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  #101  
Old 04-18-2007, 05:45 PM
mark pilato mark pilato is offline
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Re: Bronze Clay?

What we did was place cans in a circle on our kiln shelves, very much the same way if you were firing clay. we put the pyrometer cable in the powder in the can that was the farthest away from the kiln elements and hooked it up to a computer so we could see exactly what was happening making sure that sculptures were not getting to hot, also very important. You dont want to melt the bronze you just want it to fuze together. the separate cans that are in the kiln do not need tops, after the powder sucks out the wax and you vamp heat up to second hold to burn out, the wax can freely escape out your burn out tube, which by the way should have an after burner/ flame to burn the wax as it comes out to reduce emissions in the air. the bigger can or your reduction container just needs a tight fitting top, no seal. We bought ours from a company that makes stainless cans. the nitrogen creates the perfect atmosphere in all parts of the can. I have some tricks of the trade i will share later when your ready to slush cast.
all the best,
Mark
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  #102  
Old 04-18-2007, 11:13 PM
Burkhard Burkhard is offline
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Re: Bronze Clay?

Thanks Mark for the additional info - much appreciated. Can't wait to have another go!
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  #103  
Old 05-05-2007, 08:03 AM
mark pilato mark pilato is offline
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Re: Bronze Clay?

Hi all,
I have been getting a bunch of phone calls/emails on this, sorry if i have not returned them, i am overwhelmed with sculpture projects/family at the moment, I still have not reset up in my studio but I am considering starting it up again, along with a small scale foundry for my larger works. I will keep you posted, has anyone had any luck with Rand German?
All the best,
Mark
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  #104  
Old 05-22-2007, 10:38 PM
Burkhard Burkhard is offline
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Re: Bronze Clay?

Direct bronze: came across an interesting patent the other day that people may be interested in: US 4854970 (just google "US 4854970" and you'll find it). Basically describes a metal clay made from 95% metal powder (nickel in the actual case of the patent) held together with 5% bentonite. Mix, add water, voila - metal clay. Apparently the inventor, R. Wiech, died a week before the patent was granted.

Had a go using bronze powder - gave a fairly "brittle" clay, sort of like a short pastry dough (for those who cook). Sintering at 900C for 2 hours under inert atmosphere gave a reasonable result for a first trial (see attached pic, height about 5"). Probably dried the bronze clay too quickly, hence some cracks appeared. Also, ignore the colour - I tried a quick patina but ran out of time to do anything properly. The material apears to be solid metal all the way through (I cut through a small sample block that I fired at the same time - beautiful bronze colour, could be readily polished to a mirror finish).


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  #105  
Old 05-23-2007, 12:11 PM
dawn dawn is offline
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Re: Bronze Clay?

Well, now it seems simple enough for dummies like me. But...what is an "inert atmosphere"?
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  #106  
Old 05-24-2007, 12:32 AM
Burkhard Burkhard is offline
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Re: Bronze Clay?

Dawn - apologies for the jargon. If people are interested I'll post a more complete description of what I did as soon as I have a bit of time.

In brief, an inert atmosphere means firing the bronze clay in an atmosphere of either argon (slightly expensive), nitrogen or possibly carbon dioxide (I haven't tried this one yet). If the bronze clay is fired in air then all you'll end up with is a lot of annoying brown copper oxide powder rather than a sintered sculpture. Using the bronze clay rather than the "pilato method" has advantages in terms of much shorter firing time (3 hours total), no noxious gas evolution, and ability to directly produce a sculpture from the bronze clay. Drawback is still that you have to fire in the absence of oxygen just like the Pilato method (I'm currently trying to figure out a way to do the firing directly in air, and although I have some ideas it's slow progress). At the moment I've got a large steel pot sitting inside the kiln with a 1/4" steel gas pipe feeding argon gas (ex welding supply company) into the pot. The lid is not gas tight so I don't need an exit line. The sculpture goes inside the pot, gas flow is turned on a little bit and the whole kiln is heated to 900C. Ceramicist know of this type of arrangement as using a saggar, although usually the saggar is made from ceramic not metal. It may be possible to fire the clay in a gas kiln under very heavy reduction atomsphere without the need for a nitrogen atmosphere - haven't tried this yet.

