Research


by Mary Ellin D’Agostino

Some time ago, I ran some experiments to help one of my former students track down a problem.   She and some of her students experienced curling, warping, surface deformation, spots, and what they thought was incomplete sintering of the silver clay.  Was it a problem with the PMC she was using?  I asked her to come over and bring the problem clay so we could try to figure out what was going on.  On her way over, she had a light-bulb moment and realized that the work surfaces she was using might be made from aluminum.  That gave us an excellent place to start.

Most of us know that silver clays (PMC or ACS) can be contaminated by contact with aluminum, but many of us ask just what is the problem and how long must the contact be to contaminate the silver?

The essence of the problem arises from an electrolytic reaction between the silver and the aluminum (or aluminum oxide).  A full technical description is beyond me, but my understanding is that the reaction between the two is essentially the same as what happens in an alkaline battery.  What we are getting is similar to the corrosion/crud that forms on the battery contacts.

What I can give you is a description of what happens to the silver clay from a practical viewpoint.  The symptoms of aluminum contamination of PMC or ACS are:

  • Discoloration: Dark spots or areas of discoloration may appear on the unfired silver clay.  Discoloration might not appear until after firing your PMC/ACS.  After firing, discoloration can sometimes be polished away, but “ghost spots” will reappear later.
  • Warping: During firing, contaminated pieces may warp, curl, and deform dramatically.  This far exceeds the minor kinds of warping we sometimes see when firing normal silver clay jewelry.
  • Brittleness & Flaking: Pieces may be brittle and surface areas may flake or spaul off of the piece.  This can be confused with incomplete sintering of normal metal clay.  It is, in fact, due to an incomplete sintering because the contaminated metal clay is chemically changed and cannot sinter properly.

The Experiment

Going back to the original problem to test whether the problem was due to “bad clay” or had been contaminated by working it on aluminum surfaces, we tested the aluminum contamination theory by exposing silver clay from a single package to varying lengths of time in contact with aluminum. One package of PMC+ clay was broken into four pieces.  Each piece was rolled out into a slab 1mm thick and marked with a letters A-D and treated as follows:

A) The clay was worked on the aluminum and dried elsewhere,

B) worked and dried on the aluminum,

C) worked on plastic and dried on aluminum,  and

D) worked and dried on non-aluminum surfaces.

All the pieces were dried on a heating tray for about 15 minutes and then fired in a kiln at 1650°F for 10 minutes

Results

The longer the silver clay is left on the aluminum, the worse the reaction.  The contamination caused dark spots and a brittle clay with a flaking surface.  The most affected was the one worked and dried on aluminum, the next was the one dried on aluminum, and the third was only worked on aluminum.  The piece made without contact on aluminum was fine.

Generally, if you are just using a cookie cutter made of aluminum and it is not left in contact with the clay you will be fine.  If you really want to be safe, you can coat aluminum or suspected aluminum cutters with a coat of plastic or varnish.  Make sure all drying and work surfaces are coated or made from non-aluminum metals.

© MED’A Creations & Mary Ellin D’Agostino, 2010

Spring is here and lots of things are growing, including mold. Questions about mold in metal clay come up regularly this time of year.  Here is probably more than you ever wanted to know about mold in metal clay.

 The health issues related to mold revolve around breathing the mold spores and the mycotoxins they produce. Unless you have allergies, asthma, or other respiratory problems, minor exposure is usually just irritating. Mold is everywhere, so I don’t want to be an alarmist, but some molds are toxic and you do want to take some care (links to info on toxic mold are at the end of this missive). Wear a dust mask when you start working with the moldy clay–if you are “smelling” the musty mold, you are breathing mold spores. If you are sensitive, wear a HEPA grade dust mask. Be sure to wash hands and use a good air filter or positive ventilation to keep from breathing mold dust.

Personally, I am very sensitive to mold and dust and try to open up and do the initial working of moldy clay outside to keep from adding mold spores into my studio. If there is a breeze, I may not bother with a dust mask. I usually scrape off and dispose of the worst of the mold for aesthetic reasons and then knead the clay. Once it is thoroughly mixed in, the chances of getting mold spores in the air is a lot lower, and I take it back into my workshop and try to use it up quickly (within a few days).

Keeping the clay tightly wrapped so no air gets to it will help limit the mold growth as well. White vinegar can be added to kill the mold. Simply add a few drops of white vinegar to the clay and let it sit overnight; small spots of mold will often completely disappear.  Larger incursions may still be visible, but can just be mixed into the clay.

Lavender essential oil also seems to retard mold growth (it may kill it as well).  I haven’t had a problem with mold since I started using the lavender oil.  I add it to all my slip because it makes the slip stick better to both fired and unfired silver clay.

