First, I am no expert. Second, I am a curious learner who seeks to understand the basics of metal properties and how things work. I think that starting at the very beginning of any process will benefit the long term of what I am creating and producing. This leads me to dig in and find details that otherwise there is no time in a jewelry making workshop to cover.
I'll start with my understanding of sheet metal and the process of using it to create jewelry.
Sheet metal is usually created by melting metals & blending alloys that are turned into an ingot and then milled several/dozens of times through a rolling mill. This process creates thin sheets of metal which has been melted, milled, heated, milled again, heated again, milled again. This sheet is very strong after this process. The heat and compression drive the molecules together & give a very solid consistent metal matrix. You can purchase milled sheet in dead soft (annealed), half hard (work hardened slightly) & hard. Most often I have used dead soft because it's the easiest to cut with a jeweler's saw and it will bend easily. The other types are used for super delicate fret work (using a jeweler's saw) or for applications where the metal needs to be stiff in order to achieve an outcome.
Silver alloys (meaning fine silver that has been mixed with copper, nickel, germanium) are what makes the metals .925, .950, .960. It's the percentage of metal that is alloy. So .925 is 92.5% pure silver, .950 is 95%, etc. Here's why this matters- the percentage of alloy increases oxidation when heating. So, when you heat .925 the copper/nickle will turn black and this is oxidation. The copper or nickel alloy creates a layer of oxidation and that will limit the union of your solder or the molecules in sintering. Oxidation occurs with heat and oxygen. So in soldering with alloyed sheet, brass or copper, flux is used to combat oxidation by limiting oxygen/oxidation from getting on the metal near your soldering joint.
In metal clay my understanding of the process is this. Powdered metal, in many cases fine silver, is mixed with an organic binder. This binder gives the clay it's consistency and it separates the fine silver molecules from each other. With texturing, cutting, rolling there is some compression of the fine silver molecules but the binder is still in the matrix. As the metal clay dries the water is pulled out of the binder and slight shrinking occurs which then compresses the metal matrix again. If you use a burnisher prior to firing this will also compress the metal molecules. When we put fine silver metal clay into the kiln firing at full ramp it burns off the binder (shrinking). The combination of atmospheric pressure along with heat will move these molecules toward each other and they will lay closer to each other or join. Soak time is essential in full sinter. Temperature is crucial but time at that high temp (1650) allows the complete bonding of the molecules. Some porosity is still there even with full sinter. I'll attach a photo that Paddy Marcotte put up on Metal Clay Now and the article that is on AMCAW about sintering. Full sinter can only be achieved in a kiln. I know this is a bit controversial and I am certain that there are serious torch firing experts who will disagree. With all my research it points to consistent temp of 1650 degrees and 2 hours at that temp for full sinter.
This helps me understand using metal clay, the applications and the use of alloyed metal clays and base metals. Because oxidation impedes the joining of metal molecules, coconut carbon is used to limit the oxygen on the base metals and the highly alloyed metal clays like sterling or .925. However, the sweet spot is allowing some oxygen to burn off the binder. This is why some are open shelf burn off on slow ramp then put into carbon. I don't have much experience with base metals or with .925 clay and it is a real science along with being a study in patience. Fine silver is 99.9% pure. It has very little alloy and resists oxidation. This is why at high temperatures it can bond easily with minimal oxidation. Fine silver is an open shelf sintering process that requires oxygen to achieve full sinter.
Torch firing fine silver metal clay (not all are torch fire capable like- FS 999) is a quick and somewhat easy way to fire fine silver metal clay. It however, has limitations. It's great for use on personal projects & for use on small fine silver pieces. I've torched fired several pieces and found that it feels pretty sintered. However, no one can hold a torch for 2 hours and keep the temp consistent at the peachy glow. So, by torch firing it limits the "soak" time and thus limits the sintering. For earrings & pendants it works. At a microscopic level it may be less than fully sintered. It is something to keep in mind when using a torch on projects particularly if you are selling your work. Torch work during classes allows the students to see the process and moves things along time wise. It's a great way to learn about metal clay and start out at home.
My experience with all this variation in brand and type of metal clay has been confusing. There's so much to be paying attention to and thinking about but if you have a basic knowledge of how metal comes together and the effect of heat, time, pressure and oxygen it can guide decisions along the way. My advice to all students and metal clay workers it to start with one brand of clay, learn that clay inside and out. Then you can test it, push it and see how far to take it. Once you've achieved a level of understanding then move on to other clays. The fine silver clays have a lot in common but each brand/type has it's own fussiness. Find one that you like and stick with that for a while, really get to know it. There's plenty of time to play with other brands and types.
Metal work is fun, complex and challenging. There is a lot to learn and things to know. My goal in teaching is to prepare students to know the very basics of metallurgy which I think lays the ground work for more learning and good decisions as you explore. I don't know everything that is for sure! I am always willing to learn more. So if you have information that will enlighten me- please share. As always, I encourage you to find out for yourself. Explore, read and find other resources to guide your practice as a metal clayer. While there is a tremendous amount of information out there, try and stick to those who use verifiable resources, have certifications or formal education & EXPERIENCE.
May your give a damn never be broken & may your creativity shine on :)
Left to Right- Dry metal clay (prefer), torch fired or not fully sintered, fully sintered
Photo provided by Paddy Marcotte and used from the AMCAW website.