SAFETY NOTE: If you're using a metal pot with an electric kiln make sure that you cut off any protrusions such as handles etc. I believe that the heating elements inside a kiln are "live" when it's turned on i.e. 240V. If the metal pot touches the elements and you touch the pot or the metal gas line you could electrocute yourself. I also put some kiln stilts around the outside of the pot so that if I knock the kiln then the pot cannot fall onto the elements.

Also a lot of (cheaper) cooking pots have bonded aluminium bases (i.e. aluminium core inside a steel clad base) - these need to be removed (angle grinder, heat) otherwise the Al will melt during the firing. At best this will result in a mess in the kiln, at worst a small explosion inside the kiln as the pot ruptures.
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  #107  
Old 05-24-2007, 10:36 AM
dawn dawn is offline
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Re: Bronze Clay?

Whoops! I was wrong.

Are these somthing commonly practised by ceramists, I mean, the setting up of this kind of kiln? If so, maybe I could ask some of my ceramic friends to help me with it.
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  #108  
Old 05-26-2007, 07:05 PM
DanielUCM DanielUCM is offline
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Re: Bronze Clay?

Burkhard - Thank you for giving us some of the results of your research! It's appreciated. In a couple of weeks I'm going to get more time on my hands and then it will be great to try this out. I'll read the material you suggested but I just have to ask you a couple of things: Do you just let the clay dry in your mold enough so it holds up to be moved to the kiln? Do you still need to use the aluminium oxide to stabilise the green bronze?

Best wishes
Daniel
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  #109  
Old 05-27-2007, 10:09 PM
Burkhard Burkhard is offline
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Re: Bronze Clay?

Dawn – finding a potter who is also a welder (!) would probably be your best bet in terms of setting up the kiln and helping you find a supplier of gas cylinders, valves, tubing etc. I’m a lousy welder so I used a thingamajig called a “stainless steel Swagelock bulkhead union for ¼” tube” (Part No.: SS-400-61 from Swagelock.com, bought over the net for about $25) to connect the tube to my pot. This just requires you to drill a hole in the pot and then using two pliers one can connect the tubing. They also sell steel tubing but only in 6m lengths (for about $40 I think).

If you do try and set this up you should also get such a person to cast a critical eye over the set-up in terms of safety i.e. is the cylinder properly secured to a wall so it can’t fall over, is the ventilation adequate so that the kiln firing isn’t going to poison you but also that the nitrogen gas isn’t going to asphyxiate you, what happens if you bump into your kiln – any chance of the pot contacting the kiln elements and becoming live etc etc. I think I’m getting old and finally getting cautious - too many near misses due to my own stupidity.

Daniel – the two materials (i.e. the bronze wax from the Pilato/German work and the bronze clay from the Wiech work) behave very differently.

The bronze wax can be poured into silicon moulds and is quite hard when it cools. It can be readily carved but isn’t really suitable for additive modelling type of work as far as I can tell. Maybe there’s a trick to this but I haven’t found it so far. Also, I haven’t actually been able to get the bronze wax method to work satisfactorily for me so far. I suspect that filling the silicone mould while vibrating it is quite important, there are whole books written about “vibratory compaction of metal powders” for producing sintered objects.

The bronze clay acts like a very “brittle” clay and just like clay it’s difficult to use in silicone moulds. The little figure in my previous post was done by pressing it into a two-part silicone mould, carefully taking one half away, allowing the bronze clay to dry for a day, demoulding fully and then refinishing it, filling in cracks etc before allowing it to fully dry. Overall it was not that successful, some distortion and fine detail would be difficult to transfer. I think part of the reason the final figure had cracks in it was due to the uneven drying. A plaster press mould would probably work better, might even be able to slip cast this stuff – don’t know. The clay will probably be better suited to direct modelling type of work. I’m currently trying to do a couple of simple figures by direct modelling. One might also be able to make larger blocks in a plaster mould, allow to dry and then to carve the stuff.

I tried to get clever and make a bronze clay with better modelling properties by adding methyl cellulose binder and olive oil to improve handling – could improve the modelling properties of the clay but on firing all I got was powder. I suspect that the drying process of the bronze clay also serves to shrink and hence compact the bronze particles. This compaction seems to be important.

