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GaryR52 06-26-2005 12:30 AM

Bronze Clay?
I discovered a product called Silver Clay (, for jewelers, which is composed of 99.9% pure silver in powdered form, suspended in a clay-like binder. You model it like clay, then fire it at a low temperature in a kiln and the binder vaporizes, leaving only the silver. The object shrinks by 30%, so you plan your piece larger than you want the final piece to be.

I was thinking it would be a great idea if they made a bronze version of this for sculptors. You could model a sculpture, fire it like clay and have a finished bronze, ready for a patina, thus bypassing the process and expense of mold-making and casting. However, they don't make a bronze version of the product, so I'm wondering if I could make my own by mixing powdered bronze with a clay-like binder of some kind, or perhaps with a microcrystalline wax. If wax is used, the wax would burn out, leaving the fused bronze. Of course, it would have to be very well mixed so the bronze wouldn't be full of holes after the wax burns out. Any thoughts?


Arrow 06-30-2005 07:16 PM

Re: Bronze Clay?
I was thinking about this too. I was thinking about a mold filled with bronze powder and binder that could be microwaved too fusing temps. Had this idea since I saw this site.

I believe the key to the metalclay is the size and purity of of the metal particles. The average person wouldn't have anyway to grind the metal into the micro sizes. Bronze would have a higher fusing temp and mixture of different types of metals that would complicate the process.

Sinstered bronze gears and parts are possible, but they undergo a high pressure molding process before they enter a furnace for fusing.

GaryR52 07-01-2005 12:03 AM

Re: Bronze Clay?
Thanks for that link, Arrow! Great idea. I've been wondering what to do with the old microwave I've got sitting out in my garage unused. Now I know. :D

Well, I know from my research that metal clay or silver clay, as it's sometimes called, is 99.9% fine silver after firing and the gold is 24K, so, yes, they are very pure. You're right that the means to atomize bronze is beyond the capability of the average person, but that's not necessary, anyway, as atomized bronze powder is readily available. The issue of the metal content is moot, as well, since, once the wax burns out and the bronze is sintered together, you've got pure bronze that is exactly the same bronze as before, only fused. The process of firing it won't change its fundamental composition, just its state. It goes from a powder to a solid.

I'm not sure you're right about that high pressure part, either. I know that rapid metal printing can produce bronze parts (and other metals, as well, including steel) without any pressure applied at any point in the process. Also, in rapid metal printing, there is no furnace used. The metal is laser sintered. But, that's probably a different process than the one you had in mind. It does use atomized metal, though.

I'm really intrigued and excited about the prospect of using a microwave for this. I hadn't even considered it as a possibility. The 1000 C temp quoted in the article (1830 F) is perfect for use with metal clay, which needs to be fired at 1650 F (for PMC+) for 10 minutes, or 1830 F for two hours for the Gold PMC. Since silicon bronze melts at 1780 F, this would be perfect for bronze, as well. Good thing I have a spare microwave, as I'd hate to have any burnt out wax residue all over the inside of the one I cook my meals in. ;)

P.S.: well after reading the article, I'm a little disappointed to learn that the temps attained were the result of using an investment casting mold material that contains graphite and molochite. Obtaining either shouldn't be expensive or difficult, but I was hoping to be able to use the microwave for metal clay, as well as for trying my bronze clay idea. Maybe if I made a box with a ceramic/graphite/molochite shell into which I could place the metal clay object...but, then I wouldn't be able to keep an eye on it. Hmmm....


Arrow 07-01-2005 02:21 AM

Re: Bronze Clay?
I love fact that David Reid can reach in with his bare hands too invert the investment and molten metal.

Good microwave furnace pic.,00.html

I wonder how much silicon carbide bricks cost.

GaryR52 07-01-2005 03:45 AM

Re: Bronze Clay?
Well, according to this (, you can buy 50 lbs of silicon carbide for $87.40 (the black variety. The green costs much more, for some reason). I also found this, but no prices: Silicon carbide bricks are sold as kiln furniture, so they're fairly common.


