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What to use as an Investment material when casting Bronze?

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Hi all,

I recently made a "Bust of Pallas" out of plaster-of-paris and now I'm hoping to cast it in bronze. I've figured out how I'm going to make my furnace and have pretty much figured out how to construct the mould with the exception of knowing what material to use... I've read various things about it but have no idea which is the most suitable. For my first attempt at casting I'm looking for something that's relatively simple to use, so was looking at using fireclay but I'm guessing that it would be too firm to capture detail and that I really need something more liquid. I read somewhere that you can use plaster-of-paris as a mould but I would have thought that it's too fragile. I presume that if it is possible to use it, I'd need to cure it for a couple of weeks first to ensure all the moisture had gone?

I intend to use the lost wax method which leads onto another quick question; is there any particular type of wax I should use or can I just melt a load of candles down?

Hope someone can help!


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Another opportunity to give advice. :)

Having tried investment casting myself, and making a mess of it, the following observations:
- Fe-wood is right, that would be the easiest.
-Casting into plaster of paris is possible, but it need to be heated beforehand to reduce thermal shock, with doubtful results anyway.
-Melting out the wax isn't easy, especially if you've got a complicated shape. That is why I bought a burn out oven to cure the investment. The wax isn't melted out so much as burned out, and the heat remove all carbon.

- And remember the important rule: "If it works for me, it is the right way to do it."
- And if you do it, lots of pictures is mandatory.

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Pouring molten metal can be as easy as pouring it into a simple form and letting it cool to get a rough shape but investment casting is a multi-step complex process that takes some experience at each stage to make it all come out in the end. Along with a furnace, you'll need to construct a kiln that withstands temps upwards of 1100 - 1500 degrees. The temperature will have to ramp up slowly then be maintained for the number of requisite hours determined by the size of mold(s). This means using either an electric kiln with built in temp controls or gas fired with a pyrometer and thermocouple paired up to burners regulated by an electric gas valve.

You can use straight pottery plaster as an investment up to a certain size (quite small) then you need to start making mixes of silica sand and use mechanical reinforcements to produce a failure free mold. There are commecial mixes in different grades that will hold detail right down to fingerprints (Google casting supplies).

Microcrystalline wax is a common type used for lost wax casting and is available in a number of grades depending on the work being done. Proper sprueing and gating of the wax is critical to getting a successful pour and techniques vary widely depending on what the shape of the piece is.

Don't let any of this discourage you, but Fe's right - you need to work up to what you want to do. It can go wrong on you in a whole bunch of unexpected ways if you've never done it before so finding expert help or getting some training really is the way to go. You might connect with the local college and see if they have a casting dept that might be willing to take your piece on as a project.

This fella has a pretty good site on techniques for doing simple work - it might give you a little inspiration. www.backyardmetalcasting.com/

Good luck and keep us in the loop on how it goes!!

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If you've got a plaster model, the usual procedure would be to pull a flexible rubber mould (backed by plaster) of the model. Then use the rubber mould to cast your waxes, sprue your wax, invest, and finally cast.

It's very possible to do this with basic equipment, but it looks like you've got a lot of research to do.

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Safety is a huge factor!!!! That is why I suggest going to a community college or the like. Pouring 2100 degree Bronze into a mold with moisture or wax that hasn't full vaporized will have explosive results, volcanic at best.... Kinda cool to watch but not very safe. Sadly, the mold will be trash too.... I'm not trying to scare you from doing it, not at all. I'm hoping to scare you into finding someone with real life experience to help you do it successfully :D because I want to see pictures :P

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I worked in a small foundry that did dental metal ingots for the labs.