Be warned the stuff is very messy. I have no idea how toxic copper is when used in this way i.e. how much could absorb through the skin. In principle you should wear gloves. Also, although copper is not that toxic (you need to swallow about 20-30g of ionic copper to kill an adult) make sure you don’t leave any around where a child could swallow some – the copper would dissolve in the acid of the stomach and a teaspoon of copper clay could poison a child.

When I fired the bronze clay, I did pack it in some TiO2 powder that I had. This was mainly for support, but is not required as in the bronze wax process to wick away any wax. Next time I’ll try some ordinary builder’s sand for support, I suspect that it would work just as well.
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  #110  
Old 05-28-2007, 01:10 AM
dawn dawn is offline
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Re: Bronze Clay?

Thanks. I'll try to ask around. I wish there will be a step-by step- book on this whole process someday.
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  #111  
Old 05-28-2007, 02:19 AM
Burkhard Burkhard is offline
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Re: Bronze Clay?

Working on it Dawn Problem is that at the moment it's still the blind leading the blind. Well, perhaps we've managed to get to the "partially sighted" stage.
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  #112  
Old 05-28-2007, 03:35 AM
DanielUCM DanielUCM is offline
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Re: Bronze Clay?

Thanks! Yes, these types of formulas have different strenghts and weaknesses so I guess that so far the choice depends on the particular sculpture one wants to do.

(Burkhard - pm sent)

/Daniel

Last edited by DanielUCM : 05-28-2007 at 04:09 AM. Reason: Adding sentence
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  #113  
Old 06-02-2007, 06:57 PM
mahmoud haggag's Avatar
mahmoud haggag mahmoud haggag is offline
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Re: What is the composition of & Importance

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  #114  
Old 06-03-2007, 02:16 AM
jphariot jphariot is offline
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Re: Bronze Clay?

Hello Burkhard,
I am really impressed with the amount of research you have done here.
I was doing a google search for Bronze Clay and lucky me. I have been researching this on the side for about 3 years now. I am impressed with Marks process, and especially his work. But when you described the Gasses emitted during the wax burn out, I got a little nervous. But I would still prefer that to investment casting hell.
Let me mention that I have been investment casting on and off for about 5 years and hate every bit of it.But now I have no choice, because I have some projects that require it. So I have been looking at the PMC for about 4 years.
Wondered about using it for Bronze and other alloys, but realized that they would oxidize unless placed in an inert atmosphere.
I have a question: Would it work to place an electric kiln into a vacuum chamber? Wouldnt that make it inert? If I have to use Nitrogen then I will. But I would like to get a clearer picture of how that works?
So to continue, I was a bit nervous with the gasses emmited and wondered why not just use the same binder as the PMC?
Then I read your discovery of the Bentonite formula. Sounds good. I think I have used Bentonite as a sand casting binder??
At any rate, how is the process going now? I was planning to start my experiments by late summer-early fall, but I am really getting so tired of investment casting that I think I am going to push it to a sooner date.
I am working on building a resin casting machine, and I am developing my own alloys, so I am really into helping you all here in working on this.
I am wondering if Mark is very commited to the Wax-Bronze Powder system, or if he is interested in helping with some other methods? I am just trying to find an efficient, consistant, and safe system to sinter Bronze and other alloys.
I currently cast a lot of pewter into silicon. Love the undercuts and freedom. But the choice of alloys is really limited. Its left me with the dreaded investment casting.
Thanks again, and please let me know where your at with the Bentonite Formula?
Thanks Again,
John Hariot

Last edited by jphariot : 06-03-2007 at 07:31 PM. Reason: wrong word
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  #115  
Old 06-04-2007, 04:20 AM
Burkhard Burkhard is offline
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Re: Bronze Clay?

John - thanks, last summer I got as far as setting up a small foundry and learning how to do small lost wax pieces and also very quickly got tired of the amount of work and effort required.