After working with moldy clay, you will want to clean your tools and work surfaces to remove mold spores. Bleach or strong white vinegar can be used to sterilize tools and work surfaces. If you have a HEPA air filter, you should run it in your working area before, during, and after in order to remove as many of the spoors from the environment as possible. These steps may help prevent future mold outbreaks.

The absolute best way to prevent moldy clay is to not let wet clay sit around for long periods of time after it has been opened (factory sealed packages are usually sterile). So use that clay up! Alternately, if you have leftover clay that is pretty dry and do not intend to use it for some time, let it dry out completely and store that way. Dry clay, will not grow mold, but does give you the challenge of reconstituting it–break it up into small pieces or grind it in a silver clay dedicated coffee mill, add (distilled) water, mix, cover tightly, and allow to sit for one or more days to allow the moisture to fully penetrate the binder. After that, it is a matter of kneading (in plastic), and adjusting the moisture content to the desired consistency. Glycerin can be added to make the clay more workable.  Adding a few drops of lavender oil or white vinegar can give you some mold insurance.  Pick your smell!  Other essential oils (such as citrus) will probably do the trick, but I haven’t tested them.

You can also sterilize moldy clay by heating it. This means that you are going to dry out the clay and will have to reconstitute it. To sterilize the clay using heat, you will need to heat it over 130F (56C) for a minimum of 30 minutes. Heat for longer to ensure the entire piece has been sterilized. I usually use my oven or toaster oven to do this at its lowest setting–about 200-250F. A good dehydrator will allow you to adjust the heat to specific temperatures as well and will give good positive airflow. Don’t heat much hotter than this or the binder will begin to darken and burn. To read more about heat sterilization, check out: http://www.ces.ncsu.edu/nreos/wood/wpn/pallets.html.. This is written for heating and sterilizing wood, but will give you a lot of information on sterilization, heating and drying that can be applied to metal clays (or at least their binders). Apparently this is a big issue in the wood pallet industry! Who knew?

Finally, keep in mind that the mold is using the binder in your clay as a food source. If you let it keep growing on your clay, it is degrading the binder. This is probably not an issue unless you let it grow for a *really* long time though! 😉  I had a tub full of moldy clay that has been sitting in my studio for a couple of years and it was still ok to work.  Keep in mind that the binder in the metal clay does start to break down after 6+ years, so any *really* old clay you have should be used as soon as possible.  Don’t let it sit around too long.

If you know that your working environment is contaminated with toxic mold, you need to take appropriate precautions. A quick Google search found the following sources on toxic mold:
http://homebuying.about.com/cs/mold/a/toxic_mold.htm
http://www.mold-help.org
http://www.homehealthscience.com/toxicmold_inhouse.asp
http://www.toxic-black-mold-info.com/
http://www.inspect-ny.com/sickhouse.htm

I originally wrote this post in February of 2008 for the Metal Clay Yahoo group and have updated it.  It was written with silver clay in mind, but the advice should transfer to the other metal clays as well.  If anyone has additional knowledge regarding mold in the base metal clays, please let me know and I can include it here.  Bronze and copper clays are probably resistant to mold as copper tends to be toxic to growing things—it used to be used in bottom paint for boats to prevent stuff from growing on them, but has been banned because it sheds toxic residue wherever boats are regularly stored.  The replacement paints are not nearly as effective at retarding marine growth.

Send me your questions regarding this and other issues and I will research and answer them.

Mary Ellin D’Agostino 

Heating Up the Relationship: Guidelines For Firing Your Married Silver And Copper Clays, Part 2

Mary Ellin D’Agostino

This is the second part of my report on how to fire metal clay alloys made from silver and copper. For directions on how to make and use these alloys, see the post In Search of Married Metals: Alloying Silver and Copper Clays Project. In part one of Heating up the Relationship, I explained the method for firing alloyed silver and copper clay pieces made using any of the commercially available copper clays. In all tests in part 1, low fire silver clays were used.  In part 2, I will discuss firing alternatives for quick-fire copper clays.

A series of firing tests were performed on both Art Clay Copper and Hadar’s Quick Fire Copper clays. I was excited by the prospect of not having to fire in carbon.

