Apr 28 2010

The Sintering Bracelet Project

I actually call this bracelet “The Sintering Bracelet” – not only because it has undergone sintering, but also because it graphically depicts the sintering process.

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This posting has three parts: the tiles, the back of the tiles, and assembling the bracelet. This is an improvement on a bracelet project that is featured on my DVD, “Workshop at Textures Studio”. It is made out of White Bronze clay.

The Tiles

Use any design that appeals to you. Here is a pretty one taken from a rubber stamp.

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Here is my sintering bracelet:

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In the first tile from the left, the circles represent the particles of the metal powder in their “green” state. The spaces between them are filled with binder.

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In the second tile from the left, the particles are getting closer to each other.

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In the third tile from the left, the particles lose their spherical shape and fuse into each other.

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In the fourth tile (the middle one), the metal is fully sintered. The particles got as close as they could to each other without reaching the melting point of the metal, and there are spherical pores between them.

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The rest of the tiles show how the spaces between the particles change their shape. Starting from the right side of the bracelet:

The first tile from the right shows the shape of one pore between four spherical particles that hardly touch each other.

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The second tile from the right shows the shape of the pore after the particles have fused a little.

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The third tile from the left shows the spherical shape of the pore when the metal is fully sintered, as in the center tile.

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The Back of the Tiles

1. Turn the tiles over and arrange them side by side. Mark two parallel lines along the whole row of tiles. Start from the smallest tile and adjust the rest accordingly.

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2. Using a round miniature file, file a groove in each line.

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3. Roll a layer of clay 2 cards thick. Pleace a tile on top of it and cut it on both sides to determine its width. Don’t cut the length yet.

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4. Cut Angel Hair spaghetti into small pieces and place them in the grooves. The grooves help to hold them in place.

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5. Wet the back of the tile.

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6. Lay the strip that you cut in step 4 on the back of the tile. Make sure it adheres to the back at every spot. You can do this by rolling a straw over the layer, especially between the spaghetti straws.

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7. Cut away the excess from the strip at the top and bottom of the tile. Dry. When the tiles are half dry, pull the spaghetti straws out by twisting them gently. Continue drying.

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8. Seal all gaps between the two layers. Dry, and sand smooth.

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This is a side view of the tile:

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9. Fire the bracelet.

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Assembling the Bracelet

This bracelet can be assembled using only Stretch Magic. For a different method of assembly please see instructions here.


Apr 18 2010

Firing at High Altitudes – and a Project for White Bronze, Copper, and Bronze Clay

Following the question I asked in my last posting, about firing at high altitudes, here is what I understand. The warming pans and the kilns were not overheating. The temperatures were correct. However, at higher altitudes, for plastic to melt, for water to boil, and for metal clay to sinter, lower temperatures are required.

Because of lower air pressure, the liquids in the clay evaporate faster than at sea level, and the gases trapped in it expand and escape faster.

So the higher the altitude is above sea level, the lower the sintering temperature will be, and the lower the altitude, the higher the sintering temperature.

Thanks to Gail, Kim, Peggy and Mary for their input.

Here is a project combining copper, bronze, and White Bronze clay. This project is hard to do with silver clay because of the reaction between silver and bronze.

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This is a hollow form, constructed using the instructions for making a rock (see my first book, p. 69).

1. Choose a rock and cover it with a 2-card layer of bronze clay. Dry.

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2. Cut the bronze layer in half and pull out the rock. Put the two halves back together and dry.

3. Cover the bronze hollow form with a layer of copper clay, 3 cards thick. Don’t dry yet.

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4. Using a tube, cut a hole in the copper layer. Use a knife to cut a strip off the copper layer, continuing all around the rock. Now dry the rock.

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5. Fill the round hole with bronze clay. Just “smoosh” it tight into the hole to make sure it touches the base bronze layer. It’s not important if you fill too much. Dry.

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6. Use a sponge sanding pad to sand off the excess bronze, until the shape of the hole re-appears.

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7. Position the rock the way you want it to hang. Drill a hole through the top third of it.

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8. Fill the rock almost full with carbon. Fire it according to the instructions for your clay.

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9. Clean the strip part using a radial disc or any other buffing/brushing tool. Fill it with White Bronze the same way you filled the hole. Dry.

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10. Sand the excess White Bronze until the shape of the strip re-appears.

11. Fire the rock following the firing schedule for White Bronze.

12. Sand the whole rock smooth and flush.

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13. Paint the rock with Baldwin’s patina to highlight the contrast between the metals. It will react with the copper, but not with the bronze and White Bronze. Wash the rock in warm water.

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Apr 13 2010

What is the Correct Firing Temperature?