In terms of vacuum chamber - in principle yes you could set up the furnace inside a vac chamber, I think I have seen reports where this is done. However, you'd need an impressively large vac chamber to stick a whole kiln inside. You'd also need to provide access for power and control wiring and I'm guessing a very good quality oil pump. Alternatively sticking a vac chamber inside the kiln could be done, but now you need to make sure that the chamber is vac tight at 900C and isn't going to implode at these temps. Also if using a vac pump remember you'll need very good gas traps else the soot produced will quickly gum up the vac pump. If you have access to such equipment it's possible, but not worth considering to do at "home" because it would be much simpler to just connect a gas line to the chamber and flood the chamber with an inert gas. I haven't gotten around to taking pics of my current setup yet - it's our end of financial year coming up and I'm up to my ears in project planning, budgets and other $%#* stuff.

I just finished a "quick and dirty" experiment that shows that carbon dioxide gas can also be used as the inert gas. This is useful because the easiest (and probably cheapest) source of gas cylinder/inert gas combo for me are CO2 gas cylinders from the guys who supply the beer home brew crowd (apparently used to charge kegs of home brew). Maybe I can kill two birds with one stone.

Yes, you probably did use bentonite as a sand casting binder. Should also be available at most pottery supply shops as it's used in clay and glaze formulation.

I've now fired the bronze powder/bentonite (95:5 weight/weight) mix three times, all more or less successfully. My current firing regime is to load the pieces inside a can packed in TiO2 (for support) inside a 14l stockpot, place lid (lid has steel gas line attached that is connected to gas cylinder) on top, place three heavy steel weights ontop of lid (to give a reasonable seal), turn on gas. I then ramp up the heat to 900C, which takes me two hours, then hold at 900C for 1.5 hours, turn the kiln off, and allow to cool under inert gas. So far I've not been very patient, so after 30 minutes I open the pot up (wearing welding gloves, face shield and using raku tongs) and take out the bronze pieces and quench in water to see the results. I probably should let things cool down far more, the bronzes are still red hot at that stage.

Couple of things I've noticed with the bronze clay:

One - with the pieces I've done so far it looks like the very top of the surface (top 0.1 mm?) of the fired bronze clay is quite porous and pitted (using a magnifying glass) and in order to get a good finish I need to run a buffing/sanding wheel over the surface. This is not really a major problem except for very fine detail. Don't know whether this is intrinsic to the bronze clay, the firing temp inside my pot, or the size distribution of my bronze powder.

Two - my larger hand formed pieces develop cracks in them either on drying or during the firing. This may be due to bits of bronze clay not bonding when I add them together in the "wet" stage. I've tried adding some paper pulp to the bronze powder/bentonite mix (approx 1-5 g paper/100 g bronze powder/bentonite) in order to make a "bronze paper clay" and this fired as well as the ordinary bronze clay. I think it may be better to sculpt with (not as "brittle"), and I'm hoping that just like ordinary paper clay that it will allow the joining of wet clay pieces more successfully.

[I'm going to try to go back to the bronze wax soon to see if I also see the surface pitting there, but it really does produce a whole lot more soot and noxious gases than the bronze clay. (At one stage I lifted the lid of my kiln up during the firing of a bronze wax. My steel pot lid must have leaked quite badly because the inside of the kiln was full of flammable gas which gave a quite exciting whoompa as the air hit the gas and ignited in a ball of flame. Luckily I was wearing face shield and long sleeved welding gloves so no harm done. Very pretty really). Also, I've not yet managed to figure out a 100% effective way of burning all the soot that comes out of the gas outlet line when using the bronze wax, even sticking a propane torch on the outlet was not totally effective.]

In terms of why not use the same binder as used in PMC ? Good question - simply because I've been totally unsuccessful with this so far - I'd love to know why. It could be due to a whole bunch of reasons to do with the way I fired the pieces, or with the fact that the particular binders I used where slightly different and decomposed leaving too much residue behind or something else. This does not mean it won't work, I just haven't hit on the right combo. Too many things to try, alas, not enough time.
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  #116  
Old 06-04-2007, 06:39 AM
enitharmon enitharmon is offline
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Re: Bronze Clay?

I have been reading the progress here for months now, quite inspiring. Always exciting when things go boom! Motivated me to read patents and articles. Burkhard, good to see Aussie initiative! the patent for the bentonite mix suggested using an organic substance for greater viscosity...sugar was mentioned. Did you try that? Experience with ceramics, joining pieces to avoid cracks...if you cross hatch and slip the join, blend the join and the seal the sculpture in a plastic bag leaving it for a few days to redistribute the moisture across the join has been successful..might work with the bentonite mix. An article looking at the oxidation, weathering and porosity of the sintered bronze written by Dr German sounds promising too and shows [microscopic] images of the process.
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  #117  
Old 06-04-2007, 08:49 AM
Burkhard Burkhard is offline
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Re: Bronze Clay?

