Thread: Bronze Clay?
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Old 04-08-2007, 01:54 AM
Burkhard Burkhard is offline
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Join Date: Apr 2007
Location: Sydney, Australia
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Re: Bronze Clay?

This is my first post on this board – apologies that it’s so long!

I came across this thread a couple of days ago and was interested because I tried to see if it was possible to make bronze PMC about a year ago (I was stunningly unsuccessful). Then got really excited that someone had solved the problems and even got it as far as a commercial venture – wow!. Followed by immediate disappointment that the company went out of business. But if one group of people can do it then it can be resurrected and if needs be, reinvented. I’m sort of hoping that there are enough interested people here that we can resurrect the technology between us. I’d love to say I’ve got the method to work and here’s how, but sorry no. Haven’t gotten further than re-commissioning my old kiln, ordering some bronze powder and doing some background reading and musing.

I’ve put some of this info down “on paper” for my own use, but in case people here are interested, I thought I’d post the stuff I’ve got so far. Excuse the style if it sounds stilted, too much like a lecture, and contains stuff others have already mentioned in this thread - I haven’t had time to edit it properly.

DISCLAIMER #1: Why do I think I can add anything to this thread? A long time ago (sometime last century) I got a PhD in chemistry and since then have been doing research in chemistry related fields for last 25 or so years (Sculpting for the last 15 years). However, I am NOT an expert in the sintering of metals or the process described by Mark Pilato so I make no guarantee that anything I say here is mistake-free. In fact, I can almost guarantee that I've made some mistakes.

DISCLAIMER #2: The following is a current “best guess” reverse engineering of what the “Pilato Method” entails. This is based on the comments that Mark made in this forum, the links to the “CISP Fall 2005” newsletter article that Mark supplied a link to and the patent (PCT WO99/54075 “Powdered material rapid production tooling method and objects produced therefrom” Inventors German, Weaver, Thomas, Atre, Griffo) which I think is the original patent referred to in the CISP article. However, this patent does not describe the actual “Pilato method”. As far as I could see the “Pilato method” has not been patented directly (and would be difficult to patent given the prior patenting art). I don’t claim that this is the actual method used or that anything described here will give the results that Mark described, or that it will actually work!

DISCLAIMER #3: As with any process there are inherent dangers and risk. Hence, I do not recommend that anyone tries to repeat these processes. In fact I recommend the opposite – that is, don’t try this. If you hurt yourself, blow yourself up, poison yourself, kill yourself, lose an eye or two, burn yourself or do yourself or your property some other damage, please do not blame me. You have been warned. Some safety issues to consider: Any work with hot objects, kilns, unknown materials at high temperature etc carries safety risks. Also, the “Pilato method” may produce toxic gases such as carbon monoxide. The “Pilato method” may produce flammable and/or explosive gas mixtures.

Background:
The Pilato method uses sintering of bronze particles to produce bronze objects. The method is potentially very useful because it simplifies the traditional, labor intensive, lost-wax bronze casting methods. Depending on how it’s used, it could also free the sculptor from limitations due to the necessity of having to produce moulds i.e. one could work directly in bronze. Does it work? It appeared to, as a company was set up and several customers (apart from Mark) had good experiences with the company. The company no longer exists, so the question is “can the method be reproduced in a non-industrial setting by ye average sculptor?”

Put very simply, in the Pilato method, an object (e.g. a sculpture) is produced using a mixture of wax containing 60-70% bronze powder. The object is then placed in a container and packed with 1-10 times its volume of a very fine powder such as aluminium oxide. The container is placed in a kiln that can be sealed and that has an inert gas (e.g. nitrogen) line inlet and an outlet attached. The temperature in the kiln is slowly raised. As the wax melts, the aluminium oxide absorbs the wax, as the temperature is raised further the wax decomposes. As the process is carried out in an inert gas (i.e. in the absence of oxygen) the wax does not burn in the kiln but decomposes to a mixture of hydrogen gas, carbon (i.e. soot), methane, ethane, propane, butane etc (similar to natural gas or LPG type gas). This flammable gas exits the kiln via the outlet line and is ignited here to prevent build up of flammable gas.

Please note: The mixture of flammable gas produced in combination with oxygen from the atmosphere and high heat from the kiln could lead to explosive gas mixtures. If the burning of the flammable gas mix is carried out in an oxygen poor environment toxic carbon monoxide could also be produced. A very (very) rough calculation says that a kilogram of wax could produce about 300 liters of flammable gas.

The temperature of the kiln is then raised to the sintering temperature of bronze (in article it said about 840C). After cooling, the object is removed from the aluminium oxide powder and is ready to be polished, patinaed etc.