This is a question that I am often asked. The answer is that there is no absolute correct temperature. It depends on too many variables.

Some of these variables are: the type of kiln (brick or muffle), the location of the door and the heating elements, the size of the kiln, the age of the kiln, the carbon you use, the amount of carbon you use, the number of times you have used it, the box you fire in (steel, fiber blanket, ceramic, ceramic cloth, lava cloth, fiberglass cloth, etc.), the number of times you have fired in the same box, and finally, the altitude.

There is a simple way to figure out the right firing temperature for your kiln, which is trial and error.

Use bronze clay to make some test pieces, 3, 6, and 16 cards thick. If you have a muffle front loader kiln, fire them at 1550°F/843°C. If you have a brick kiln, fire at 1470°F/799°C. You can refer to my instruction manual for other conditions, such as box, carbon, and length of firing.

Take note of the firing conditions in which you did your test: carbon, box, etc.

After firing check the pieces. If they look curved, blistered, swollen, or somewhat textured, it means that this temperature is too high. Lower the temperature by 20°F (10°C) and repeat the test. Continue testing until you get smooth, strong test pieces.

To check sintering, use slight pressure to try to bend the thinner piece with your fingers. If it breaks, it means the temperature was too low. Raise the temperature by 20°F (10°C) and repeat the test.

A thicker piece may not break easily, even if it is not fully sintered. To check thicker pieces, sand the surface with course sandpaper. If the piece is not sintered, you will immediately see a non-metallic mass under the surface. Again, raise the temperature by 20°F (10°C) and repeat the firing until you get a strong, fully sintered piece.

Once you have found the correct temperature, try as much as possible to stick to the firing conditions you noted prior to your testing.

Next, look at the different schedules for the other clays, and adjust them accordingly. Now you have a customized firing schedule for your kiln.

Conditions may change, of course, so it is a good idea to test sintering after every firing. Buff or sand a hidden spot of the fired piece to see if there is non-metallic matter under the surface. If there is, you haven’t lost the piece. Just re-fire, or repair and re-fire.

I have just come back from teaching a workshop in Grand Junction, Colorado. It’s high desert, 4,700 feet above sea level. Two things happened that I had never encountered before.

First, the same candle warmers that I use in my studio (at sea level) were a lot hotter. They melted plastic, while at sea level I can dry a piece with a straw in it with no problem. Also, some of the kilns were firing too hot.

This seems somehow counter-intuitive, since from what I understand, because of lower air pressure, the higher the altitude, the longer it takes to heat; cooking rice takes longer, and baking bread requires adding dry ingredients.

Since this question may be relevant to metal clay users who live in high altitudes, I would appreciate any input on the subject that I can post on this blog.

The other thing that happened, was that Baldwin’s patina darkened the copper a lot more than I am used to seeing. This may be related to the composition of the air at higher altitudes, so again, if you happen to know the reason, I would love to hear it and let other people know.


Apr 7 2010

Troubleshooting Copper

This posting is in response to a question that I have been asked by many people. There seems to be some misunderstanding about the firing schedule of copper, and I would like to clarify.

The problem that people are having is that although they get fully sintered pieces of bronze, they are often able to break the copper pieces. There is no powder at the cross section, but the pieces are not very strong.

Here is my answer: copper, in order to be as strong as the bronze, needs to be fired at higher temperatures. If you look at my instruction manual for Quick-fire Copper and Bronze, you will see that if you fire copper alone you can go as high as 1800°F/980°C. In fact, I fire copper and steel clay in the same batch.

Therefore, there is no point in testing the strength of copper if it was fired at the sintering temperature of bronze. The binder may be gone, but the metal has not reached its highest density and the copper has not shrunk all the way. It’s exactly the same as when we torch-fire silver clay: you have probably noticed that if you re-fire it for repair, it shrinks more. That’s because it hadn’t reached its highest density the first time around.

So what do we do? When I have just copper to fire, I fire it by itself at 1800°F/980°C. But what if I have mixed pieces?

Luckily for us, mixed pieces are stronger even when they are fired at the sintering temperature of bronze. Even though the copper may be under-fired, the bronze gives the piece its strength.

Under-fired is not necessarily a bad thing. When we add silver to fired copper and torch-fire it, it’s because we want it under-fired. We don’t want it to shrink too much and crack. And the fact that it is under-fired is not important, since it is not a structural part of the piece.

If you want to make mixed pieces even stronger, make sure that the structural part of the piece is made out of bronze. For example, inlay copper in bronze rather than bronze in copper.

I must say, though, that I don’t usually find it necessary to worry about this. Most of the time mixed pieces, if fired properly, are strong enough for normal wear and tear. This is especially true for hollow pieces, since hollow pieces in general are stronger than flat ones.