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The Ultimate Silver Metal Clay Firing Guide

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The Ultimate Silver Metal Clay Firing Guide YOU ARE HERE: Home (https://www.cooltools.us/) > Learning Center (https://www.cooltools.us/Learning-Center-s/772.htm) > Firing Guides & Schedules (https://www.cooltools.us/Metal-Clay-Firing-Guide-s/782.htm) > The Ultimate Silver Metal Clay Firing Guide (https://www.cooltools.us/The-Ultimate-Silver-Metal-Clay-Firing-Guide-s/1219.htm) Silver Clay Firing Guide Tucked inside every package of silver metal clay is a neatly folded sheet with basic instructions for using the clay type at hand. Included in the instructions is a firing table that lists (in devilishly small type) the various firing time and temperature combinations that the clay type can be fired at. For instance, the firing guide included in a package of PMC 3 lists the following firing options: 1110F for 30 minutes, 1200F for 20 minutes, 1290F for 10 minutes, and 1650F for 10 minutes. The insert from a package of Art Clay Silver 650 lists: 1200F for 30 minutes, 1290F at 15 minutes, 1380F for 10 minutes, and 1435F (and higher) for 5 minutes. How confusing! How do you know what combination to pick? The instructions don't really say. Minimum vs Ideal Here's the rub...those little firing schedules list minimum times and temperatures, not ideal times and temperatures for firing silver clay. The variety of times and temperatures for each clay type makes it possible to co-fire embedded items along with the clay, and it also allows for a variety of firing methods. What the guides don't mention is that there is a trade-off in the strength of the final product when firing at anything less than the optimum schedule of 2 hours at 1650F in a digitally controlled kiln. A common misconception about silver metal clay is that after firing it's solid metal, just like a casting, but it's not. Castings are made by pouring molten metal into a mold and allowing the metal to cool in the mold. The result is a very strong, solid metal object with very little porosity (porosity is the technical term for air spaces). But silver clay is not at all like a casting. Silver clay is sintered, and sintered objects are a whole other animal. The Sintering Process Sintering is a process where tiny particles of metal are brought to a temperature just below their melting point, a point where the metal is able to bond to itself. Sintered objects are always able to bond to itself. Sintered objects are always porous because the metal particles are not like blocks that stack together nice and tidy, they are various sizes and shapes, and they don't fit together perfectly. To visualize the structure of sintered metal, imagine a dish of lemon drops. Each candy is touching another one, but because of their shapes, they cannot make complete contact with each other, leaving open spaces between each piece. At sintering temperatures a bond is formed only where particles touch, so a fully sintered object is not completely solid even though it appears to be on the outside. To understand how porosity impacts the strength of silver clay, imagine those lemon drops again. Sugar can be sintered, just like metal, so if we heated up the dish of lemon drops to a point just before the sugar melts (when the candy is sticky and tacky), the candy will bond everywhere the pieces contact. When the candies cool, the sugar recrystallizes and the candies will hold together in a mass. Because of the air spaces the mass is weak and can be broken if it's dropped, bent, or receives a blow. It's the same with silver clay, but on a much smaller scale. When a sintered object breaks, the break is irregular and the inside appears to be grainy and is a lighter color than the outside. Silver clay objects have the same look when broken. Many mistake this typical breakage pattern for "underfired" silver clay when what they are Minimum Optimum actually seeing is the telltale sign of sintered Temperature/Time Temperature/Time metal. Time, Temperature & Strength Sintering does not happen instantly, which is why it's called a process. It takes a while for the metal particles to form a strong bond. The longer silver clay is held at its sintering temperature, the more fully the individual particles are able to bond to each other. When fired at the lowest temperature for the shortest time, the bond that is created is not nearly as strong as when fired at the ideal temperature and time. Here's an eye opening experiment to show how time and temperature affects the strength of a silver clay object: Roll out 2 identical strips of PMC 3, same length and thickness. Fire one at the minimum schedule of 1110F for 30 minutes. Fire the other at the optimum schedule, 1650F for 2 hours. Take the strip fired at optimum and form it into a ring on a ring mandrel with a rawhide mallet. Now you've got a dandy little ring. When you try the same thing with the strip fired at 1110F, it will crack and break as you try to fired at 1110F, it will crack and break as you try to form it. The difference in strength by firing times can be pretty dramatic, so it's important to understand that you are making a trade-off in strength when firing at lower than the ideal schedule. I find that it helps to think of silver clay objects as being made of ceramic rather than metal. We all know what to expect of a ceramic object, and silver clay is very much like this in its fired state. Most of us wouldn't give a second thought to lifting a heavy suitcase while wearing a sterling silver or gold ring, but we'd avoid lifting heavy things while wearing a ceramic ring. If you think of silver clay objects as being made of ceramic, you'll make them more sturdy in the first place and treat them with more care when you wear them. When to Lower the Temperature Lower the temperature and shorten the firing time only when you have to. If you want to fire glass, heat-sensitive gemstones, or sterling silver along with silver clay, the temperature and time will need to be reduced for these items to survive. This is where the various firing schedules come in handy. Use the firing guides to find the clay that is compatible with the item you want to co-fire, and use the highest temperature and longest time you can within that range. Teachers will often fire at less than ideal temperatures in a workshop or classroom situation because they have to budget their time to be sure your work gets in and out of the kiln and still leaves time to finish. When they have the luxury of time, however, they will fire at the optimum schedule. It's a good idea to re-fire at optimum anything that was hastily fired in a classroom as long as there are no heat-sensitive inclusions (glass, sterling, heat-sensitive stones). When firing at less than the optimum schedule, be mindful of the trade-off. Avoid delicate tendrils and lacy details that are not supported. These can be easily broken during finishing, tumbling or in daily wear. Beef up ring shanks and bracelet links. Reinforce connections so you have a larger area of contact on components such as jump rings, bails, and pasted on elements. Remember that thicker pieces are less prone to breakage. Consider multiple firings if you need to include something that requires low firing, especially in the case of rings or bracelet parts. Quick Reference Minimum vs Optimum Firing Schedules Schedules PMC Standard 1650F/2 hrs 1650F/2 hrs PMC Plus 1470F/30 min 1650F/2 hrs PMC 3 1110F/30 min 1650F/2 hrs Art Clay Silver Standard 1470F/30 min 1650F/2 hrs Art Clay Silver 650 1200F/30 min 1650F/2 hrs Art Clay Silver Overlay Paste 1470F/30 min 1650F/2 hrs Art Clay Oil Paste 1470F/30 min 1650F/2 hrs Maximum Firing Temperature The melting point of fine silver is 1761F/961C. You may find slightly conflicting numbers, but the fact is, at this temperature, give or take a degree, fine silver will be a molten blob. Obviously, in order to transform our carefully crafted treasures into silver works of art, we need to avoid anything close to 1761F or our hard work will be just a forlorn memory. At 1110F fine silver becomes "open" to fusing, but is cool enough to maintain its form. Up to 1650F, fine silver will fuse and still hold it's form, so this is why the range of temperatures for sintering all forms of silver metal clay is between 1110F and 1650F. Digitally controlled kilns will sometimes creep up above their target temperature and a rise of a few degrees above or below your target temperature is nothing to worry about. Kilns usually go just a little over their target temperature, but will fall more than that. If your kiln overshoots your target temperature by more than 20 degrees, check the thermocouple to be sure it's functioning properly because temperatures just a few degrees above this will show the early stages of melting and will be detrimental to your silver clay objects. Maximum Firing Time Each type of silver clay has a minimum firing time and temperature, but what's the maximum? We know the maximum temperature for all forms and types of silver metal clay is 1650F, but what's the maximum time? Technically, there is no maximum firing time.
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