Araich 07-01-2005 05:11 PM

Re: Bronze Clay?
That is really clever, using silicon carbide. I'd tried many years ago to make use of a microwave but couldn't work out how to build up such high temperatures.

How well does the inside of the microwave cope with that heat I wonder?

GaryR52 07-01-2005 09:17 PM

Re: Bronze Clay?
I wonder the same, Araich. Meanwhile, I emailed someone at the PMC Guild and he said he didn't think it was a good idea to try it at all. I figured he'd say so, since he began by saying they are reluctant to tell anyone they can do this because they don't want to be held liable for the consequences if someone burns down their kitchen trying it.

I would imagine that, since the silicon carbide absorbs the heat, it would be mostly contained by the bricks. But then, even so, it has to radiate that heat to somewhere, also.


Arrow 07-03-2005 08:58 PM

Re: Bronze Clay?
More microwaved metal info.

"Powder metals do absorb microwave radiation and can be heated and sintered, using microwaves... Because microwave sintering takes less time and lower energy levels, it is cost effective... We obtained essentially fully dense bodies with substantially improved mechanical properties compared to identical bodies sintered in the conventional manner"

Now I'm on the hunt for the right binder formula. I wonder how long before we will see the first Microwave powered fine arts foundries :)

GaryR52 07-03-2005 11:12 PM

Re: Bronze Clay?
Well, that was encouraging. I just wish the author would say
powdered, not "powder metals." ;)

The other article didn't mention the properties of powdered metals that this one does. Now it makes even better sense. So, basically, all I'd need is some powdered bronze, a binder to mix it with, and some silicon carbide bricks to form an enclosure with. I wonder if you have to completely enclose the piece, or if you can leave a little "window" to view it and reach in to get the part out? It kind of looked that way in the photos that were published with Reid's article.

I'm in favor of microcrsytalline wax as the binder, myself. It's readily available, can be melted down for mixing with the bronze powder, then can be cast in a mold while still liquid. Once cooled, it can be placed in the microwave, where it will readily burn out, leaving the sintered bronze powder. My only concern about wax is that it might burn out too quickly and the bronze, if not sintered yet, could collapse without the wax to support it. Then again, I think the wax burning would probably accelerate the sintering process, so it might work out fine. Other than wax, what could be used as a binder that is relatively inexpensive?

Another thing about using wax is that it can be worked after the bronze powder's been mixed into it, thus, you wouldn't have to pour it into a mold, you could just model it directly. I wonder how easily it would model or carve with bronze powdered infused throughout? It might be a little more dense and less pliable, I would think. But, I remember seeing mention of a product called "gold wax," which is powdered gold suspended in microcrystalline wax and what little I read about it said it's workable by hand.


Arrow 07-03-2005 11:28 PM

Re: Bronze Clay?
Silicon carbide Rub Brick:

Don't know if this form of silicon carbide brick would work the same as kiln bricks.

Or silicon carbide metal cut-off saw blades. I don't know about the rub bricks, but the blades are bonded with some type of resin that would probably burn.

For support I was thinking a layer of plaster. Sort of like a thin lostfoam coating. A sandbed might aid in support too. That is if the microwaves can penetrate through a sandbed.

Stevem 08-23-2005 12:25 PM

Re: Bronze Clay?
Hey all . I am fairly new to casting, I have been paying the foundries to mold and cast my pieces up untill recently. So at the sake of sounding ignorant, has anyone tried this process? I don't understand how the binder melts out and leaves the powder solid in the same form it was before. Wouldn't the binder melt before the metal? Seems like the wax would melt way before the metal had a chance too! Would this not create a puddle of metal at the bottom? at the very least would this deform the sculpture? I am very intrested in this if it is plausable.

