# Issues with creating Gold Bullion - help required



## daki (Jun 9, 2013)

Hi guys,
I'm not sure if this is the right place to place my question but I'll give it a go. Moderator please feel free to move my post to relavant section .

We have been casting gold bullions for a local supplier. There are a few issues they like to get someone option to make improvements to the mold. The questions and issues are as follows:

1. The mold only lasts up to 10 pouring of bullions. The mould is made out of iron cast. So when the gold is pored, after number of pouring the holes are created in the mold, making it unusable again.

2. Also we don't really know how to get consistency on all of the bullion edges to be flat and shiny. It's probably one of the biggest challenges we face as a small business to compete to some of the bigger mobs in the area.

We think we need a different mould materials with some sort of new coating to and processes to improve on bullions. 

Any tips, books, links, personal experiences would be greatly appreciated to get us into direction to solve these few issues. 

Cheers,
daki


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## Geo (Jun 9, 2013)

molten gold is a solvent for other metals.the holes are being formed by the gold dissolving the iron.your gold is not bullion grade.use a graphite mold instead,they are inert and can withstand the temperature and is very durable.

did you by chance happen to soot the inside of the mold before you poured the gold? the soot (carbon) acts like a lubricant and helps keep the bar nice and shiny.


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## daki (Jun 9, 2013)

Hi there,
Thanks for your answers. 

Why do you think it's not bullion grade? Can you elaborate more on this and how it affects molding?

We tried graphite and steel moulds but it seems like the iron cast with graphite works the best but not sure why it's still creating this problems.

As for the soot, the rubber soot is used before the gold is poured. 

cheers,

daki



Geo said:


> molten gold is a solvent for other metals.the holes are being formed by the gold dissolving the iron.your gold is not bullion grade.use a graphite mold instead,they are inert and can withstand the temperature and is very durable.
> 
> did you by chance happen to soot the inside of the mold before you poured the gold? the soot (carbon) acts like a lubricant and helps keep the bar nice and shiny.


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## patnor1011 (Jun 9, 2013)

It is poured to cast iron mold therefore it is contaminated with iron. 
What is size of your bullion? If it is smaller then maybe you need to change process and try to get some press to have blanks cut out and stamp them with logo.
You may get rid of imperfections by simply hammering it to desired shape.


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## patnor1011 (Jun 9, 2013)

Something like this
http://www.hydropneumaticpresses.com/coin-press-machine.html

and adventurous person can try to make this design (advantages: it can be quickly upgraded to popular french justice tool :mrgreen: )
http://www.panoramio.com/photo_explorer#view=photo&position=274&with_photo_id=703709&order=date_desc&user=51891


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## Claudie (Jun 9, 2013)

patnor1011 said:


> *Something like this*
> http://www.hydropneumaticpresses.com/coin-press-machine.html
> 
> and adventurous person can try to make this design (advantages: it can be quickly upgraded to popular french justice tool :mrgreen: )
> http://www.panoramio.com/photo_explorer#view=photo&position=274&with_photo_id=703709&order=date_desc&user=51891



I want one!


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## Geo (Jun 9, 2013)

daki said:


> Hi there,
> Thanks for your answers.
> 
> Why do you think it's not bullion grade? Can you elaborate more on this and how it affects molding?
> ...



it is fairly common knowledge here that molten metals in general and gold in particular is considered a solvent for other metals.im sure you understand the melting points of individual metals but apparently no one has told you that metals with a high melting temperature can be dissolved into or "melted" by metals that has a lower melting point just by introducing the two while the lower melting point metal is in a molten state.every time you poured molten gold into the steel mold,the gold dissolved some iron which mixed with the gold making it impure. 99.5% is considered bullion grade. im not saying it is absorbing that much iron but not knowing these simple things about casting gold, to say the gold was pure to begin with may be a big claim at this point.


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## daki (Jun 10, 2013)

Agreed!

The bullions are 12 kg, something must have changed in the whole process which is hard to identify at this stage. 

First casting mould that was created lasted approximately 500 over quote a few years (this one was German made), every one after that one was locally made, and it lasts under 10 times.


