The one thing you need to know about firing silver metal clay is that it is fine silver. Fine Silver’s melting point is 1761°F or 960°C. As long as you stay at 1700 or below, you will not change the shape of the silver piece. However, if your kiln is firing hot, or over firing, your kiln could be over shooting this temperature as much as 20 degrees. This is why the manufactures of metal clay chose a temperature well below any temperature that would risk you melting out your piece.
So the manufacturers, AIDA and Mitsubishi, have set the goal temperatures at 1600°F and 1650°F respectively.
The PMC Guild in cooperation with Mitsubishi has posted several tech reports on www.pmcguild.com. We recommend joining the guild to receive the four tech journals and the gallery book, PMC Annual once a year.
One report, the Relative Strength Chart, shows the 3 types of PMC being fired and tested for strength at 5, 10, 20, 30, 60, 120 minutes. There is noticeable strength difference between firing at a higher temperature for a longer period of time.
We recommend firing all the silver metal clays at 1650°F for a minimum of 30 minutes. If you have time, fire for 2 hours. The difference between firing for 30 minutes and 2 hours is about 5% in strength. However, firing for the top strength is optimum in every situation. You want the silver metal clay to be as strong as it can possibly be.
Why? Silver metal clay is fine silver. So not only is it soft but because of it’s structure, it’s brittle. Remember it is not melted and cast and then fabricated. It is microscopic particles of silver held in an organic binder. This creates space in between the silver. Because it is not melting but fusing or sintering, these particles are not becoming homegenous, but instead are “glued” together. If they are not bonded or sintered fully this can cause breakage and even weak spots that can later break.
Because of this structure it is more brittle than fine silver that has been fabricated or milled. You cannot hammer or bend it as much as you could traditional fine silver without annealing occasionally. Think of it as a heat massage. The workout for metal clay is much harder than the same workout for traditional fine silver. So you need to give it a heat massage must sooner to relax those crystals so you can continue to hammer and bend it.
If you crack the metal, the nice thing is with silver metal clay it can be repaired and refired. The one thing you want to know about multiple firings with silver metal clay is the Theory of Diffusion. A good analogy that Tim McCreight stated, is to think of canvas versus fabric. It takes much longer for water to seep through canvas. Metal clay is much the same. If you add a clay repair or an additional piece of clay to an already fired piece, by firing it for 30 minutes you may sinter the new clay, but the previously fired clay and the new clay will not be molecularly bonded. By firing for a minimum of an hour you are allowing the molecules to intermingle and sinter or bond.
So we want to fire metal clay the highest temp so the binder is completely burned out and it’s as dense as possible. The metal is not fully sintered if you do not fire hot enough or long enough.
When do you not fire to 1650°F? I only make an exception when I need to fire something that cannot be fired at 1650°F, for example a glass inclusion. I will try and fire the piece at 1650°F for 30min to 2 hours without the glass. Then incorporate the glass and refire to 1200 for 45 minutes to an hour. You can certainly fire for longer if desired.
The reason the manufacturers have on the packaging to fire to a lower temperature is that they are trying to make it more accessible to those that do not own kilns. Is is questionable whether someone can torch fire a piece properly without experience. It’s one thing for Tim McCreight, an experience metalsmith with torching experience versus a hobbyist to torch fire their metal clay pieces to a fully sintered state. There are what I call human errors involved. It’s about learning to see the right color of the metal when it’s at the right temperature and holding it for the right amount of time. This is an experience issue.
I don’t believe its in the best interest of the final product to fire lower or for less time. I understand the manufacturers desire to sell more product and make it more accessible, but not at the cost of the final product being made properly. To send masterpieces out into the world to be enjoyed, when they end up broken doesn’t send a good message about our medium. It’s up to all of us to put our best work out there.
There are other considerations also when making your piece that will effect strength. One is learning to wedge or compress the clay particles prior to rolling it out. If you roll the clay into a ball you are folding in hairline cracks and air pockets. By wedging or pressing the clay you force the air to the outside and you get rid of cracks. This is a technique used in pottery because their medium also sinters and does not melt into a homogeneous state. So they take great care to expel the air and get rid of cracks. Metal clay is no different. You can find an example of this wedging technique on my video on how to start with BronzClay.
When you are finished cutting your piece from the slab you rolled out. Instead of picking it up and folding it into a ball, cut strips from the slab, stack them and then pinch it into a ball or oval shape. This will eliminate air and cracks.
If you see a hairline crack in your slab, do not continue to make your piece. Strip, stack and add a little bit of water. Then roll it back out again.
Lastly, use the right firing equipment that has been properly maintained. Do not fire on cordierite or “hard ceramic shelves”. These are a heat sink. The heat inside the kiln goes to the shelf and not to your piece until it is at temperature. They get so hot that it can also cause slumping during firing. It is best to fire on a Hard Fiber Shelf.
Don’t forget that every time you fire your kiln the thermocouple oxidizes or sheds a layer of metal. This results in a thermocouple that does not fire perfectly and with each firing will be a little farther off accurate. In most instances the kiln will fire 5-20 degrees hotter.
I recommend keeping a journal of your kiln firings. This way you’ll know if it’s time to test your kiln, replace the thermocouple or when it was maintained last. This is similar to maintaining your car. A must for creating perfect artwork!