Hi enitharmon - thanks for the encouragement to all the people who posted in this thread. Hopefully slowly but surely some more people will have results (positive and negative) to share.

I haven't tried the sugar additive (I thought he used that to increase the amount of microorganism in order to increase slipperyness. I'll have to reread the patent. I sort of assumed this was just one of those throw away lines used in patents as I don't think it was actually listed as one of the patent claims, but you're right - worth a quick experiment).

With regard to the joining of pieces of bronze clay - I sort of tried cross-hatching (although not as well as I could have), adding bronze clay "slip" to the parts before joining, and also left the small figure in a plastic bag for a couple of days to equilibrate the moisture, followed by slow evaporation in the plastic bag with only a few holes, before removing the bag etc. No joy. Cracks still formed. However, this may still be just a matter of technique. Read in another article that in order to avoid cracking of ceramic greenware, they did the initial drying of their green body at 90% humidity for 24 hours when most of the shrinkage occured, followed by normal drying.
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  #118  
Old 06-07-2007, 06:01 AM
enitharmon enitharmon is offline
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Re: Bronze Clay?

Hmm if the bronze paper clay works whole areas of possibility are opened, modelling, shaping, carving, embossing. Inspired again Burkhard!
Was in situ strength maintained? Firing paper in ceramics causes problems but you say it worked with the metal, do you think it increased the porosity? Burnishing prior to firing ceramics is an old technique to compact the particles, I wonder if this might apply to the metal clay and reduce porosity.
Did you use a variety of particle sizes for the bronze powder somewhere I think I read a ratio of 7:1 small to large gave greater sintering strength and reduced porosity, same article said the firing cycle for the wax bronze was 24 hrs which would imply a longer cooling period. Is the proportion of crystalline wax to parafin critical?
Could anyone help with where to get metal powder in the UK coz I think its time for the burning boom! stuff here too.

Last edited by enitharmon : 06-07-2007 at 06:32 AM.
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  #119  
Old 06-07-2007, 10:54 AM
Harryman Harryman is offline
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Re: Bronze Clay?

Any thoughts on forming the clay over a core that could burn out? Less mass of clay would mean less shrinkage, maybe less cracks. Or is this stuff too friable to hold up in a thinner layer?
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  #120  
Old 06-08-2007, 02:32 PM
jphariot jphariot is offline
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Re: Bronze Clay?

Hello Burkhard,
Thanks for the response. Now your saying that you have not had good results, but that initial bronze statue that you have photographed is not bad for a start.
Quite encouraging really.
Now, I wasn't sure about your response to using Methyl Cellulose, Like PMC uses. Do you mean that you tried it, and it did not work well? Or you have not tried it yet?
The PMC seems to be really easy to work with, and basically full proof. Actually, I ordered some Methyl Cellulose from Dow and should get it today or Monday. I already got some Bentonite. What doesn't make sense about Bentonite is that its a ceramic right? So I would imagine that It would NOT burn out, but stay in with the Bronze and bond or "alloy", if you will.
Also, I made an Aluminum Powder-Bentonite clay test peice. It is drying right now. I imagine that Aluminum would not oxidize prior to sintering?

Good Idea about using the paper pulp. Theres a Japanese product called Paper Clay, I think it also has volcanic ash in there. I have it in a drawer, in an opened package. Got to get around to it, but that has been in the back of my mind for a few months.

Also, enitharmon has an interesting idea with the sugar. Keep us posted of any developments.
And Harryman, about the core issue. I think that PMC makes a cork material that they use as a core. As far as I have read it works fine. See Rio Grande catalogue. Or maybe Riogrande.com I think is the site.

