Wednesday, July 13, 2011

Glass, Heat, & Metal Clays

by Mary Ellin D'Agostino
Technical Advisor

This week I am at the Metal Clay World Conference giving a talk on mixing metals. I will post about that next time, but as long as I am in the mixing mode, I wanted to discuss mixing other stuff with your metal clay creations. I get asked a lot about combining glass with metal clays.

Glass and silver clay is a beautiful combination and can work very well because glass and silver have very similar Coefficients of Expansion (COE). That means they expand and contract at similar rates and temperatures. Alas, they do not expand and contract at exactly the same rate, so some care needs to be taken when combining glass elements in a silver clay piece.

First of all, you need to know that glass needs to heat and cool slowly so that it won’t crack. Everyone has a different favorite firing schedule and you have a fair amount of leeway when firing small pieces of glass, but don’t expect to be able to have a happy piece of un-cracked glass if you heat on a full speed ramp and pull the kiln door open to cool the pieces rapidly.

Above: Dichroic glass and fine silver pendant by PMCC Senior Instructor Marlynda Taylor

If you are firing at a very low temperature (say PMC3 at 1110°F) and the glass doesn’t soften (start to melt), you are in great shape! The silver will shrink around the glass and hold it in place as long as your silver is well constructed, with no bad joins or cracks that might open up when it shrinks around the glass and can’t shrink any more in that direction. The clay will thin out or find other directions to shrink, but if there is a hairline crack, weakness, or poorly joined seam, the clay may use that weakness to shrink in that less desirable way. You can often fix these cracks by filling with more silver and re-firing, but sometimes it takes more than one round of repairs. The really low temperature firing is great for the glass, but the silver is not very strong. Be sure to hold the kiln temperature at least 45 minutes (I usually let it go for 2 hours) when firing at 1110°F so your silver will be stronger

If you are firing a little hotter, to the point where the glass surface gets tacky and fuses to the silver, you will have a greater failure (cracked glass) rate, but a stronger silver piece. At slightly hotter temperatures, 1200°F-1300°F, the glass gets a little tacky and fuses to the silver. At these temperatures very thin layers of slip painted silver or syringe work will fuse to the surface of the glass and allow you to create a lot of cool design effects. This does, however, put stress on the glass and if it is too much stress, the glass will get a stress fracture (it will crack). There are many factors in what does and does not work. I really recommend taking a class or trying some of the excellent projects available in books and online before branching out on your own for this technique.

If you are taking the glass to a full fuse (melting it), you have to be even more careful. The guaranteed success method for doing this is to make an open-backed frame around the cabochon that fits a little loosely after drying and is perfectly smooth on the inside where it touches the glass. This ensures that the glass is stressed evenly around its edges and minimizes the chance that it will fracture from the stress. We used to use this method a lot before the low fire clays were available. The percentage of failures using this method is a lot higher.

What about copper and bronze? Copper has an expansion rate a lot closer to glass than silver, so it makes a really great fit. Glass artists have been fusing copper with glass for years. The problem with copper and bronze clays, of course, is the carbon firing method. Open firing won’t work with glass because the glass needs a slow heat and cool and the open firing for copper usually involves quenching (really rapid cooling) and is guaranteed to crack your glass. Glass and copper can be combined and fired in carbon if you can come up with a good way to keep the carbon from getting to the glass. Attempts to come up with a good technique for this usually involve copper mesh and ceramic fiber blanket or thinfire paper. I don’t think anyone has worked out a really good technique for this yet.

Bronze will probably work well with glass because it is 90% copper, but I haven’t gotten into researching how it interacts with the glass yet.

I hope this answers some of the questions about glass and metal clays. Post questions if you have them.

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