Class Notes!
by Stacey on July 17th, 2010
filed under
I’ve been working to answer some of the questions I’ve gotten about working with bronze. Don’t hesitate to email me, paula@onesweetbead.com, if you have any other questions! – Paula
Bronze Metal Clay Notes 7/2010
ADDING STONES:
Most lab grown stones can be fired into bronze clay. These include CZ’s, spinel and lab grown corundum (rubies & sapphires). Be sure to read the descriptions of any lab grown stones before you buy because some of them may be called doublets or triplets and will not fire well. These are lab grown stones but constructed in layers with epoxy, which suffers heat damage in the kiln. You can also fire mosaic tiles in bronze clay. Test fire a piece at 1516 degrees for about 30 min. to be sure the glaze doesn’t burn off. And remember to set anything in bronze clay deep enough to allow the clay to shrink in around the top edge slightly. Bronze clay shrinks about 25% overall.
FINDINGS:
Any bronze, brass or copper metal can be fired in place with bronze clay. This includes wire & pre-made findings. The metals are similar enough to fuse together during the firing process and not suffer any heat damage. Be sure to embed them securely and allow for shrinkage. You can create bails or even use wire as a decorative element on the surface of your pieces.
PATINAS:
Activated carbon creates a natural patina that ranges from copper, reds and purples to blues and soft greens. It is random and depends on the particular batch of carbon you buy. The first bag I bought gave me wonderful colors and lasted about 25-30 firings, with the color fading and becoming lighter with each firing. I recently replaced it and am now getting very little color and the color I do get is a muddy looking mix of pinks, reds and copper. So I decided to look into patinas that I can add to my fired pieces.
One of the easiest ways to get color is to fume your fired pieces in ammonia. Find a container with a tight fitting lid big enough to hold a small glass dish and your fired pieces. Fill the glass dish with about ¼ c. of household ammonia and set it into the container. Wet all of your fired bronze pieces with water and coat with regular table salt. This is the same process as breading food! When the bronze is coated place it into the large container next to the dish of ammonia, not into it. Close the container and leave it for at least 2 days. The fumes from the ammonia react with the salt to create a beautiful green and blue patina.
Another way to get a different look to patina is to place cedar wood chips into a baggie, pour in enough ammonia to wet the chips and then put the bronze directly into the chips and seal it for at least 2 hours. Check your piece and if the color isn’t strong enough close up the bag and let the ammonia work its magic. You can leave the pieces in the bag for up to 2 days. This method will give a mottled look to your piece because the chips are in direct contact with the bronze and will leave a stronger color where its touching. There are also commercial patinas available through Rio Grande. Just be sure to follow directions and use safety precautions with any chemical.
HOLLOW FORM CONSTRUCTION w/ CORES:
You can use any burnable core when creating hollow forms in bronze. Cork clay, and paper clay are the two most commonly used. My substitute for cork clay is soft white bread. Squeeze it to compress it, create the desired shape, then let it dry thoroughly before applying any clay to the surface. This method is safe and non-toxic.
Be sure to work in a ventilated area when firing hollow forms because there is usually some smoke created when you fire. Cork clay, foam and many other materials produce toxic fumes when fired. Please do your homework when using any material other than bread.
Just a quick note. If you aren’t making a piece that has a hole, such as a bead, you‘ll need to create a hole at least 3mm wide to allow the core to burn out. Try to place it in a spot that can be patched without disrupting the pattern or design. After following the firing schedule below, use fresh clay to patch the hole, sand as needed, then fire at the normal schedule of ramping at 250 degrees, target temp is 1516. Hold for 3 hours then let it cool naturally till the pieces can be removed.
The firing schedule for bronze hollow forms with a core is different than the normal one you already use. Because bronze is fired in charcoal without oxygen, the core does not burn out completely and prevents the clay from firing properly. So to be successful with this type of hollow form you will need to pre-fire, or open fire, the piece to burn out the core. Put the piece in your kiln using a kiln shelf on posts. Ramp the kiln at 280 degrees per hour, target temp is 560 degrees. Hold time is 15 minutes. Let it cool naturally, don’t quench with water. If any part of the core remains, gently use a needle tool to remove it before firing in the charcoal. The bronze is fragile at this point so be careful not to break it open.
