By Mary Ellin D’Agostino

Overview:

Since copper and bronze clays have become available, many people have combined the different clays together.  Most consistently successful methods involve firing one metal first adding the second metal and re-firing.  In this class, we will take a slightly different approach.  When I first came to this project, I simply swirled the pure copper and pure silver clays together, burned off the binder, buried the piece in activated carbon and fired for different lengths of time between 1400°F/788°C and1470°F/799°C.  Why so low?  Although both silver and copper can take hotter temperatures, alloys of these two metals melt at lower temperatures.  If you have ever accidentally fired some sterling at 1650°F/899°C, you know that the end result is a blackened crumbly mess.  When fired in carbon too hot, you get a melted alloyed mess.  If you don’t fire hot enough, any areas with a high copper content are very weak.  In addition, pure copper and pure silver areas don’t bond well together and tend to separate.  Not to mention differences in shrinkage.  There were lots of issues to deal with!

To overcome these problems, I started mixing the two types of clay to create alloys and tested their various firing characteristics.  I also started researching the properties of powdered metals, alloying silver and copper, and how metalsmiths create married metals and Mokume-Gane.  Once one understands the properties and characteristics of silver, copper, and their alloys, creating pieces using combinations of these metals in a single piece and successfully becomes relatively easy.

One great advantage to this method over adding silver to already fired copper pieces is that there is not the tremendous clean up and loss of texture.  Pieces should come out of the kiln bright, with color shading stemming from the variations in the copper-silver mix.  I love this method for its ability to create contrast that is inherent in the material rather than through patinas that are less enduring.  On the minus side, extended firing in carbon is required. Shorter and no-carbon firings have been unsatisfactory no matter what brand or version of copper clay is used.

Keeping Clean:

When you work with the combined metals, you need to keep the tools and work surfaces separate from your other metal clay endeavors.  It is fine to wash and use the same basic tool set, but be sure to clean the tools when switching metals.  Some tools, such as paint brushes and sandpaper cannot be easily cleaned and should probably be dedicated to the particular material.  Contamination with copper will make your claims that a piece is “pure silver” dubious.  Contamination with bronze, which contains tin, could present problems in sintering your pieces.  See Hadar Jacobson’s books, Silver and Bronze Clay: Movement and Mechanisms and Mixed Metal Jewelry from Metal Clay, or her blog (www.artinsilver.com) for more information on the problems of combining bronze and silver.

The Process

  1. The first step will be to mix a selection of different alloys.  You can use pure silver as one of your metals, but you cannot use pure copper because the pure copper will not sinter at the temperatures necessary when combining the two types of clay.   The chart below lists a few of the possible alloys that can be used alone or in combination.  Note that pure copper and the Shibuichi alloy (95% copper) did not perform well in firings and should only be used as accents or where fully supported and protected by surrounding metal.
Ratio by weight in grams

Cu:Ag

% of Cu % of Ag Alloy Expectations
1:0 100% 0% Copper Weak, possible if it is fully supported by stronger surrounding metal
10:0.5 95% 5% Shibuichi Weak, possible if it is fully supported by stronger surrounding metal
3:1 75% 25% Shibuichi Strong; allows bending
1:1 50% 50% Strong; allows bending
1:3 25% 75% Strong; allows bending
1:12.75 7.5% 92.5% Sterling Strong; allows bending
0:1 0% 100% Fine Silver Strong; allows bending

There are three simple alternatives to measuring out the clays:  First, you can use the smallest of your kitchen measuring spoons or the mini-measuring spoons that can be found at kitchen specialty stores or on the medacreations.com website.  A second alternative is to roll out clay between slats and use a cutter to cut out equal sized pieces of clay and use these to create your mixes.  For example a 75% copper mix would use three pieces of clay to one piece of silver.  The third method is to just wing it—make your own mixes of clay by just eyeballing the amounts of copper and silver you use.  This works fine as long as you are using more than 10% silver to 90% copper.

Note on Hallmarking: Hallmarking rules were not made with married metals projects in mind and it is very difficult, if not impossible, to place a legal USA hallmark on married metal pieces.  Add an explanatory tag to the piece instead.

  1. After choosing the alloys you will make, measure out the clays by either volume or weight.  If you want to claim specific alloy percentages in your pieces, you should probably measure by weight, as this will give you a closer approximation of how alloys are traditionally calculated.  For more accuracy, you could take into account and subtract the weights of the water and binder in the clays when measuring because the two clays do not have the same binder and water content.  These vary by type of clay and manufacturer.  For practical purposes, I just use the raw clay weights or volumes to create my alloys.  Sometimes, as when I have random mixed bits of clay left over, I go with the winging it method and just make up a mix that looks good to work with.

