Casting Bronze

Hi Everyone.

I want to do some small castings in iron, aluminium and bronze.

Is it possible to make your own bronze?

I want gunmetal which is approximately 86% copper and 11% tin + 3% zinc according to the machinerys handbook.

I've got plenty of copper and can get tin and zinc.

BUT, looking at the vastly different melting points of these metal, will the molten copper vaporize the tin and zinc??

Maybe the makers of such alloys use a special process.

Can anyone advise me please.

Given that our ancestors made Bronze, very successfully, using primitive facilities … I am sure it is far-from-impossible.

That said … I have never tried.

MichaelG.

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Edited By Michael Gilligan on 07/07/2023 20:25:43

Melting Tin and Copper isn't a problem given enough heat and a suitable crucible, and the Tin won't vaporise unless the furnace is run far too hot. (Shouldn't be a problem unless Oxy-acetylene or an Electric Arc is used.)

However, Zinc fumes easily, and it's dangerous to breath it. 'Brass Founders Ague' was one of the first industrial diseases, and it's still a problem today - chaps poison themselves welding Galvanised Iron. The 'special process' is keeping the workforce away from the fumes. I don't think melting a small quantity in the open air whilst standing well up-wind of the furnace is too risky. Doing it in a small garden might worry the neighbours though!

Scrap Brass (such as old water taps), melts more easily than Copper, so I'd start with that. Then add Tin. It's usual to add a little Zinc during the melt to keep the alloy proportions about right because Zinc slowly evaporates.

How to get 86% copper, 11% tin and 3% Zinc? I think the Ancients got the mix right by experience - either able to judge how much Zinc was lost over time from a given mass of metal in their particular furnace, or perhaps by recognising the right moment from flame colour or how the liquid poured. I guess a modern foundry would use a horribly expensive spectrographic gun to measure the proportions accurately.

The 'gunmetal' may not need to be accurately made for amateur purposes, but if it is, might be done by weighing a known volume and checking the alloy's density.

Dave

WHY do you need such a specific alloy ? Why not melt bronze/gunmetal from scrap ? Almost any boatyard will sell you marine pumps Etc from the scrap pile. OR as dave has said, melt brass and add tin.There are a few tricks to melting copper alloys - if you want help in this area let me know ! I take it you are well voiced in foundrywork and the pattern making and molding ? For iron you will need a LOT of heat ! Noel.

I don't know much about casting bronze, but a little about casting lead alloys. To add antimony to a lead tin mix the method I was told about is to have the antimony on top of the pot and use a propane torch to add more heat to the antimony. In the advice I had, there is a warning that there could be a bit of an explosion and molten metal would go everywhere! Being faint hearted, I never tried it!

As to scrap bronze, taps and the like have a small percentage of lead to make the molten metal fill the mould better. We had this problem with a new government hospital where I live. The builder purchased all the plumbing fittings from China, and when tested the water showed traces of lead! All had to e replaced! Also, bronze can be made with a percentage of iron.

Church bells are made from a bronze with high tin content; around 77% copper, 23% tin - and hundreds of years ago, some bells were cast in pits in the churchyard.

I have no idea how they did this, but melting and casting bronze must be relatively doable, without modern furnaces. And of course the "Bronze Age" before those times cannot have had any modern technology either.

We had two extra bells, of 4 and 5 cwt respectively, made to augment our 6 bells to a ring of 8, and we watched our new bells being cast at the Whitechapel bell foundry in London, (where the "Big Ben" hour bell was also cast). A fascinating process.

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Edited By John Doe 2 on 08/07/2023 06:57:23

Hi, Thanks for the replies.

I asked the question because I had watched a few You-Tube videos and seen the zinc evaporating in a cloud of white (dangerous) smoke. I can operate outside and keep well away from the zinc fumes and I don't have people living close by.

I quoted the percentages from the book but realize I can only get an approximation of the mix. I want to try for gunmetal to make loco cylinders and parts, plus a bit for the Quorn. I've got plenty of copper and brass (from plumbing work) so buying scrap bronze seems pointless if I can make it.