Thanks for any information on this,
my small and inquiring mind would like to know,
Steve Miller

GaryR52 08-23-2005 03:56 PM

Re: Bronze Clay?
Steve, the wax would burn out and the bronze powder would sinter (fuse) together, forming solid bronze. The process would be identical to that used for Precious Metal Clay (Silver Clay), which consists of powdered silver in a resin binder. The piece would be fired in a kiln at a temperature high enough to burn out the wax, but not high enough to melt bronze.

Your response suggests that you're assuming this is a casting process. It is not. The "metal clay" would consist of powdered bronze suspended in either a resin binder or, as I advocated, microcrystalline wax binder. It would be pliable and you'd model your sculpture with it directly. To get a "bronze," you'd then fire the piece in a kiln, vaporizing the binder and leaving only the sintered bronze powder, which becomes solid bronze. The object is not to melt the bronze and pour it into a mold (which would make the whole modeling process pointless). This is intended as a cheap, direct substitute for arriving at a bronze casting, as it totally eliminates the casting process altogether.

I haven't tried this, as there is no such commercially available product; just silver and gold "clays," which are used in jewelry making. They shrink by 30% as the binder vaporizes, leaving behind sintered silver or gold. The same process should be applicable to any metal, though. All you need is powdered metal, suspended in some type of binder that can be easily burned away at relatively low temperatures. You can even "hand fire" Precious Metal Clay with a butane torch. If you substituted bronze powder, the firing temperature would probably have to be modified, but the process and result would be the same. Gold clay fires at a higher temperature than silver clay, so I would imagine bronze clay would fire slightly higher than gold.


fritchie 08-23-2005 10:03 PM

Re: Bronze Clay?
Steve - At the risk of sounding ignorant myself, because all I know about this process is what I’ve read here, I suspect the rate of heating is critical. The metal particles have to begin fusing together or “sintering” before the clay matrix is gone. If heating is essentially a “flash” process, that could happen.

I’m a little surprised at the high degree of shrinkage, but that probably reflects the maximum amount of metal powder that can be suspended in the matrix, possibly also with heating characteristics in mind. If the metal and the “clay” suspending agent were heated as separate powders with identical heat sources, the metal would heat up to 2 - 4 times faster because it would have a much lower heat capacity.

On the matter of shrinkage, I’m a little puzzled also. We are talking about 3D objects, so a 30% shrinkage in volume is about a 10% linear shrinkage. (That is, a 1 inch diameter ring, for example, would be about 0.9 inches in diameter after sintering. On the other hand, if the shrinkage is 30% in linear measurement, that same 1 inch diameter ring would be about 0.7 inches in diameter. But the volume of a 1 inch cube of this stuff would have a volume of about 0.7 x 0.7 x 0.7 cubic inches after sintering. The volume would be about 0.35 cubic inches instead of 1 cubic inch.

I suspect it’s volume shrinkage that is 30%. And a basic problem is that both silver and gold have much lower melting (fusing) temperatures than bronze. This process might not be feasible with bronze or copper. It might with aluminum, which also has a relatively low melting point.

GaryR52 08-24-2005 02:31 AM

Re: Bronze Clay?
Fritchie, I believe you're right that the shrinkage is 30% by volume, rather than linear. I haven't tried it yet, but, based on what I've read about silver clay, I think that's correct.

You may be right about bronze and copper not being suitable. Again, I don't know, yet. Aluminum powder would be ideal, though.


ERB 09-03-2005 08:44 PM

Re: Bronze Clay?
I have had several pieces "cast" using the sintered bronze process by Aesthetic Materials a "foundry" in State College Pa. I gave them a clay and they made a silicone rubber mold. A compound of wax and bronze was poured into the mold the piece was removed from the mold and "fired". The final bronze shrunk about 10% and was solid metal that could be brazed. The foundry is no longer in busniss. It was owned by a professor at Penn State who holds the patent on the process. I wish they would start up again because the pieces were great ans a fraction of a poured metat foundry.

