Tuesday, July 19, 2011


Only certain gemstones can be embedded in unfired silver metal clay and withstand the temperature and firing times needed.   There are suppliers who test their gemstones and stand behind them as being "kiln-safe."  Any inclusion or air-space in the stone can cause the stone to fracture while firing, however nice “clear” stones can be bought from other sources but test-fire a "spare" stone by itself (cover it with a piece of fiber blanket in case it shatters) and see whether it fractures or changes color.  However, even a stone that appears to survive a test firing successfully may be weakened and fracture later on.  Setting firable stones in silver metal clay pre-firing is never 100% guaranteed and even “kiln-safe” tested stones can occasionally crack.   Therefore you should need set extremely valuable stones in metal clay prefiring. 

Setting Gemstones in "Wet" Clay

To set faceted stones directly in metal clay, ensure that the girdle of the stone (the widest part) is embedded in the clay about 1 mm below the surface of the clay, and the table of the stone is level. The clay will lock around the girdle of the stone during firing, so the setting must adequately capture this.  For cabochons, the clay must come up over the shoulder of the stone. Cutting a hole in the clay under the stone helps minimize the amount that the clay pushes up on the stone during firing, as well as conserving clay, and allowing some light through transparent or translucent stones.

Check the depth of the stone vs. the clay thickness. You'll need to add enough clay in the setting area to cover the stone's girdle or shoulder.  If setting a faceted stone, the depth of clay must cover the bottom point of the stone, otherwise it could potentially “poke” the wearer.

1.  Simple setting: Cut a hole and just press the stone in so the girdle or shoulder is about 1 mm below the clay.  Cut the hole slightly smaller than the stone.

2.  Ball setting:  Cut a hole in the base clay slightly smaller than the stone's diameter. Roll a small ball of clay and then flatten it into a disc that is wider than the stone. Using a pencil or pointed shaper, make a cone-shaped hole for the stone and press the stone into the clay. Make sure the girdle is covered, the table is level and if the stone is faceted that it does not protrude below the base of the piece.

3.   Tube or Bezel setting - Roll out clay to the thickness of the stone from table to point.  Cut a hole slightly smaller than the stone.  Press the stone into the hole.  Take a larger cutter, center over the stone and cut out around the stone, leaving the stone in a “tube” of clay.  These can be made ahead and added to clay pieces as desired.

4.  Syringe bezels. Cut a hole in the clay slightly smaller than the stone to be set. Set the stone in the hole and extrude a line of syringe clay to create a rim surrounding the edge of the hole. If needed, add a more rows of syringe as needed to make the bezel tall enough to capture the girdle. Alternatively, the syringe bezel can be made first and the stone pressed in until the table is level and the girdle captured by the syringe bezel.

5.  Syringe prongs. Start with the simple setting above. Moisten the clay around the stone and add syringe “prongs”around the stone.  These are simply “dots” of clay that attach both to the clay base and the stone itself.

6.  Coil settings. Extrude a coil of lump clay and form the coil into loops just slightly smaller than the stones you want to set. Place the stone in the loops and press into the clay so that the girdle is captured and the table is level.

7.  Channel setting.  Take double sided tape, put a strip of tape on a piece of Teflon, then place stones table side down on the tape.  Cut strips of clay marginally deeper than the full depth (table to culet) of the stone to account for the shrinkage of the clay in the kiln.  Press these strips along the sides of the stone to capture the girdle.  If this will be “free standing” you will need ends, otherwise these strips will simply fall off the stones.  If this will be added to another piece of metal clay that will hold everything together, this can be left as two separate strips.  Once dry, lift tape off Teflon, trim down the tape and adhere clay strips to your metal clay piece in the usual fashion.  The tape can be left on the piece if its removal might make your piece fall apart.

8.  Marquise or odd shaped stones – strip setting.  This setting is much like the one above, you will place the stone table down on double-sided tape.  Cut clay strips marginally deeper than the full depth of the stone.  Add the strips to one side of the stone, pressing clay to the stone to capture the girdle.  Angle cut the strips flush to the ends of the stone, then add another strip to the other side of the stone.  Moisten and paste the join and press firmly.  Let dry and then pop off the double-sided tape and add to your metal clay base.

Setting Gemstones in Dried Clay

1.  Gypsy settings. Make sure the clay is thick enough so the girdle and the point of the stone will be covered after pre-finishing. When the clay is dry, use a very small drill bit in a hand drill/pin vise to drill a pilot hole all the way through the clay.  Once you have a hole, use an exacto knife to “drill” cone-shaped hole.  Test-fit the stone in the hole frequently making sure the girdle is slightly below the surface of the clay.  Extrude a very fine line of syringe just inside the edge of the drilled setting hole or brush a little paste inside the hole. Place the stone into the hole then wipe the edge of the setting with a damp brush to make sure no clay squeezed out above the stone.

Prefire cleaning

If any clay ends up on top of the stone, scrape/flake it off the stone gently once it has dried.  Carefully brush off any loose dust from the clay and from the gemstone. Make sure it's level and clean the top of the stone with alcohol and a sponge-tipped cosmetic applicator or cotton swab.  Stones must be absolutely clean prior to firing, as any clay on the stone will fire to silver on the stone’s surface and alter its appearance.

Firable Stones Chart

This listing is from current available data and should be used as reference only.  There is always a chance that a variety of stone that has tested safe will have inclusions, fractures or other features which will cause a specific stone to fracture in the kiln.  If your stone is valuable, set after firing!

1650 for 1 hour:  Alexandrite, Black Star Sapphire, Padparadscha, Ruby, Hematite, Zircon, Spinel, Lab Alexandrite, Lab Ruby, Lab Sapphire - blue, orange, yellow, Lab Spinel, CZ’s - Amethyst,  Champagne, Garnet, Olivine, Orange, Pink, Yellow, White
1560 for 1 hour: Almandine Garnet, Demantoid Garnet, Pyrope Garnet,
1470 for 1 hour: Rhodolite Garnet, Tsavorite Garnet, Peridot, Lab Emerald
1200 for 1 hour: Amazonite, Labradorite, Moonstone - Grey, Sunstone, Chrome Diopside, Star Diopside, Tourmaline - Green, Chrome Green, Chatham Opal,
1100 for 1 hour: Moonstone - Peach, White, CZ - Bright Emerald, Tanzanite.

The photos taken for the stone setting are in the following order:  Wet pressed in stone, strip clay setting, channel setting,  pre-cut bezel, faux prongs with lift on back, syringe and dry setting - gypsy setting.


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