In order for bronzes to be produced by a sintering method, three main problems needed to be solved. Firstly, sintering produces porous objects – how can one increase the density so that the strength, weight and feel of a bronze are obtained? Secondly, metal powders oxidize very rapidly at high temperature. If you heat bronze powder in air you’ll get copper oxide and tin oxide powder not a bronze statue. Thirdly, the bronze powder has to be mixed with a binder in order to form and mould it in its green state (i.e. in its pre-sintered state) – how can one remove the binder if we cannot burn it out in the presence of air/oxygen?

1. Sintering:
Sintering is a process that has been used to produce billions of objects that we use every day. It is mainly used for producing ceramics, everything from sinks to cups to bricks.

In the process, a material that is made up of very fine particles with a binder is formed into a suitable shape. In the case of clay, the binder is usually just water. The material is then heated to a point where the binder is removed and the surface of one particle fuses with the surface of another particle without actually melting.

As an analogy, imagine going into a freezer room and collect a bucket of ice cubes, about an inch wide, placed in bucket. Now take the bucket outside and allow the ice cubes to warm up just enough so that each ice cube has a thin film of water on the outside. Now place the bucket back inside the freezer and allow the thin film of water to freeze again – you have just sintered the ice. You could in principle invert the bucket and de-mould your ice-bucket object. If you did, you would immediately notice how many holes there are left between the ice cubes. This is a consequence of the sintering method – sintering gives porous materials.

There are two ways of getting around this:

Firstly, increase the temperature to as close to melting point as possible without actually melting the stuff. This is what one does with ceramics – the low-temperature, bisque firing leaves the pots very porous, with low mechanical strength, whereas firing to near the melting point of ceramics at 1300C gives hard, vitreous (glass-like) non-porous ceramics (or if you’ve pushed the temp too high it leaves you with a slumped mess). Down-side of this is that the higher the sintering temp the greater the overall shrinkage of the object. Too much shrinkage will result in distortion of the original object.

Secondly, use a mixture of particle sizes. For instance, if you take another bucket of ice cubes and mix in several handfuls of crushed ice, you can fill in most of the holes left between the large ice cubes. If you sinter the ice in this bucket, you’ll end up with a much denser object, fewer voids. The ratio of large to small particles is important in order to obtain maximum packing density but can be approximately calculated (I’ve got the numbers somewhere but haven’t looked it up at this stage) or can be gotten by experimentation.

The Pilato method makes use of the second point above in order to reduce the porosity to a low 13%. That is, one needs to use a mixture of particle sizes. The low porosity of 13% would make it difficult to tell a sintered bronze apart from a cast bronze just by looking at it and holding it. Presumably one could still use single-size bronze particles, but the resultant bronze would be far weaker than using a mixture.

(By the way, the statement in the CISP newsletter that a scientist would not be able to easily tell a cast and a sintered bronze apart is not true – just measure the density, the sintered bronze will be 13% less dense. Another by the way – using mixtures of particles in order to increase density is not something new invented by the Pilato method, this has been around for a very long time).

2. Sintering bronze – the problem of oxidation

Most people would have heard of PMC – Polymer Metal Clay. PMC comes either as silver PMC or gold PMC. PMC is vaguely similar to clay in its wet state. When dry one can simply heat it with a torch or in a kiln, burn out the binder and sinter it, and voila – a silver (or gold if you’re rich enough) object is created. If you’re a sculptor and have heard of PMC you immediately think “wouldn’t it be nice to have a few kg of this in bronze”.

Alas, life is not that simple. PMC is made up of fine particles of silver (or gold) in a polymer binder (methyl cellulose), with a few additives such as detergent to improve its handling ability, and certain amount of water. Gold and silver are part of a group of metals known as “noble” metals (other metals include platinum, palladium, iridium etc). Gold in particular doesn’t oxidize easily, whereas silver does oxidize but still much slower than metals such as copper, iron tin, etc. In fact silver is just inert enough to be used in PMC whereas copper oxidizes enough that it can’t be used. I had a look at the patents on the PMC stuff to see whether these guys thought they had a trick for doing copper, but couldn’t see anything (doesn’t mean it’s not out there somewhere – patents are deliberately obscure and difficult to read at times).

[Oxidation: reaction with oxygen to form an oxide e.g. iron plus oxygen equals iron oxide commonly known as rust. Air is approximately 21% oxygen, the rest is mainly nitrogen with small amounts of carbon dioxide and various other gases.]

