. – TheCodeNovice Dec 14 '19 at 16:19

A fully encapsulated void in thin glass is difficult to impossible to do hot in a kiln. A carved out indentation open on one side is trivial. You could the encapsulate it by cold-fusing (i.e. gluing) another sheet of glass to the back.

For a design such as you show I’d kiln carve it by cutting the design out of ceramic fiber paper, placing a double sheet of glass over the fiber paper in the kiln, and firing to a full fuse.

Trying to tack fuse a cover into this will have problems because the tack fuse temp is high enough that the glass will move and distort the carved pattern, and because trapping air in the pattern is likely to cause bubbles. You can avoid these problems by filling the pattern with enamel or powdered glass to form a sold block. Either way will end up thicker than the your 2mm final size by default, but you could create a blank for pressing this way. The pressing will likely distort the image some.

Bullseye has a TipSheet on kiln carving at https://www.bullseyeglass.com/methods-ideas/tipsheet-1-kilncarving.html

Another option (similar to previous suggestions of engraving or acid etchjng) would be to sand-blast the pattern. It really depends on the effect you’re looking for and the tools you have available. You could get your single layer look by sandblasting, filling the sandblasted cavity with a low fire enamel and then firing to a temp that’s high enough for the enamel to mature but low enough that the glass doesn’t deform.

If I understand you correctly, you just want to cut out the hollow of that shape in your glass square.

I would just use a Dremel rotary tool with grinding bits and simply carve out the design. I've carved glass before with a Dremel - it's slow going, but it works. You'll need different grade bits, starting with coarse and then with finer grits, finishing with rubber polishing tips to get it super smooth.

But with the dremel, you can use whatever kind of glass square or shape you want.