Now, Burkhard, I have a few more questions about the Gas-vacuum issue:
What I did have in mind for vacuum was as you mentioned to place a small electric furnace inside of a steel vacuum chamber. Its easy to just drill holes into the vacuum chamber and run wires through, Pyrometer etc and seal them with epoxy. I do have a nice welch vacuum pump.
BUT!! if nitrogen is easier?? Then why am I going to build a vacuum right away. I am ignorant about nitrogen. What kind of safety issues is there with nitrogen? So I would place a stainless Steel pot with lid inside Furnace, where I then place the Metal clay piece into, correct? And theres a stainless steel tube running into the stainless pot. Correct? I guess this means that I cant sinter stainless steel parts with this system. Got to stay with CU based alloys or silver- gold etc.
But, If CO2 works fine??? I remember using CO2 for sand casting core molds that I tested a few years ago. Does the CO2 work the same way as the Nitrogen? Any safety cautions? CO2 explosive in 1800F environments? I should have taken chemistry in college. Would you suggest CO2 instead of Nitrogen?
Also, do you have any more photos of your more recent sintered Bronzes?
Lastly, If I can get this to work with these binders in question, I think that the next step would be to maybe use a binder that will not air dry. Time consuming and conducive to cracking. Good to start with, but I am already wondering about Sodium Silicate, which cures immediately under CO2, or they have some one part epoxies now that set at 250F. I am getting ahead of myself here, so for now, I am going to focus on making this work. I will post photos when I get my first success.
Thank You,
John Hariot
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  #121  
Old 06-09-2007, 03:27 AM
Burkhard Burkhard is offline
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Re: Bronze Clay?

Hi enitharmon

Was in situ strength maintained? Firing paper in ceramics causes problems but you say it worked with the metal, do you think it increased the porosity? Burnishing prior to firing ceramics is an old technique to compact the particles, I wonder if this might apply to the metal clay and reduce porosity.
Did you use a variety of particle sizes for the bronze powder somewhere I think I read a ratio of 7:1 small to large gave greater sintering strength and reduced porosity, same article said the firing cycle for the wax bronze was 24 hrs which would imply a longer cooling period. Is the proportion of crystalline wax to parafin critical?


WRT the bronze paper clay: I tried 4 samples with 95g bronze powder, 5 g bentonite and approximately 0.25 g, 0.5 g, 0.75 g, 1g of paper pulp added (paper pulp was simply tissue paper that I blended with water using a paint mixer attached to an el;ectric drill). After firing I couldn't see any difference in external appearance. I haven't cut the samples in half yet, and looked at them under a microscope, to see if there are any internal differences, but all seemed to be "solid" bronze pieces and couldn't be twisted/broken by hand. The idea came from the fact taht I use "paper clay" routinely for the "normal" sculpures that I do - this stuff is just ordinary clay mixed with some paper. The paper allows you to join clay that is almost dry and then fires quite normally.

The bronze powder I used was what was available to me over the internet from the local sculptors supply company and is the stuff used for cold cast bronze where bronze powder is mixed with resins to produce the bronzy looking resin casts. I have no idea at this stage what the size distribution of the particles is. A quick look under the microscope only revealed that there were aggregates (50-100 microns) as well as particles below 5 micron in diameter. I figured this was good enough for a first attempt.

I think for the firing of the bronze wax, the firing cycle is far more important than for the bronze clay. Especially the decomposition phase for the wax.

In terms of wax composition - sorry, no idea. I know that when I used "jewellry" wax, used for casting jewellry findings, all I got was powder, but this could have been due to other problems at the time.

I came across a paper that looked at slumping of sintered bronzes. Two findings were that at about 880C the sintered bronzes had achieved their maximum density (about 90% of theoretical maximum) with virtually no slumping/distortion but that the amount of slumping/distortion increased rapidly at temps above 900-910C. In future I'll take the temp to 880C not 900-910C as I have been doing. Given that pyrometers are also not that accurate I may have been overheating my kiln.
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  #122  
Old 06-09-2007, 03:36 AM
Burkhard Burkhard is offline
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Re: Bronze Clay?

Hi Harryman,

Any thoughts on forming the clay over a core that could burn out? Less mass of clay would mean less shrinkage, maybe less cracks. Or is this stuff too friable to hold up in a thinner layer?

When I did the hand formed figure I did use a small core of tissue paper wrapped in a small plastic bag. The figure was formed over the top and after drying a little bit I carefully removed the core, and then allowed the bronze clay to fully dry. I suspect that just like ordinary clay, allowing the clay to dry over a solid core would lead to cracking. I've never used the cork core used with PMC.

