There are many opinions about how to properly fire your metal clay. Today I am going to give you my OPINION about firing your silver metal clay and some tips that may help you out.
First, silver metal clay is made up of a binder, water, and fine silver particles that when fired the metal sinters and binder will burn off. The process of sintering is "to cause (ores or powdery metals) to become a coherent mass by heating without melting". So in this case we are heating our fine silver to below its melting point to bond together to become solid metal. The way I understand this is that when metal clay is fired it is made up of a mass of silver particles bonded together.
Now in order for silver metal clay to properly sinter two factors need to be taken into account: temperature & time. Both these factors are important in to making sure the metal sinters properly. The metal clay need to reach at least a minimum temperature and it needs to be held at this temperature for at least a minimum amount of time.
There is a philosophy out there that in order for metal clay to sinter properly you need to fire the clay at 1650F for 2 hours. I agree with this philosophy in most part - I do believe that you will get the best possible sintered metal clay if you do this but other methods are options although not ideal and I will explain this as I write. (by the way there are other methods of firing metal clay which I am not touching on today)
KILN FIRING SILVER METAL CLAY
No matter what brand of metal clay you use the manufacturer will recommend firing times and temperatures. For example a Art Clay Silver 650 (low fire clay) it is recommended that a 5 gram piece could be fired at 1200F for 30 minutes or at 1475F for 5 minutes. Hmmm...these manufacturers recommendations can seem confusing as to why I said earlier that I agree that you should fire your metal clay at high temperatures for long periods of time.
The main reason I believe that longer at higher temperatures can give a better product. If I fired my metal clay piece as recommended and then fired it again for longer at a higher temperature the second firing may very well result in the piece shrinking more. This example was highlighted in a class in which I was a student that I took quite a while ago where a student made a ring and the instructor had fired it for the minimum requirement and when it came out it was too big for her, the instructor proceeded to say well lets fire it again and it will shrink. I was a student at this time and was trying to figure out the whole issue of firing metal clay and this was a great example. IF a piece is fired at the minimum temperature and them is re fired and it shrinks more then the increased in firing has made the metal to bond much closer and in my opinion giving you a stronger piece.
There is an extremely detailed explanation of firing guide from Cool Tools that will give you even more explanations The Ultimate Firing Guide
Unfortunately it is not always ideal to fire at high temperatures or for long periods of time (examples of this are when you are firing certain stones, if you include sterling silver or glass). Hopefully after reading the above reference and what I have said will help you make up your own mind.
TORCH FIRING SILVER METAL CLAY
So now on to torch firing silver metal clay. After explaining the above what about torch firing? Do I think torch firing is sufficient for metal clay? Yes and no, will be my answer. One of the joys of metal clay is that it brings people who may not otherwise find there way to working with metals. The supplies for silver metal clay, if you don't purchase a kiln, are affordable and people can make some great simple pieces. Traditional metal work is labour intensive and intimidating to many. Metal clay has it's challenges as well but simple roll and cut charms are manageable for many. Torch firing pieces for those who have no access to a kiln can make it all possible. Torch firing is deemed an acceptable means of sintering your metal clay and hopefully when you are finished reading this you will be provided with enough information to make your own decision.
I will not got through a tutorial on how to torch fire metal clay here instead I will refer you to this decent description Art Clay Silver Torch Firing Instructions. I will highlight that not everything can be torch fired, this reference will give you most of the details. Briefly, the size of you piece (if it is larger than a silver dollar, or if it is large than 25 gram also if you include other things in your metal clay such as glass or organic materials they shouldn't be fired. Refer to this reference for more details please I have not included everything here. One thing that is missing from this reference is that when torch firing pieces that are shaped such as a domed item they may flatten or loose their shape, I don't know all the limitations here as I rarely, if ever torch fire these days but I know that domed pieces can flatten so be aware (a solution to this is to place the dome so that the flat edges of the dome are on the block and the dome is up, then the friction of the edges on the brick will reduce the pull of gravity on it and will lessen if not eliminate the flattening).
Torch firing has many downfalls and I do believe many people do not do it properly, which reinforces the issues and questions as to whether torch firing is sufficient to properly sinter.
Remember earlier that I said that proper sintering requires both time and temperature to be correct in order for torch firing to work. When you torch fire it is difficult to measure both of these items. The temperature you need to determine by visual changes in colour and the time well you need to time it properly both these things can become problematic. Add on the factor that you may accidentally melt your piece and well torch firing can be tricky for the beginner.
Temperature is determined by the colour of the piece. It is often described as a salmon glow. This is where you need to keep your heated. Often you need to move your heat source to adjust the temperature...if it becomes glossy red it will start to melt.
Timing is initiated once the piece is the salmon colour (the colour indicating you have reached the proper temperature). You only start timing once it has hit this temperature and you need to HOLD the piece at this temperature for the recommended length of time. If any situation arises where your piece slips to being more white or you have to stop in the middle such in the case of running out of gas, YOU NEED TO START OVER AGAIN. The piece needs to be held for a continuous amount of time not cumulative. Hopefully this makes sense. The amaunt of time you hold the piece depends on the size of your piece(there is a list on the Art Clay reference) but my philosophy is to firing it at at the end range of the recommended time.
Tips to avoid melting: Torch fire in low light, it is hard to see colour changes in bright light. Set a countdown timer beside you when you are torch firing that you can start the timer when the piece reaches the appropriate colour so that you don't have to be glancing at you clock and taking you eyes of your piece while you are firing it.
I think that in the situation of a hobbyist who has no access to a kiln and understands the concept of torch firing and it making simple piece that won't get a lot of stress and strain then torch firing although not ideal may work for them.
In the situation of someone who plans on selling their work and is making rings, bracelets and more then they should SERIOUSLY think about kiln firing their pieces.
Now to answer the question. What do I do to fire my silver metal clay???- I tend to fire at 1600F for 2 hours and I tend only to put fine silver findings in my metal clay and use manmande gemstones that can take the heat. Rarely do I stray from this. Hopefully this gives you some information to make your own descisions.