Pre-Burning Binder (and Why)

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There are silver-metal-clay creations that require activated charcoal or carbon firing. The classic that comes to mind is incorporating silver with another metal, such as copper or a copper alloy like bronze. 

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Bronze Feathers in Fine Silver


Some stones, even some colors of CZs, require carbon firing to maintain their color.

13-800 Ohm-8 copy

I do believe I began with a green CZ. It turned deep red during the firing process. Darn.


There are likely more instances in your creative process in which you might need to know what activated carbon does and how it works. And how the carbon’s process might dis-allow the silver in metal clay from sintering.

Activated carbon prevents oxidation — the oxidation that causes corrosion on the surface of copper and copper alloys — by trapping oxygen molecules in the carbon’s microscopic pores. If the oxygen is removed from the environment, such as in your firing container, then burning cannot happen, since combustion of any type requires oxygen. This means then the binder in the silver metal clay does not burn off because that reaction requires oxygen of which there is none. If the binder cannot burn off, then sintering does not occur because the silver molecules cannot get close together with binder surrounding them. 

Thus, while the activated carbon prevents non-silver metals from oxidizing during the firing process, it also prevents any binder from burning off. Did you ever wonder why the fist-step firing for copper and copper alloys takes place on top of carbon or on a steel mesh shelf?

Therefore, the silver-metal-clay binder must be burned away prior to activated-carbon firing.

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To burn off the binder I tried two methods. 

ONE

The first one I simply torch fired the silver and non-silver piece until it caught fire. 

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Remove the torch after the fire starts. Let the fire burn to completion.


Here is how I did this. I moved the torch’s flame in slowly so as to not shock the silver or non-silver metal. When I saw smoke, I knew the fire was soon to come. When the piece caught fire, I simply removed the torch and allowed the fire to burn out. You can kind of tell if the flame goes out prematurely. Some suggest you allow the flame to burn itself out, then turn it over and repeat on the backside to be sure no binder remains. You could do this, but I have found this to be unnecessary and might add too much heat to the non-silver metal.

Use a flame with no oxygen added to it, such as a butane torch. A cool flame is needed so the non-silver metal is not overheated during burnout. The non-silver metal might darken or turn a rainbow of colors. The low temperature of the flame is not enough to oxidize the copper or copper alloy beyond polishing.

TWO

The second method I tried to burn off the binder is a kiln firing at 1000°F / 538°C on an open shelf for 30 minutes at full ramp. 

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Ready for the quick kiln firing at 1000°F / 538°C

I did this a number of times with many pieces. After this binder burn-off the pieces looked fine. But during the final activated-carbon firing, each one melted to one degree or another.

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Toast. These were some of the more recognizable pieces. The others were simply globs of silver and bronze.


I got tired of melting my pieces and gave up on this method of pre-burning binder. Maybe a lower temp would work? Like 700°F / 371°C? 

If anyone knows why this is happening to me, outside of consistent bad luck, do let me know!

I’d also like to hear what other circumstances you run into in your metal clay work in which you need to pre-burn off binder.


For other online courses in metal clay, go to I Love Silver, where you learn how to design and create your own silver creations.

© Kris A Kramer 2017