Bronze Casting 3. Equipment.
by John
I have been unwell for 1-2 weeks with a respiratory disease. I twice requested Covid-19 testing, but was declined because I fell outside the guidelines. At the same time my wife fell ill with similar symptoms, but her situation rapidly worsened with severe asthma, and she required a hospital admission. She was given the Covid test, but it was negative, and it turned out that she has a different virus named RSV (respiratory syncytial virus) which causes croup in infants. So it seems likely that I have the same virus.
The problem is that we are coughing constantly, and sleep is very interrupted. And we need to continue self isolation just to avoid coughing near other people. I feel some empathy for infants with croup.
So not much happening in the workshop.
But I have been accumulating various bits and pieces that will be used for bronze casting pieces for the Armstrong cannon project.
First, the metal melting furnace. 10amps, 240v, 2600w, 1200ºc. Graphite crucible.
This should melt alu, copper, bronze, but not steel. Is there a town named “Italy” in China?AUD$405
And some quite reasonable gloves.
A second furnace is needed to prepare the mould. This was a quite old pottery furnace, used by a lady for ceramic painting. Purchased by me second hand, (AUD$700) and knowing that some repair work would be required.
It is a good size, and heating coils are intact and well seated. I do not know if the thermocouple works so I have ordered a spare.
The firebricks are in excellent, almost unused, condition.
It is using the rated 2600w.
The electrics work, but most of the joins and fittings are rusty. I will clean up the joins, and replace the fasteners. I also intend to replace the power switch with a digital control.
To make the mould I have chosen (on advice) a jewellers investment powder, normally used to mould rings and brooches with very fine detail. It is not cheap (AUD$130), and must be handled carefully and not inhaled.
The steel mould cylinder, and rubber end piece.
Wax cylinders to be used as supports, sprues and vents.
And finally, the vacuum unit, for removing air bubbles from the investment powder mix, (AUD$200)
A significant financial investment, and not finished yet. And no guarantee of acceptable results. I did obtain a quote from a professional caster, but it was even more expensive. So, I will be giving bronze casting a trial soon.
johnsmachines 
Hello John, Sorry to hear you are unwell.
Years ago a mate and I decided to make a 2 kg cast bronze trophy at work. He was a metallurgist though his experience was all ferrous. As a chemist, I was in charge of the binder system and moulding. We used the laboratory gas fired crucible furnace and a graphite crucible. My mate poo-pooed the idea of a melting cover flux and de-gasser and our casting came out full of gas holes.
The moral of the story is that for copper alloys, cover flux and degassing is essential. This was especially embarrassing since our company made both.
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Hi Rob, what is a “melting cover flux”? Borax?
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John, all the best in healt also for your wife. Hope you will recover soon.
In Europe we are all in the ban of corona, in the Netherlands there is allmost a lockdown.
Regards
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Hello John, A cover flux is a powder that acts as physical cover (by melting over the scrap or ingots) to minimise metal oxidation and the pick up of hydrogen. It was not my area of expertise and I don’t know what the constituents were for copper alloys but borax sounds like the kind of thing.
I do know that copper alloys use lithium (enclosed in copper tubes) to remove hydrogen gas and that a dross coagulant is used to aid skimming dross from the melt before pouring.
The company I worked for was Foseco, the largest manufacturer of metal treatments world wide, it became part of Burma Castrol in 1991. They did produce a book called “Foseco Foundrymans Handbook” it referred to the application of their proprietary products for best melting practice. The company is located in Padstow NSW and is now owned by Morgan Crucible.
My advice is to try to get a bit of whatever is currently used by talking to a metallurgist at a bronze casting foundry or talk to someone at CABIC the copper and brass information centre (Sydney)
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Hi John,
It looks like you have your bases covered equipment wise. The vacuum pump is the best thing you could do. Those tiny bubbles you get in castings can drive you nuts.
I used to use this plastic domed kitchen appliance called a “Minute Marinater”, it was a plastic dome on top of a vibrating table that drew down a fair vacuum. It did a fantastic job of pulling out the air snd only cost me forty dollars American.
Some kind of vibrator helps a lot. If your setup does not have one you should consider it,
Another trick I’ve used is to paint on multiple thin layers of investment, made into a thin slurry, onto my wax moulds before filling the flask with investment.
Jewellery investment sets up real fast so have everything ready before mixing it.
Glad you fo not have the real nasty bug and hope you recover fully soon,
Jenny
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Thanks Jenny, I will use both of those (the vibrator and slurry coats). john
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Hello John, I just spoke to a friend who regularly casts aluminium bases for his model stationary engines using a gas fired crucible furnace and sand moulds. He has cast bronze without gas holes using crushed glass as a cover flux and degassing with hexachlorethane tablets.
Hexachlorethane produces chlorine gas in the melt that reacts with the dissolved hydrogen. It is plunged below the surface and used to be standard aluminium practice in foundries. Large scale operations used chlorine gas directly pumped into the molten aluminium.
I had not heard of chlorine degassing being used on bronze but it worked. Naturally it should only be done in a well ventilated area with appropriate filter mask.
He recommended two foundry practice books by Steve Chastain (volumes 1&2) but having loaned his copies couldn’t remember the titles.
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Hi Rob,
I am not enthusiastic about using chlorine gas. Nasty stuff. But I will try to obtain the books which you mention, to investigate further. John
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Hi John,
Apparently the current degassing tablets for small aluminium melt size release nitrogen rather than chlorine. Presumably forming ammonia in the melt to remove hydrogen.
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