Open Shelf & Torch Tests:
Alas, no-carbon firing is not a good option for mixtures of silver and copper clays unless they contain only the smallest amount of silver or the alloyed clays. Torch firing produced a very weak product in all of the alloy mixtures. Paradoxically, in a complete turn around to alloys made from the first generation copper clays (Metal Adventures CopprClay and Hadar’s “traditional” copper clay), the sterling alloy was very weak and the shibuishi alloys were the strongest. None of the open shelf or torch firing tests resulted in a work product I would consider using in a final product. Even the strongest (the shibuishi alloys) broke when bent as little as 30°. Most of the test pieces snapped long before that point. This was true for pieces torch fired for a few minutes and those kiln fired for as long as 3 hours. The maximum firing temperature for these alloys is approximately 1450°F/788°C. If you exceed this temperature, the pieces melt. See Part one of this series for a discussion of firing and melting temperatures for silver-copper alloys.

An attempt at firing with a Speed-Fire Cone also resulted in failure, but may have been due to not keeping the pieces hot enough as the gas tank was a bit low. If the other results had been more promising, I might have rushed out and gotten a new tank of propane and tried again. Since all the torch and open shelf tests were dismal failures, I have not been in a rush to try again with the Speed-Fire Cone system. If anyone tries it and gets better results, let me know.

No-carbon firing is not recommended for alloyed copper and silver clays. However, it is possible that if you make the piece very thick and don’t need a really strong (bendable) end product, you might be able to use one of these methods. Hadar Jacobson reports some success with a shibuishi clay that she torch fired, but doesn’t go into detail as to how strong her piece was. Click here to go to her report.

Carbon in a Fiber Blanket Box Tests:
Again, both Art Clay and Hadar’s Clay brands of quick fire copper were tested. For a full description of the fiber-blanket box method, see Hadar’s blog on the subject. A box was made of fiber blanket and filled about ¾-1” with carbon. The test strips were placed in the carbon and covered with another ¾-1” of carbon. (All carbon tests have been done with the coconut based carbon).

In the first test (test series C5 & D1), the kiln was heated to 1450°F/788°C and held for 2 hours. Binders were not burned off in advance. Approximately half of the carbon burned away. Pieces came out black and the alloy strips with the most silver melted. Some bending was possible, but the maximum bends ranged from 45° to 90°. Bending beyond this point resulted in the test strips snapping. Results were similar for both brands of clay with one small difference. The Art Clay copper pieces with more than 25% silver melted, while only those with more than 50% silver melted in the pieces made with Hadar’s copper clay.

In the second test (test series D3 & C7), I reduced the temperature to 1425°F/774°C and held for 3 hours. Results were similar to the first. Alloys with more than 25% silver mixed with Art Clay brand copper melted while those with more that 50% silver mixed with Hadar’s brand copper melted. In all cases where the pieces didn’t melt, the metal failed at or before a 45° bend. Again, this firing schedule may be adequate if pieces are thick and are not likely to be subject to much stress.

One complicating factor is that in these final tests (C5-C7 and D1-D3), reconstituted silver clay was used that contained some PMC+ in addition to PMC3 silver clay. This was an economy measure on my part as these tests are getting expensive! If anyone gets better results using only low-fire silver, let me know.

Steel Box & Carbon Tests:
In these tests (test series D2 & C6), the pieces were placed on carbon in a 3.5” diameter stainless steel box, the binder torched off, covered with more carbon, then sealed with a stainless steel lid, heated at full ramp in an Evenheat E91 kiln to 1450°F/788°C and held for 6 hours. These tests also used the reconstituted clay rather than pure and fresh low-fire silver clay. Results are consistent with use of the first generation copper clays (Metal Adventures CopprClay and Hadar’s Traditional copper clay). In these cases, the pieces with more silver were the strongest while those with the most copper were somewhat weaker. Maximum bend achieved before breaking were 45°-90°. An 8 hour firing would probably improve the strength. Firing in a larger box and kiln might also improve matters as I achieved the best results in earlier tests using a large kiln with a significantly larger firing box with a lot more carbon to create a better reduction atmosphere. I will report more after further testing.

Tnanks go to Jackie Truty and Art Clay World for the Art Clay Copper sample.

© 2010 Mary Ellin D’Agostino

Heating Up the Relationship: Guidelines For Firing Your Married Silver And Copper Clays, Part 1

Mary Ellin D’Agostino

The experiments are long and ongoing, so I am just going to publish this in bits and pieces before re-uniting all the parts into a whole.  I see that in the earlier project post, the firing guidelines got left out since I wrote it as a project to go with the longer article I am still working on.  So here goes:

There is not any “one true way” for firing mixed metal clay projects, so what I give you here is a starting place for you to begin.  There are several factors to keep in mind when firing mixed metals projects.

The Basic Firing Process:

The type of copper clay can make a difference.  The following will work with both first and second (quick-fire) generations copper clay.  I will cover firing alternatives for quick-fire copper clays (Art Clay Copper and Hadar’s Quick-Fire Copper) in Part 2.