Linear shrinkage of the bronze clay on drying was approx 9%, after firing it shrunk by another 10%.
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  #123  
Old 06-09-2007, 06:36 AM
Burkhard Burkhard is offline
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Re: Bronze Clay?

Hi John,

Sorry if I tended to sound too negative about the results so far, I'm actually quite pleased with it and you should have seen the jump for joy when I took the very first sample out and it looked solid and like bronze. I guess I'm a cautious type and don't want to give people a false impression that everything works perfectly. Also I'm trying to be cautious in terms of saying what will and what won't work - quite often something that won't work for one person will work for the other, who does things just a little differently.


I tried adding some methyl cellulose to the bronze clay. I made up a 1% wt/vol solution of the Me cellulose in water and used that instead of pure water when making the bronze/bentonite clay. I also tried adding a small amount of olive oil to see if I could make it less sticky. On firing, a small figure made from this was just dust after firing, even though another sample next to it made from the ordinary bronze clay was fine. I read somewhere that depending on the type of binder used and the heating regime it is quite possible to coat the bronze particles with carbon residue from the decomposition of the binder which can prevent them from sintering. This may be the case for the Me cellulose. Remember that the silver PMC is fired in air, hence the binder can burn out, whereas for the bronze powder, we're heating in absence of oxygen, i.e. no burning only decomposition of the binder. Also, in the bronze wax method, the wax is first wicked out into the aluminium oxide powder (leaving a bronze powder with virtually no organic matter in it) and then it decomposes in the aluminium oxide powder not while touching the bronze powder. I'm very curious to see if you have any luck with it.

Yes the bentonite won't burn out so the bronze clay will give an "alloy" that's about 85% copper, 10% tin and 5% bentonite. The original patent actually calls the water the "binder" and the bentonite the "plasticiser" for the metal clay.

WRT the aluminium - aluminium oxidises so quickly that all aluminium has a layer of aluminium oxide on the surface. If you make pristine aluminium powder and expose it to air it can actually spontaneously catch fire because the rate of oxidation is so quick and vigorous. So, the Al powder that you have will certainly already have this layer of oxide on it. Whether the oxide layer is thin enough, porous, brittle enough etc to allow sintering betwen the Al metal cores of the particles - hopefully you'll be able to tell us soon .

In terms of the gases: I really don't know enough about engineering of vacuum chambers to give good advice here, except if you use a vac pump I really don't think you'll be able to do the bronze wax stuff because the soot etc will gum up the pump (also not sure whether the pump is rated to pump flammable gases).

If you can set up a vacuum chamber you certainly should have no problem setting up a gas line and a steel pot inside the furnace. I'll try and take some pictures and post them in the next day or so.Trying to sinter steel in a steel pot would be touch and go. You might be able to get away with it as for sintering you want to stay clear of the melting point (and even the slumping/distortion temp) of the material your sintering.

Common inert gases are argon, nitrogen and carbon dioxide. I've tried argon and carbon dioxide and am sure that if these work then nitrogen will as well. An advantage of argon and CO2 is that they are heavier than air and so will form a better gas "blanket" in the bottom of your kiln pot.

The gases are not toxic by themselves i.e. you won't be generating toxic fumes by using them, they won't explode at 900C. I was initially worried that the CO2 might chemically react with the copper to form copper carbonates at high temp and hence stop sintering - this doesn't seem to be a problem.

The main hazard with these gases are that in enclosed spaces they will displace air (oxygen) and and if you happen to be in that enclosed space you will asphixiate and probably die. However, if you're setting up a kiln to operate in an enclosed space without good air circulation then you probably shouldn't be doing this stuff anyway .

As asphixiant nitrogen and argon are more dangerous than CO2, because you can't tell when you're breathing pure nitrogen (or argon). When you hold your breath and you feel the need to breathe that's the CO2 level in your lungs increasing. The body has sensors that can sense this increase in CO2 and it tells you to breathe again. The body has no sensors for nitrogen or argon (or oxygen for that matter) so if you breathe pure nitrogen you will continue to breathe perfectly normally with no sense of asphixiation - and you will have (literally) just enough time to say "oh bugger" as you fall to the ground, never to get up again. Unfortunately this happened a couple of years ago at work (three fail-safes all failed at the same time, the poor guy walked about two steps into a nitrogen filled room, end of story).

