No Pre-firing for White Bronze; One More Update to the Manual

Last week was dedicated to one of my New Year’s resolutions: improving the firing schedule of White Bronze. The result: White Bronze does not require pre-firing. Just fire in carbon at 1250°F/677°C (brick kiln); 1325°F/718°C (muffle kiln). Hold for 2:00 hours. Optional: 1:30 hours for less than 100 grams.

The Instruction Manual has been updated and is marked February 10, 2013. For your convenience, the changes are only on page 14 and 29 (the quick-reference firing schedules)

Very thick pieces fired this way with full sintering, no distortion, and no cracking. They were fired in two different kilns, so it seems that with the one-step firing schedule White Bronze is not so sensitive to slight changes in temperature.

The following pieces are 13 cards thick – 10 cards a piece plus a 3-card bail on the back:

Doors

Slate

To test the brittleness of these piece I made a circle of the same width. Here it is next to a penny:

Circle

I threw it hard (not just dropped) a few times on a concrete ground and it did not break. Don’t get me wrong: this is not a good way to test your pieces! Although people report that they regularly tumble White Bronze without breaking it, White Bronze is still more brittle than the other clays, and cannot be hammered, bent, or drilled. The test is always to sand it with course-grit sandpaper (150 or 220) and see if it becomes all metallic or has pits of powder. You will never be able to perform the finishing process if the piece is not well sintered.

It seems that thick pieces are less brittle than thin ones. However, if you are still worried about strength, here is a way to protect a piece. For the following piece, a frame of copper was fired first at high-fire schedule. This is the structural part of the piece.

Room

1. Cut 3 identical-size shapes. Leave one of them solid (6 cards thick). In the second (2 cards thick) cut another, smaller, identical shape. In the third (4 cards thick) cut a smaller, identical shape.

3 identical shapes

2. Place the second layer on the solid one.

2nd layer on solid layer

3. Place the third layer on the second one. You have now created an undercut, an empty space between the first and the third layer.

3rd layer on 2nd layer

4. Seal the gaps between the layers.

5. Insert fiber paper into the undercuts to keep them open during firing.

Insert fiber paper

6. Fire the piece following high-fire schedule for copper.

7. Clean the undercuts and much as you can. Press White bronze into the undercuts to fill them completely. Dry.

Clean undercuts, press White Bronze

8. Roll a layer of White Bronze, 1 card thick. Lay it on the copper piece.

Lay White Bronze on copper

9. Cut away the excess clay from around the inner frame. Dry.

Trim and dry

10. Make your piece as if the White Bronze were the backing layer.

Make your piece

11. Fire following the new low-firing schedule.

Notes:

  • The moon in the window is made with Brilliant Bronze, which sintered just fine at this low temperature.
  • The frame can be made from any other clay. Here is a piece made with Low-shrinkage Steel XT and White Bronze.

Undercut earrings

Why is the undercut necessary?

First, high-fire metals may not fuse to White Bronze at low-fire schedule. The undercut is a mechanical device that prevents the metals from separating.

Second, the White bronze pressed into the undercuts prevents the inner (White Bronze) part from shrinking away from the frame.

And last, just for color comparison, here are similar pieces side by side, one with White Bronze, the other with Low-Shrinkage Steel XT.

2 slates

Both pieces above were fired with one-phase firing schedule, no pre-firing.

2 rooms

Both pieces required pre-firing, since a large amount of copper is involved.


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