Measure out the clay for each alloy and mix thoroughly by rolling and folding until the clay is a uniform color without streaks in it.  Label a salsa cup and place the clay in the cup and seal with a lid or place the clay on a tile with the cup up-ended over it to keep it moist.

  1. Select your alloy-color palette, texturing and working tools. Once you have decided on your sculptural or jewelry form and general design, you can begin to work.  There are, of course, many techniques and methods of combining the alloys in your piece.  A few of these are:
    1. Single alloy piece: You can just create a piece that uses a single alloy rather than combining different colors of metal.
    2. Overlays: Create a base piece of one of the strong alloys or pure silver and then use water and paste to attach embellishments of differing alloys.
    3. Embedded elements or Inlay: Press wet or dry elements of one alloy into a wet base of another alloy or press a wet alloy into or onto a prepared dry base.  The dry base may have depressions or spaces carved out to be filled by the second alloy
    4. Adjoining alloy base: You can create a base that joins two or more alloys by joining the alloys before rolling out, or by butting them against each other after they are rolled out (use paste to ensure a good join).  Joining can be done either wet or dry as desired.  If the alloys are far apart in content, you may want to include a thin band of an intermediate alloyed clay.  For example, if I am making a dome that will be part pure silver and part a low silver alloy Shibuichi, I should probably place a thin coil that is a 50:50 mixture of the two between them.  If I don’t do this, there may be a distinct warping and stepping evident after firing because of the different shrinkage rates of the clays.  Copper clays shrink anywhere from 10-35% while the silver clay shrinks 10-15%.
    5. Marbled or swirled base: Two different alloyed clays can be partially mixed to create a marbled effect. Do not try to use pure copper as one of the clays as it will leave weak areas that will break.  Use at least 10% silver in the copper used in any of these processes.
    6. Married Metal or Mokume Gane look: Layers of different alloys can be joined in a billet, carved into, cut, folded, and rolled to create a piece that has the look of Mokume Gane.  If you want a specific look, I would suggest practicing with polymer clay or play-dough to determine what will work best for your intended result.

I like to create pieces with pure silver in combination with varying copper alloys so I can keep a bright, pure silver look in conjunction with darker areas.

  1. Make sure that all joins are strong and well executed because the differing shrinkage will mean that poor joins are likely to crack or separate.
  2. Warping happens.  This is not a good technique for precision pieces where no warping is allowed.  If that is what your design calls for, you should make and fire the elements separately and then join them by soldering or cold connections.
  3. Repairs.  Due to the long firing times, repairing pieces can be tedious and require multiple firings, so if you are concerned about a particular part of the piece, try to anticipate and pay extra attention and fix potential problems before firing.

Firing:

  1. Test, test, test, before you commit your best work to a firing schedule.

Every kiln is different and will need to be tested prior to committing to a firing schedule for your work pieces.  Kilns change over time, so you will have to periodically test your kiln to make sure you do not need to alter your firing schedule.  Typically, as the thermocouple becomes coated or degraded, kilns fire hotter than the controller reports and the same schedule that worked before may melt your pieces at a later date.  I had just such a disaster as I was preparing for these retreat classes.

Remember that the firing times for base metal clays is extended, so you will see changes in your kiln’s firing characteristics much more rapidly than when working with silver clays.

The firing for this technique involves flirting with the melting point of the alloyed metals.  For best results, we want to go as hot as possible to ensure that the pieces sinter properly, but not so hot as to melt them.  Lower temperatures necessarily translate into longer firing times. You can read more about firing on my blog at http://www.medacreations.com (February 2010).

Silver alloys are known to start bonding at about 1350°F/732°C and melt at 1450°F/788°C.  So we want to work within this temperature range.  Learning how to read and understanding the phase diagram for the Ag-Cu system (appended).  The lowest melting alloy for Ag-Cu is one that is about 71.59% Ag and 28.41% Cu, so you may want to avoid mixes that are close to this mixture.

A basic procedure for establishing a firing schedule is to

  1. Create test strips of a selection of alloys or the ones you intend to use.
  2. Firing the clays requires a two step firing.  First the binder needs to be completely burned off.  Then the pieces need to be sintered while buried in a reduction atmosphere.  A reduction atmosphere is one with little or no oxygen in it and is achieved by burying the pieces in activated carbon during the firing.   Heating is generally slower than in open-shelf silver firing because the mass of the container with the carbon takes a long time to heat.  The entire mass needs to be brought up to temperature and held for long enough for the metals to sinter properly.  This means that firing times are long.
  3. Binders in the silver clay and some types of copper clay require oxygen to completely burn out, so you need to burn off the binders using a torch or in a kiln.
    1. i.      Kiln Method.  Place the pieces on an open kiln shelf or on top of a layer of carbon in a firing sink and fire to 800°F/427°C to 1000°F/538°C for 2-10 minutes, depending on the size and thickness of the pieces.
    2. ii.      Torch Method:  Place pieces on a firing surface or on top of the carbon you will be firing them in and slowly heat with a torch until the pieces catch on fire and binders burn off.  If you heat too rapidly, the pieces will poof up or pop.