The ingots were cast in a hard red wax that was then treed up to make the pattern. A ceramic pour cup was added them the dipping began. A very fine slurry was first applied, then it had to dry. As it came back around on the conveyor system it was removed, and dipped in another slurry to wet it, then dipped in a medium sized silica sand. On the consecutive trips around it was dipped in a coarse silica to add strength. We removed most of the wax in an autoclave. The pressure exerted by the autoclave helped to keep the molds from cracking as the heated wax expanded inside the mold. The molds were then put upside down into a large gas burn out furnace, an run up to 1,600 for most alloys, 2,100 for the chrome cobalt alloy we poured. We had an induction furnace that melted 25kg at a time. When we were ready to pour someone would open the furnace, and remove 2 molds, and put them into a small cart full of sand. As the metal was poured 1,450 on the NPG alloy, 3,250 on the chrome cobalt (white hot) you could see the level coming up the mold as it turned translucent under the heat. The mold would start to crack off as the material cooled, and we finished it off with a 90# jack hammer to get it all out. We had a couple of molds crack during a pour, and the cart usually caught the metal coming out. Safety gear is very important with this type of work.

Now there are other ways to do this. Any carbon based item can be used to make the pattern; wood, plastic, flesh (I have seen small frogs cast in silver), and even styrofoam. The styrofoam doesn't need to be burned out, just pour the metal in. The foam burns out as metal is going in. Some engine blocks are done this way now.

The main thing is that there is absolutely no moisture in the mold when it is poured, and that it is sufficiently strong enough to contain the metal.

There are castings done in third world countries without fancy equipment, so it is possible for you to do this, BUT there are better/safer methods than what they use. Don't forget about ventilation too, as metals can give off some nasty fumes.

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Thanks for all the advice guys, it's very much appreciated! :)

I think I will take the advice to start small as I have a number of other ideas I want to try out, none of which are anywhere near as complex as what I described above.

As soon as I manage to create something worth showing, I'll be sure to put some pics up!

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Hi CB,

Fe-Wood sugested the right 'way to go' however if you are typical of the breed(Blacksmith) you will want to give it a bash anyhow.......................SO, B)

If you have never cast before, a good start in to try casting some items with lead, then try zinc(the type used for carburetors etc. All of this is availible quite inexpensively from scrap.(The fumes are TOXIC)

Start by moulding a positive with candle wax and adding sprues! then dip a number of times in plaster of paris(gupsum plaster) leaving it to dry between layers. them use your oven to SLOWLY melt out the wax then bake at 220 for 2 hours.

Now bury the mould in DRY sand and try casting in it. It's a process to learn and slowly build up the metal type as you learn more!

Good luck Ian

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  • 4 months later...

I must have been in a drug induced coma at the time of this one, oh well here goes. For really important jobs I used a commercial product call Kerr T-64. It gave me really good detail along with great strength. I also use Kerr Satin Cast for very small and delicates objects, wonderful stuff and used by most jewelers and dentist because of the lack of cleanup work left to do after casting. Both of these investments will pick up fingerprints off of the wax so that is pretty cool to see on you final product. Other manufacturers of investment make similar products to these but I always found Kerr to be just fine with me and there wasn't that much difference with the price to the other guys.
I also used a home madeformulaa that worked pretty well but was given to cracking but that didn't bother me all that much. Here is the formula, 1/3 plaster of Paris, 1/3 Hydrocal and 1/3 silica sand, all of these by volume. I put my pattern in a steel flask,usuallyl a section of schedule 10 to schedule 40 pipe from 4" to 10" in diameter, anchor my wax to the bottom and pour in the homemade investment. I seal the bottom of the pipe with dry plaster of Paris to prevent leaks. I also paint the waxes with a thin coat of Satin Cast prior to pouring in my homemade investment. This investment is all about saving money. One of my ancient casting buddies would coat his waxes with Satin Cast and then just mix up some new Hydrocal in a 50/50 mix with old investment. It worked but he had a lot of flash. Some of the African, Indian and Asian native investments use cow dung, clay and sand as their investments with great success, oh, and some even add in a little charcoal dust. Their wax is mostly bees wax too. It's to bad my old computer has crashed, I had some really nice sites for Africa and India, they had some neat tools for working their own brand of casting magic.

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