As I said, with normal ventilation (e.g. open window or doors) there should be absolutely no problem. I like CO2 because for me it's cheaper, it's heavier than air so will probably be better at excluding air in a pot with a leaky lid, and if by some absolute freak occurance the CO2 levels increases where I'm working I will know.

I was wondering about the sodium silicate but wasn't sure how alkaline/corrosive the stuff was. Certainly worth a shot though. I'm still curious whether part of the reason why the bronze clay work is because on drying the bronze particles are pushed close together until they touch.

Good luck everyone!
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  #124  
Old 06-09-2007, 11:23 AM
Harryman Harryman is offline
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Re: Bronze Clay?

Appreciate all the effort your putting into this project Burkhard, you're making great progress. I'm really enjoying watching it come along.

I've built a few vacuum systems and it would be a lot easier to flood an area with an inert gas than to try to eliminate all leaks in a vacuum system, especially one operating at a high temperature. Not to mention getting a pump to operate well under those conditions.

I agree that CO2 is a great way to go, it's easily available, relatively cheap and pretty hard to kill yourself with.

There's carbonated springs in my area that some of the local kids used to use to get high. They'd lay in the hollow by the water where the CO2 had displaced some of the O2 untill they felt light headed. Great fun untill someone lay a bit too long. Ignorance or evolution in action, take your pick.
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  #125  
Old 06-09-2007, 02:12 PM
jphariot jphariot is offline
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Re: Bronze Clay?

Burkhard,
So, the aluminum tests I did proved what you are saying. In fact thats what I thought yesterday. I looked at the powder and it really did not seem shiny. So I figured that it is already oxidized.
But is there an aluminum powder that you can get that is NOT oxidized?? I am pretty sure that they do sinter aluminum,so there has to be, I would guess.

As far as the Methyl Cellulose. It sounds like you didn't try it alone without the Bentonite, so do you think that there might be a chance that it works under vacuum with bronze, or with CO2? Or it won't work?
If that is the case, maybe we could just use it with silver powder in an oxygen atmosphere like PMC. Why not just buy PMC then? Well, it is about twice the price of silver on the market. Not sure its worth making an in house version with available powders?

Now as far as the Gas vs Vacuum, I think you have sold me on CO2.
My kiln is outside to top it off and when I have one inside, I will always open the doors. But to make sure, I am not sure what it is about CO2 that you could detect? Do not quite follow that.
Now what might also be an added benefit to CO2 is if by chance we could use a sodium Silicate binder, then the CO2 could be used to cure the binder, and it cures quickly. I mean in maybe like 30 seconds or 60 at the most.
I was attempting to make small sand molds with undercuts. So I had urethane molds that fit into a wood box. Then I would pack the molds with sand and Sod. Silicate. Then you put a lid with a hole and stick the CO2 tube into that hole and give it gas. Any way you can figure to impermeate the sand mold should work. And soidium silicate is cheap and water based. The only question is, what happens during sintering?? Thats the million dollar question. Well, I can try it and find out. But first I need to try the Bentonite, CO2 and Methyl Cellulose.
Now that first sculpture, ( the image you posted), you said that it cracked coming out of the silicone mold. I figured you already thought of this, but still, what if you placed it in a thin dryed out plaster mold? Like in Ceramics, I should suck the moisture out of the bronze clay. But a hard mold of plaster will not be forgiving as to demold the Bronze Clay. So you would have to break the plaster, Dissolve it in Muriatic Acid???, or place the whole thing with bronze clay still inside the plaster mold , into your Kiln or furnace. Think that might work?
I can totally believe how excited you were when you pulled out your first piece. I plan on getting that CO2 this week, if I can, or the next and making it happen here. One last question about CO2. How much are you using? Or how much do you introduce into your stainless steel pot? You must have a regulator with an amount?
Thanks again, and I am looking forward to seeing more pictures of your Bronze pieces.
John Hariot

Last edited by jphariot : 06-09-2007 at 02:56 PM.
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