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## FrugalRefiner (Jun 10, 2013)

daki said:


> Agreed!
> 
> The bullions are 12 kg, something must have changed in the whole process which is hard to identify at this stage.
> 
> First casting mould that was created lasted approximately 500 over quote a few years (this one was German made), every one after that one was locally made, and it lasts under 10 times.


That sounds more like it. Cast Iron molds have been the standard for much longer than graphite has been available. From your original post I couldn't understand how a cast iron mold would fail in only 10 uses when it should last far longer. Properly sooted, a cast iron mold should serve perfectly well with no contamination of the ingot.

Now that you mention that the ones that are failing are locally made, I have to wonder about the quality of the "cast iron". I wouldn't be surprised if there are other metals in the iron alloy that might allow it to melt and be cast at lower temperatures. Those same metals would be more prone to failure when you're pouring 12 kg ingots. I wish Harold_V was still more active on the forum. He might have some insight into why a "cast iron" mold would fail so quickly.

Failing that, my advice would be to buy another German made mold. It has to be less than 50 times the cost of the locally made versions and it won't contaminate your gold.

Dave


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## chlaurite (Jun 10, 2013)

daki said:


> The bullions are 12 kg



If you didn't throw in an extraneous "k" there - Want to be my friend?

Wow.


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## Harold_V (Jun 11, 2013)

I was asked to comment. I'd like to start with the fact that there is NOTHING wrong with iron molds, and, in fact, I recommend them highly. They have been used with success for a very long time. They simply must be handled properly. 

I have limited experience in pouring large ingots of gold---with 10 ounce being the largest I cast. I did pour many 100 ounce ingots of silver, but they don't offer the same problems, due in part to the lower melting temperature of silver. 

Here's some things to consider. 

Cast gray iron suffers from thermal shock. A better choice for a mold would be ductile iron, which is, for all practical purposes, the same material as gray iron. The difference being gray iron must have a low sulfur content, and has been inoculated with magnesium, causing the free carbon to form as spheres instead of flakes. The net result is an iron that has far greater tensile strength and greater ability to resist thermal shock. So much difference that ductile iron can be welded much like steel, unlike gray iron. 

Regardless of any preconceived thoughts, a mold used to receive molten gold should be well sooted. The smoke from an acetylene torch can serve the purpose quite well, or you can buy prepared lampblack from foundry supplies. It is painted on the mold surface and dried before pouring the molten gold. 

As has already been mentioned, molten gold is a strong solvent. If you have been pouring ingots and creating cavities in your mold, you are contaminating the gold. The displaced metal has to go somewhere, and, unlike silver, gold will readily alloy with iron. It destroys gold's ductivity, however, so it's really not a good thing. 

Industry grade standard for gold is not 995, but 9995. You can rest assured, if you're dissolving part of the mold in the process of pouring, you are most likely turning out a product that is under industry standard, even if it met standard before being poured. 

Creating cavities in a mold is caused by pouring in one place alone. If you'd like to minimize that problem, start pouring at one end of the mold and move to the other end in the process. That will allow for heat distribution over more of the mold instead of concentrating all the heat in a common place. Under no circumstance should the gold be poured in one place until the soot layer has been eroded. You can't (and won't) undercut a mold if you keep that layer of soot present. 

How smooth you find your ingots depends on several factors. Pouring temperature can be critical, as it's important for the gold to remain molten until all of the gold has been poured. That, of course, adds to the problem of localized heating, as you must pour at an adequate temperature. Moving from end to end during the pour helps offset the problem. That's true even of small ingots, by the way. Also, keep in mind, if there's any movement of the mold or flame heating the gold that can cause movement of the gold as it cools, you're most likely going to have ripples on the gold surface. 

How flat an ingot turns out is dependent on proper cooling. If you can heat the surface such that the gold cools from the bottom of the mold to the top, not from the sides to the center, you may find a much nicer finish on your ingots. That will also eliminate the familiar pipe, assuming you control the cooling well. 

A parting thought. 

Cast iron is formulated to cast either thick or thin cross sections. In order for thin cross sections be possible to be poured, the iron itself has a higher free carbon content. That is accomplished by raising the silicon content of the iron, which, in turn, allows for more carbon to be absorbed. Overall, this lowers the melting temperature of the iron. So then, a low carbon content will resist dissolution somewhat better than a higher carbon content. It would be wise to discuss this issue with the foundry in question, but most importantly is to discuss the possibility of them pouring ductile iron instead of gray iron. They may not be willing. 

