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

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