  • Silver clays require oxygen to burn out their binders, so you cannot just bury them in carbon and fire in a covered steel box.  A 2-stage firing process is essential.  The initial burnout of the binder can be done with a torch or on an open kiln shelf.
  • First generation copper clays also benefit from a 2-stage firing schedule to fully burn out the binder.  Second generation clays require oxygen to burn out the binder.
  • Copper-Silver Alloys all require that the binder be burned out in an oxygenated atmosphere.

This means you can’t just bury a piece in carbon in a covered steel container.  Binders need to be burned off first.  There are several options for this:  A torch can be used to burn off binders of pieces placed on carbon before they are covered.  Pieces can be left exposed or just barely covered with carbon and heated to 800-1000°F (427-538°C) and held 30-60 minutes.  Alternately pieces can be placed on an open kiln shelf and binders torched or burned off in a kiln.  The disadvantage to the open shelf method is that pieces may be very fragile and difficult to transfer from the shelf to the carbon without damaging them.  Discussions on 2 stage firing for copper clays can be found at http://www.artinsilver.com/blog and http://www.squidoo.com/bronzclay

After binders are burned off, the pieces are then covered with more carbon, the lid is placed on the stainless steel container, placed in the kiln on posts, and heated at a medium to fast ramp to the firing temperature and held for the specified amount of time.   Because kilns vary a great deal, the schedules I that work for me will probably need adjustment to match the kiln being used.

Temperature:

Alloyed clays have different firing temperatures than pure silver or copper clays.  Sintering temperatures must be well below the melting point of the metal.

  • Pure copper sintering temperatures range up to 1780°F/971°C; the copper melting point is 1981°F/1083°C).
  • The highest standard sintering temperature for pure silver is 1650°F/900°C; the silver melting point is 1761°F/960.5°C.
  • Sterling silver melts at 1640°F/893°C and the highest temperature that we can typically get away with when firing sterling into silver clays is 1470°F/799°C, but many people prefer a much lower temperature when heating sterling embedded in silver clay due to the oxidation and deterioration of the sterling that occurs when it is held at high temperatures in an oxygenated atmosphere.
  • A 75% Copper-25% Silver Shibuichi alloy melts at 1742°F/950°C

The key, of course, in firing the alloyed clays is to find a temperature-time schedule that will adequately sinter all of the alloy mixes included in the piece and encourage a strong bond between the different alloys.  Creating bonded layers in metal clay is easier than doing the same thing with rolled metal sheets.  In making Mokume-Gane billets from sheet metal, a billet of layered metals is heated and held at temperature 3-12 hours, copper/silver mixtures are known to start bonding at ~1350°F and melt at ~1450°F (See Binnon’s Mokume-Gane Workshop).

The key to successful firing is both time and temperature.  A lower temperature will succeed given a longer firing time, while a higher temperature will work more quickly.  The most successful results I have gotten are as follows:

  • Small Evenheat E91 kiln, 4”x4”x4” firing chamber, kiln with a 3.5”diameter x 2.75”h stainless steel firing box.  First stage: torch fire off binders or heat exposed pieces in kiln ramping 1000°F/538°C per hour to 800°F/427°C, hold 0 min.   Second stage: Covered with carbon and heated at full ramp to 1450°F/788°C, hold 6 hours.
  • Large Sierra 1100F kiln, 11”x11”x8” firing chamber, with a 6”x5”x4” stainless steel firing box. First stage: torch fire off binders or heat exposed pieces in kiln ramping 1000°F/538°C per hour to 800°F/427°C, hold 0 min.   Second stage: Covered with carbon and heated at full ramp to 1430°F/777°C, hold 6-8 hours.

Note that the different kilns required different temperatures.  Firing to 1450°F/788°C in the larger kiln resulted in melted pieces.  Every kiln is different and will require testing to find the acceptable temperature range.

Longer hold times definitely result in stronger pieces that can be bent 180° without breaking.  When using first generation copper clays, the pieces with higher silver content were strong after shorter firing times.  The alloys with the greatest percentage of copper required the longer firing to achieve strength.

The newer generation of copper clays exhibit greater strength when alloyed with small amounts of silver and show great promise for shortening the firing times required for this technique.  Firing schedules and results with these clays will be reported in Part 2 of this article that will be published next week.

A full listing of all experimental results will be available in the article In Search of Married Metal Clays: Experiments in Alloying Metal Clays that I hope to complete and post soon.

End of Part 1.

Click here for  Part 2 of Heating Up the Relationship:  Guidelines For Firing Your Married Silver And Copper Clays.

© 2010 Mary Ellin D’Agostino

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