The simple answer is of course to heat in an atmosphere of gas without oxygen e.g. in a nitrogen atmosphere, or even better nitrogen containing 10-20% hydrogen which will reduce any copper oxide back to copper metal. Using hydrogen can be done in the lab but is difficult for the average punter due to hydrogen’s high flammability and because it can form explosive gas mixtures at high temp if there’s a leak in your system and oxygen from the air seeps in. However, if the bronze particles aren’t too small and not too oxidized prior to sintering an inert atmosphere such as nitrogen or argon will suffice to keep the particles from oxidizing.

For the Pilato method this means that you need a kiln with an air-tight door and a steel tube nitrogen inlet going in one side of the kiln and an outlet at the other side of the kiln to let the nitrogen and any other gases produced during the heating out.

An alternative may be to place the object and the aluminium oxide into a separate steel container that has a nitrogen inlet and outlet. The container is then placed inside an ordinary kiln. This would mean that one doesn’t have to try and make a gas tight kiln.

For the average sculptor without access to nitrogen cylinders, gas fittings, tubing etc this may be the most challenging part to set up.

3. Removal of binder.

Speaking strictly from a patent point of view, the above two points aren’t particularly novel, the idea on how to remove the binder without burning it away in an oxygen atmosphere is/was the novel patent-able bit of the Pilato method.

However, this was already disclosed in the patent by German, Weaver, Thomas, Atre, Griffo in 1999, where they used the process to produce far more complex materials, consisting of a ceramic-metal, with a second infiltrated metal, than what is required for sculptures. In patent language this would mean that the Pilato method would be “obvious to those skilled in the art” after reading the original patent, but that the process isn’t actually covered by the patent. The good news is that, if there are no other patents out there, (but I didn’t look that hard), then this could mean that the Pilato process is not covered by a patent and hence available for exploitation/use by anyone.

Anyhow, back to the clever bit – how to get rid of the binder. What the Pilato process does is to use a melt-able material as the binder rather than a non-melting polymer and to pack a “wicking” powder around the object which adsorbs the molten binder. In the Pilato method the binder is simply wax. As the temp is raised the wax is soaked up by the wicking powder and then as the temperature is increased further the wax decomposes to a variety of gases such as hydrogen, methane, ethane, propane, butane etc (similar to natural gas) as well as soot. The decomposition occurs in the wicking powder, not in the bronze particle matrix, hence no problem with the soot contaminating the bronze powder and no chance of the bronze being damaged by internal gas build up. I think Mark Pilato mentioned in one of his posts that a flame is lit under the nitrogen gas outlet in order to safely burn off the decomposition gases.

The wicking powder mentioned in the Pilato method is alumina (aluminium oxide). However, the patent also mentions other ceramic powders can be used as long as the ceramic powder does not sinter at the highest firing temperature used. An important point is that the wicking powder must be fine enough so the wax preferentially is drawn into the wicking powder by capilliary forces i.e. the particles of the wicking powder should be smaller than the bronze particles.

Patent mentions that between 1 to 10 volumes of wicking powder to the volume of the bronze/wax is used.

Additionally, the binder also provides some mechanical stability to the object until the temp is raised to the sintering point. I.e. the wicking powder is packed around the bronze/wax object by “uniaxial tapping” to ensure intimate contact with the bronze/wax and to ensure maximum stability and support. I assume this means that one takes a stick and gently rams the powder around the object from above.


Other Stuff

The time of the firing i.e. wax melting, wax decomposition, metal sintering time is important, however, this will vary depending on the size of the bronze/wax being fired and will probably need to be fine-tuned by trial-and-error.

The foot-vibrator and the vacuum oven are probably used in the first phase where bronze/wax mixture is poured into the silicone mould. Given the amount of bronze powder used, the bronze/wax mixture probably has the consistency of concrete. In order to get a cast with minimal air pockets you ideally want to place the mould on a vibrating table inside the vacuum oven, fill the mould with the bronze/wax mixture and vibrate the whole thing under vacuum while the oven keeps the wax molten. This would give the best packed wax.

(I don’t know whether a foot vibrator would survive in the oven, so it may be easier to heat the mould, add the bronze/wax while vibrating, and then place in vacuum oven? Problem is that if there are any air-bubbles trapped in the bronze/wax you’ll end up with all the bronze/wax coating the inside of your oven as you apply the vacuum! Mmh, I suspect that you do need to stick the vibrator in the oven. Alternatively, the people who pour concrete often simply ram their concrete into moulds, live with imperfections or patch them after demoulding. Biggest danger is of course having a large air bubble trapped inside your bronze/wax object as you heat up the object while it’s still full of wax – could cause the object to crack or bits of it to explode. Once the wax is removed the bronze powder may be porous enough that trapped air can escape if the heating is carried out slowly enough.)

If I get any further I'll let people know.
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Burkhard
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