GaryR52 09-03-2005 09:50 PM

Re: Bronze Clay?
Thanks, Dick. I had no idea anyone was using this commercially. Well, I guess they aren't anymore, but it is encouraging that someone was using it. It at least proves my theory about using bronze powder suspended in microcrystalline wax actually works, if nothing else.


GaryR52 09-04-2005 01:00 AM

Re: Bronze Clay?
In the discusssion we had on using a microwave oven as a kiln, there was a link to an article in which the author had used refractory bricks inside the oven to acheive high temperatures. The oven, by itself, isn't capable of temperatures high enough to melt metal.


Arrow 09-05-2005 12:28 AM

Re: Bronze Clay?
Found little info.

Aesthetic Materials: polymer-powder

Randall M. German
Aesthetic Materials (1998-Present

Too busy with NASA/hightech jobs to deal with sculptors :confused:

Arrow 09-05-2005 12:50 AM

Re: Bronze Clay?

"A process for sintering green powder metal, metal alloy or metal composition parts employing microwave energy is described.",805,835

Direct metal fabrication,517,773

GaryR52 09-05-2005 02:39 AM

Re: Bronze Clay?
I'd like to find out more about Aesthetic Materials, the company mentioned in the PDF. It says they "...simplify the production of bronze art via use of hybrid polymer-powder forming and sintering technologies. The company provides a variety of replication technologies for artists, designers and engineers...", unfortunately, I was unable to find anything on the company.


Arrow 09-05-2005 04:28 AM

Re: Bronze Clay?

GaryR52 09-05-2005 04:59 AM

Re: Bronze Clay?
Thanks, Arrow! :)

This sounds exactly like what I had in mind. It says you can carve directly in blocks cast from the material. The shrinkage, during sintering, is only 5%, too. One limitation I'm wondering about is the maximum height of 20". I wonder why that is?

They have an intro offer whereby you can order a free block of the material for carving, then send your carving back to them for sintering and they won't charge you for the process. Sounds like a sweet deal.

I'd like to find out if I can fire my own, though. According to another website, "Sintering temperatures can range anywhere between 1450-1550 degrees for bronze." An electric kiln can easily acheive such temperatures and so can a butane torch, so, it's entirely possible to fire your own bronzes. A little experimentation with mixtures of wax and bronze powder can probably duplicate their product, also. It would be cheaper to do so, not to mention faster (there is a 4 week turnaround time using their services).


bluedogshuz 09-05-2005 07:59 AM

Re: Bronze Clay?
Gary, Did you contact this company? I thought aprevious post stated they were not in business? Sounds great to do smaller pieces particularily if through direct carving vs molds. Also you are probably correct that if you had a large enough kiln you could produce larger works.

GaryR52 09-05-2005 01:42 PM

Re: Bronze Clay?
I haven't contacted them yet, Blue. As for the size factor, that's going to be limited by the confines of the kiln or furnace used for firing and that would explain their maximum height limitation of 20".


Arrow 09-05-2005 10:33 PM

Re: Bronze Clay?
Pilato Process:
"Pilato covers the clay miniature with several layers of silicon rubber, each of ten coats becoming progressively thicker with subsequent applications. In turn, he slathers the rubber mold with ceramic slurry encasements made in sections. These sections are then puzzle-pieced together to create a workable Mother Mold, which is then filled with wax. The wax miniature is then delivered from the Mother Mold and refined and detailed further. This rendition is then dipped in ceramic slurry 13 times, creating a solid, sturdy, layered mold. During the Lost Wax Process, Pilato vaporizes the wax replica and fills the vacant mold with powderized metal that turns to bronze. He then buffs, polishes and patinas the sculpture, producing a portable miniature of the most-often-times life-sized models-to-come.....cutting 20 steps out of bronze foundry processing..."

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