Hadar’s Blog

Did you ever want to add one or more steel elements to a bronze/copper piece? Whenever I am asked whether it can be done, I have to say: Only if you use it in very small amounts; it has to be fired at a temperature which is too low for a considerable amount of steel to sinter. Bronze needs to be fired at a mid-firing schedule: 1470°F/800°C (brick kiln); 1520°F/827°C (muffle kiln). Steel has to be fired at a high-firing schedule: 1700°F/926°C (brick kiln); 1780°F/971°C (muffle kiln), and will not sinter at a lower temperature unless it is used in very small amounts, as in mokume-gane patterns.

Up until now, if we wanted larger amounts of steel in a piece, we needed to fire the steel first at a high-fire schedule, then add lower-firing clays with some kind of mechanical connection, and re-fire the piece at a lower temperature.

This, of course limits our design options.

Bronze XT changes all that. It looks like regular bronze, but can be fired at a high-fire schedule, at the same temperature as steels, copper and Rose Bronze. That means that it is possible to fire a mix of copper, Bronze XT and steel clays in the same piece without limiting the design and the amount of steel.

The following pieces were all fired just once:

Inlay of steel in bronze as in the photo above is possible only by using Bronze XT.

Wraparound as in the photo above, when regular bronze is used, is only possible with two firings.

And “onlay”, with the majority of the piece being steel. is only possible with Bronze XT.

With architectural pieces the range possibilities is much wider with Bronze XT.

Since Bronze XT is fired at a temperature as high as copper and steel, under-firing is not an issue anymore. If a piece did not sinter, you can be sure now that it is because something went wrong in phase 1 (pre-firing), not because of low temperature in phase 2.

Testing

Fire pieces of Bronze XT at 1700°F/926°C (brick kiln); 1780°F/971°C (muffle kiln). If the pieces blister, swell, or melt, lower the temperature gradually until you reach a good result.

Firing

Firing Bronze XT is the same as with the Quick-fire copper, steels, and Rose Bronze. The Instruction Manual has now been updated to include the Bronze XT firing schedule.

Note: The current version of the Instruction Manual shows all of the changes highlighted in yellow, so those of you who already have the manual can easily see what has changed. The yellow highlights will be removed around the middle of May 2012.

Shrinkage

The shrinkage is the same as that of the steels, 28% by volume. When firing steel, copper, and Bronze XT together, copper will shrink the least. “Onlay” design, with Bronze XT as the major part of the piece, will not work well due to its high shrinkage.

A ring made from Bronze XT shrinks 2½ sizes.

The following section was stricken out on May 10, 2012. For details, please see my posting entitled “Three Metals, One shot: Bronze XT Now Available – Part 3.”

Important note:

With both regular bronze and Bronze XT, copper tends to alloy with the tin content of bronze at the contact points between the metals. The alloy is closer to bronze in color, so the fired piece looks like it has less copper then it had before firing. When we fire copper next to regular bronze, at mid-fire temperature – 1470°F/800°C (brick); 1520°F/827°C (muffle) – we can compensate for the alloying by doubling the amount of copper, as described in my book, Patterns of Color in Metal Clay.

This reaction is even stronger at the high firing temperature of Bronze XT. As a result, the bronze may “bleed” onto the copper part.

If the copper is used in small amounts, as in mokume-gane patterns, the distinction between copper and bronze may disappear altogether.

To avoid the bleeding, we can do one of two things:

1. Separate copper from bronze with an empty space:

2. Or, separate copper from bronze with steel:

Separation of the second kind is also needed when you load your extruder to create a mokume-gane pattern. The order of the circles to be extruded should be: copper, steel, bronze, steel, copper, steel, and so on (see my book, Patterns of Color in Metal Clay, pp. 9 and 12.

Since the bronze and copper are now separated, there is no longer any need to double the amount of copper. All three metals can be used in equal or different amounts, depending on the desired design.

A nice surprise is that when Bronze XT is used in a mokume-gane design instead of regular bronze, Baldwin’s Patina does not color the steel black. It darkens the copper only, and the steel looks blue.