And no Noel, I don't have much foundry experience (yet !!!) but am ok with the woodwork and the furnace is taking shape. It was me who gave you the small sample of "Bromsgrove sand" some years ago at the model engineering show, Leamington. Will you be there this year ??

I think the brass plus some tin and a bit more copper may be the way to go as I have quite a lot of brass without the chrome plating and it's old stuff (not Chinese!!)

So thanks again for the comments.

I'm away now till Monday so won't be able to reply straight away.

Casting Bells..... There is a thread on the Australian Metalwork Forums about casting bells. The thread is called "They are getting big." Here s the web address:

https://metalworkforums.com/f267/t207736-getting

If you are not a member, you can't look at the pictures, BUT, there is a video of a pour, that is available on Youtube. Here is the link:

https://youtu.be/Xy8KGZCjgUg

Not a lot of help in the original enquiry, but shows that bell casting is alive and well!

Hello Stephen, No this year I will not be at the Midland, at least not working, I will go up one day as a visitor. One reason for lead in brass fittings is to give hydraulic soundness. If I can be of assistance in your project, you know where to find me ! Noel.

Hi Stephen,

I make alloys from base metals often, with no issues. The pickup (amount that goes into the charge) for the bronze alloy elements is high so you don’t need to worry about losses. Fade (the amount of additive/inoculant or alloy you loose during melting) is not significant with Zinc so you won’t have any issues there. The mass loss due to the fumes typically seen is very small. 3% Zinc will dissolve very quickly and should have very little fuming. When you do the brasses you should add a capping inoculant, I placed a recipe for one somewhere on this forum. If I don't have Zinc granulate then I use brass as a zinc source, but you need to do the calculations for the amount needed on a mass basis.

Cast iron is a little different. I melt from pig iron and scrap and then you need to add carbon which has a 50% pickup. FeSi is added at the end of the charge to prevent fade.

Melting alloys is easy; the trickier part is in the moulds. If you’re going to cast patterns with large surface areas and low volumes (plates etc.) you’ll be fine, the trick comes in when casting, for example, cylinders (large volume/surface area ratio) which require careful feeding to prevent draw etc.

PS: your furnace should be built in such a way that the fumes are carried with the exhaust up away from where you stand when casting etc. Be aware of the fumes but don’t let it deter you. I have a tradition of going for a 10Km run after a day of casting to ‘wind down and relax’ and it’s always a pleasant run…

Hi Everyone.

Thanks again for all the comments, and Noel and Luker for your detailed words.

I will now get on and finish the furnace and set up for some trial melt/pour practice,

It will be some time before I can get this done but will come back (eventually) and report my results.

Thanks again all.

Since you do not have a furnace yet and little experience can I offer some advice ? Temperature control in casting all really but iron is one of the keys to success. A hand held electronic reader and an inconel sheathed K type thermocouple of 3mm dia and 1/2 metre long will remove one of the many variables you will face.Trust me when I say that the money this will cost will be repaid many times over in time and fuel saved, this does not include the satisfaction of a sound casting ! For aluminium and its alloys you will be looking for about 700c ! For brass between 1000cand 1050c, exceeding this figure will result in high zinc loss, zinc boils at 907c so some zinc loss is to be expected but is not significant at lower temps. For bronzes with NO zinc then 1150c is the upper limit.The degree of super heat above melting point will be dictated by the shape and thickness of what you are casting. A casting that pours short may need more superheat ! OR the melt has been badly over heated and the metal has lost it's fluidity. Molten metals will disolve metals of a higher melting point ! One reason for not over heating the melt is to reduce as much as possible the risk of gas absorbtion and the problems that go with it ! Good luck. Noel..

Edited By noel shelley on 12/07/2023 09:59:10

Thanks Noel.

I will certainly get a reader when I get towards firing the furnace up.

Have you any recommendations for a particular reader or should I just troll e-bay and google ???

If I get to melt iron, is there a need to check its temperature ?

Have made notes of all the detailed stuff in my book.

Thanks again.