Take care because the pieces are now quite delicate and can easily break. For this reason, I prefer to fire on top of carbon using a torch or the kiln.  Be sure to remove any white ash residue on top of the carbon after this burn out step.

Once the binder is burned off, bury the pieces in carbon as usual in the stainless steel firing sink.  Sink should be completely filled and covered with a stainless steel lid.

  1. Fire them at a selected temperature and test them for breakage.
    A relatively safe starting temperature is 1400°F/760°C to1425°F/774°C hold at temperature for 6-8 hours. If the pieces break, you need to fire either longer or hotter.  If the pieces melt, you need to lower the temperature.  If you fire lower with a test piece, you have a chance to re-fire hotter or longer.  If you start hotter and the piece melts, then you need to make new pieces for the next test.

When establishing firing parameters for a kiln, you can begin with the following potential temperature/time combinations

1400°F/760°C           1425°F/774°C           1450°F/788°C

12+ hours                     8 hours                       6 hours

  1. Try again until you get it right.

After firing, the pieces should be shiny metal.  If not, you can re-fire or test for breakability by bending with pliers.

  1. In each firing batch, try to include a test strip that you can do a bend test on so you won’t have to test and possibly break your “real” pieces.  To avoid waste, test strips can be simple pieces you can use as tags, charms, or simple pendants.

Finishing

  1. Polishing: Happily, when pieces are fired in carbon, the copper and silver both come out bright, so only a little polishing is necessary. You can brush, burnish, or tumble your pieces as usual.  If your pieces come out black, it is a sign that your pieces may not be sintered properly and that your carbon is wearing out or your pieces were not sufficiently covered by carbon during firing.  In that case, you may want to re-fire the pieces.

One thing to keep in mind is that a tumbler that has been used for bronze and copper has minute particles of bronze and copper in it.  If you shift back to tumbling pure silver pieces, you will need to thoroughly clean your shot and barrel to avoid embedding non-silver metals in the surface of your silver piece.  If you switch back and forth a lot between metal types, you might want to have a separate tumbler or barrel and media for the different types of metals.

  1. Patinas: You can add either chemical or heat patinas to your pieces.  I prefer heat patinas on alloyed pieces if additional contrast is needed because they allow me to get the brightest finish on the pure silver for the highest possible contrast with the darkened copper alloyed areas.

Both chemical and heat patinas will change over time by becoming darker or wearing off, so if you want your piece to remain stable, you will have to apply a wax or lacquer finish to them.   Many metalmiths use Renaissance wax, carnuba wax or a mixture of beeswax and alcohol.  Others swear by the various acrylic sprays available.  Most of the metal clay suppliers sell one or more of these products.  You can go to http://www.ganoksin.com to search out different surface protectors, where to purchase them, or how to make them.

To read more about the process, please see In Search of Married Metal Clays from my February 2010 posts.

I am working on a comprehensive re-write of this process that will include more information and hope to publish it soon.

© 2010 Mary Ellin D’Agostino

4 Responses to “Alloying Copper & Silver Clays”


  1. Hi…
    Thank you for this information!
    You know, I cannot find the percentage of copper to silver clay for mixing….There is one source but you have to download and pay for the magazine…Isn’t this now becoming open to the public?

  2. Michelle Says:

    The reason the copper and silver mixed sinter at a lower temperature is called a eutectic reaction. At 28.1 weight percent copper this mixture will melt signifincantly lower than copper or silver alone (1434F, vs 1750F for fine silver and near 2000F for pure copper). This is speaking traditional metallurgy, but must also remain true for powdered metals.


  3. The eutectic point is the temperature where an alloy mixture goes directly from solid to liquid. In the silver-copper system, the eutectic temperature is 779.1C and the alloy proportions are 71.59% silver and 28.41% copper. If your alloy is any other proportion of Ag-Cu, there will be a temperature range where the metal is ‘slushy.’ To see phase diagram of the silver-copper system go to:
    http://www.metallurgy.nist.gov/phase/solder/agcu.html

  4. vicki Says:

    Thank you for lovely, clear information 🙂

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