In Search of Married Metals: Alloying Silver and Copper Clays Project

Mary Ellin D’Agostino

In this workshop we will combine silver and copper clays to create alloys and “married metal” projects.  I have developed this technique through extensive experimentation and testing.  For background information see “In Search of Married Metal Clays: Experiments in Alloying Metal Clays.”

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

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 Shakudo 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 gramsCu: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 (5-25% Ag) 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 yourself 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.

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.

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:

  • Single alloy piece: You can just create a piece that uses a single alloy rather than combining different colors of metal.
  • 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.
  • 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
  • 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 need 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 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 typically shrink about 35% while the silver clay shrinks 10-15%.
  • 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.
  • 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.  For info on traditional Mokume Gane, see the work of James Binnion at Mokume-Gane.com and his 1996 Mokume-Gane Workshop article.

Joins. 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.

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.

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:

Test, test, test, before you commit your 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 if you have been working with silver clays for a long time.
  • 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.  My process for establishing a firing schedule is detailed in the appended technical report.  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. Fire them at a selected temperature (take a guess–either high or low) and test them for breakage.  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.
  3. Try again until you get it right.
  4. 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.
  5. 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 an 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.

Finishing

Happily, when pieces are fired in carbon, the copper and silver both come out bright, so only a little polishing is necessary.  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.  You can brush, burnish, or tumble your pieces as usual.

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 get a separate tumbler for the different types of metals.

For more on firing your project see the following posts on Heating Up the Relationship Part 1 and Part 2.

© 2010 Mary Ellin D’Agostino

Advertisements