Posted by Delia Marsellos-Traister
I personally enjoy placing stones in my work. I am a fan of agates, opals, and other ‘don’t you dare put that in the kiln’ stones. I am a bigger fan of ‘fire in place’ gems. But I’ve not taken the opportunity to test fire a lot. In fact, I don’t actually ‘test fire’ anything. I have a lot of knowledge about gemstones (I’ve been a gem dealer since 1994), so I’ve simply set stones in metal clay and then found out what would happen.
When placing a stone in metal clay there are several criteria I consider:
- Will it fire okay, at what temp, how long, in the kiln only, is a torch okay, and is it quenchable?
- Will the material have durability (which is often an element of how it is set, and exactly what the stone is); and does it hold up under use or when someone drops or bangs it against a file cabinet.
- How lovely it looks, the variety of colors I can choose from and how much it costs.
Some of my favorite gems for fire in place work are Cubic Zirconium [CZ], Yttrium Aluminum Garnet [YAG] and Corundum, aka sapphire and ruby (both laboratory grown and natural).
Cubic Zirconium (CZ). Does it fit my criteria? Sure.
CZ has a base of zirconium and was originally developed for laser use. It’s REALLY pretty and comes in a wide range of colors to play with, has a similar dispersion to diamond (dispersion = how bright and shiny it is) and a beautiful refractive quality (how it responds to light or how sparkly it is).
It’s been my experience that some green and yellow CZ’s might cloud during firing. This has been especially evident when they are fired in copper and bronze clays. Some CZ 's may show abrasion after being rubbed up against things like, for instance, other jewelry in your treasure box. I believe both of these issues may be dependent on where you get the stones. (Editor’s note: CZ’s are made by many companies each of which have a proprietary chemical recipe. Some stones fire perfectly, some not so much. Black cz's are notorious for changing color in the kiln. Which is why you should always buy lab grown stones from retailers that have tested them for use in metal clay.)
According to metal clay instructor Terry Kovalcik it may be the phosphorus in some base metal clays that's causing cloudiness. One expert recommends using magic marker to avoid this anomaly: “You can prevent the cloudiness of CZ's (and probably YAG's) by coating them with magic marker or India ink prior to firing in the copper clay.” (editor's note: Some people on the Yahoo! board have reported success and some failure when trying this trick. Again, this might be due to the exact stone used.)
Here are my suggestions when using CZs:
- Avoid using CZ with copper or bronze clays unless you have tried one of the above methods and found it useful.
- If you enjoy torch firing, be prepared to throw a fiber blanket over the piece when you're finished to allow a slower cool down.
- Avoid quenching a piece straight from the kiln. If you are really hot-to-trot, leave it in the kiln until it’s about 900 degrees and quench in warmish water.
Now, let’s take a look at Yttrium Aluminum Garnet (YAG).
Many of us are familiar with both aluminum and garnets, but what the heck is Yttrium? It’s a rare earth mineral, also a metal. Like CZ, YAG was originally developed and manufactured for laser use.
I like it because of its dispersion (brightness) and durability, and overall it fires in place well in silver metal clay - up to 1650ºF for an hour (or two if that’s the way you like to fire). YAG wears well, with little to no abrasion seen over the time of wear.
- Avoid setting YAG in copper or bronze clays, again unless you’ve tried techniques that have worked. It has been a disappointment for me to have both yellow and green YAGs fracture.
- YAG stands up to quenching and quick cooling in silver clay! But use room temperature (or slightly warmer), not cold, water.
- If you enjoy torch firing, go for it. Let it cool to room temperature on it’s own.
- (Editor's bullet point: If you want a true emerald green color - you can't do better than YAG. It's a bit pricey, but it's gorgeous and fires perfectly.)
Okay, last but not least, my very favorite fire in place gem - Corundum (sapphire and ruby) fits all my criteria! Laboratory sapphire has also been manufactured in rods for laser use!
I use faceted laboratory grown stones more often because they’re far less expensive than natural sapphire and have no inclusions (flaws). Lab grown corundum has the same physical and chemical properties as natural corundum. Corundum is made of Aluminum Oxide, which creates “white” or colorless sapphire. In nature, trace elements create the colors you see. Natural sapphire has about 400 color variations.
In my personal experience, all corundum can be safely fired in place in silver, copper and bronze clays at all of the popular firing temperatures. And you can always find plenty of information by simply overwhelming yourself with a Google search to find out if your stones will fire well in the exact type and brand of clay you're designing with.
Corundum can be kiln fired, torched and quenched. More durable than a diamond, it is very useful and valuable. When I say ‘more durable than a diamond’ I’m speaking about commercial grade diamonds that have fracture points (referred to as cleavage) that may cause the stone to break apart on impact. Sapphire has no such cleavage and in fact sapphire powder (grit) is what is often used to polish diamonds.
So there you have it. Some tips, ideas and information that you can draw on when you’re making a choice about what stones may work for you. Have fun, try everything!