Kiln Firing, Sintering and Silver Metal Clay
Updated: Sep 4, 2019
It seems the bigger the equipment in the jewelry studio the more intimidated newcomers are of that equipment. But you need not be afraid if you test your kiln and the clay you will be using first to get to know your kiln and how it behaves. Once you perform these tests you are good to go.
And when I say test, I don't mean putting the masterpiece you spent hours on in the kiln to see how things turn out, I mean test small simple earring components not giving you the same angst of failure. I figure if I mess up, no big deal and if I don't mess them up, well I have a nice pair of earrings. But why test if the manufacturer's instructions have a given temperature and a percentage of shrinkage right on the package? Because each kiln is ever so slightly different. Temperatures on a kiln can vary, and even new kilns at high mountain levels for example, may need to be adjusted for the elevation difference. Also overtime a kiln may need to be adjusted. For instance, my 10-year-old kiln needs 10º more added to the temperature to register as accurate. The only true way to know the shrinkage of a given brand of clay is to measure your before greenware and after fired piece.
It’s all about the sintering. Understanding sintering basics: Metal Clay is made up of particles of fine metal, and an organic binder, and water. The binder and water is what allows the metal to be flexible and moldable. It is the ability for the binder and water to be removed, and the remaining metal particles to sinter together and form a solid piece of metal, albeit a porous metal — this is the magic of Metal Clay. The binder and water are removed with intense heat, either by the heat of a torch or kiln and other devices on the market. Next, the small particles of metal sinter together. My definition of sintering is to form a coherent mass by heating without melting. In this case, joining particles of metal to create a solid mass.
Key Concept: Fire as hot as you can and as long as you can allowable "for your project" for the strongest most durable end product. For fine silver that is1650°F for 2 hours (PMC3, Art Clay Silver, Cool Tools FS999, FYI Silver, and Metal Clay Powder*. (Other clays may have not been tested or have different schedules, and other unknown properties - Prometheus is a good example as it's highest temperature is 1292º F. It bubbles at 1650ºF, which scientifically speaking baffles me as it is stated to be pure fine silver)
The sintering of the metal is the key element in the process. At the lower temperatures of the firing schedule, the silver particles sinter together, but it is NOT a full sinter, leaving microscopic spaces. At higher temperatures, the metal DOES fully sinter and becomes a more dense material, thus there is more shrinkage at a full sinter as well. Ok, what does this mean? This means at all levels of firing on the firing schedule you will get a hard piece of metal, however at the lower temperatures the metal will not hold up to the same amount of abuse, wear and tear, and stress as a piece fired at the higher temperatures. Pieces fired at the highest temperature not only fully sinter, but the metal can actually gently bend without breaking. There are reasonable uses for some of the lower firing temperatures such as glass, gems, and fire-in-place inclusions, but the usage of the final item, construction, and design should be considered. For instance, earrings and pins get less wear and tear than rings, so firing at low temperatures for a ring would not be prudent. Also, delicate parts or construction like filigree or prongs need high firing also. In fact, I never fire at the 1,110°F firing schedule since my “personal” feeling is the strength is compromised. I hear too many reports of breakage and have experienced some myself in my newbie days way back when.
We were told firing at 1,650°F for 15 minutes is almost as good as firing for 2 hours, is this true?
There had been tests conducted by the PMC Guild when they were in existence suggesting when firing at 1,650°F for 15 minutes compared to a 2 hour firing, there was a small advantage to firing for 2 hours, but the difference was very small - a 2% difference I think it was. For this reason, teachers may choose to use the shorter firing time during a class setting without compromising the strength of the given project due to time constraints.
So Why A Full Firing Schedule?
There are factors that need to be considered before selecting your firing schedule. One firing schedule does not fit all projects:
• The Sturdiness of the design. Is your project delicate with filigree and very thin parts and appendages? Is it sturdy enough to hold up to wear and tear or should you fire at higher temperatures to avoid breakage?
• Manipulation and use of the end item. Will the piece need to be worked further requiring shaping, bending, or hammering? Will it get a lot of abuse such as a ring or a bracelet would?
• Are there inclusions? Are you firing in place glass, gems, or inclusions that require a lower firing schedule? If so, you may need to alter the clay thickness, design of the piece, and consider the stress on its end use. A new sturdier construction plan may also need to be considered.
Let me again emphasize that all projects should be fired as high as they can with consideration of the inclusions for the strongest most durable results. Check out this Metal Clay Comparison Chart which lists the firing temperatures of ALL available clays. This is a public document kept up to date by manufacturers and other contributors https://tinyurl.com/y62tl5m4
*Please note: Some of the metal clay manufacturers forget to mention the 1650ºF temperature for Fine Silver in their written pamphets.
If you like this article you will love my classes selection. They are packed with design tips, techniques, and information so you will get the best out of your Metal Clay experience.
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