Harold


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## daki (Jun 11, 2013)

Hi Harold,

That is very detail explanation and very close to what's currently done. Your comments have actually raised a few very good points that theart of t attention can be pointed to for improvements. I guess can I get your opinion on the following:

1. What is the recommended temperature of the mould before gold is poured.

2. Did you try anything else but soot solutions you mentioned?

Our part of the job is to create moulds, but we are having arguments back and forth between the departments to resolve the issue, hence your post will serv as guideline for people to read and educate themselves as it very well summarises the process .

Another thing which we recomended is the use the spoon during pouring process to spread the impact across the mould and soften that initial impact onto the mould. 

Keep you updated.

Cheers




Harold_V said:


> I was asked to comment. I'd like to start with the fact that there is NOTHING wrong with iron molds, and, in fact, I recommend them highly. They have been used with success for a very long time. They simply must be handled properly.
> 
> I have limited experience in pouring large ingots of gold---with 10 ounce being the largest I cast. I did pour many 100 ounce ingots of silver, but they don't offer the same problems, due in part to the lower melting temperature of silver.
> 
> ...


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## Geo (Jun 11, 2013)

Harold_V said:


> Industry grade standard for gold is not 995, but 9995. You can rest assured, if you're dissolving part of the mold in the process of pouring, you are most likely turning out a product that is under industry standard, even if it met standard before being poured.
> Harold



thank you Harold.i had asked about this when i first joined and was citing from memory.i just couldnt remember if it was two nines or three.


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## FrugalRefiner (Jun 11, 2013)

Thank you Harold!

There is NO substitiute for experience. 8) 

Dave


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## MysticColby (Jun 11, 2013)

daki said:


> 2. Also we don't really know how to get consistency on all of the bullion edges to be flat and shiny. It's probably one of the biggest challenges we face as a small business to compete to some of the bigger mobs in the area.



top edges will be rounded if you pour into open faced molds. molten metal has surface tension, same as water does. The only way I know of to avoid this (aside from milling it afterwards) is to use a 2-part mold like the one pictured below. This will allow for sharp edges all around, but will require the sprue to be cut off after cooled, which may make potential buyers cautious of what happened.


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## Palladium (Jun 11, 2013)

I use only graphite, but this thread does bring up an interesting question for me. Soot not only provides for a mold release agent, but for passivisation or protection of the metal surface from erosion as i understand it. I know that WD-40 or motor oil can be used as a release agent, but does it also offer some kind of passivisation or protection layer from erosion?


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## rusty (Jun 11, 2013)

Palladium said:


> I use only graphite, but this thread does bring up an interesting question for me. Soot not only provides for a mold release agent, but for passivisation or protection of the metal surface from erosion as i understand it. I know that WD-40 or motor oil can be used as a release agent, but does it also offer some kind of passivisation or protection layer from erosion?




When I poured my molten copper into a cast iron frying pan, I had brushed a layer of motor oil onto the pan. The hot copper when it hit the pan immediately converted the oil to carbon aka soot.

Yes the oil created a protective layer on the cast iron pan.


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## Palladium (Jun 12, 2013)

rusty said:


> Palladium said:
> 
> 
> > I use only graphite, but this thread does bring up an interesting question for me. Soot not only provides for a mold release agent, but for passivisation or protection of the metal surface from erosion as i understand it. I know that WD-40 or motor oil can be used as a release agent, but does it also offer some kind of passivisation or protection layer from erosion?
> ...



I wondered if the oil would undergo carbon conversion and act as a barrier like that.


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## Harold_V (Jun 12, 2013)

daki said:


> 1. What is the recommended temperature of the mould before gold is poured.


I never poured under very scientific conditions, but preheat of the mold was of the most importance. If it is not heated, you can expect cold shuts and a generally low quality finish on the ingot. MY rule of thumb was to preheat the mold to the point where it would not burn off the soot layer, but was close to doing so. 



> 2. Did you try anything else but soot solutions you mentioned?


No. They served my purpose adequately, including in pouring my anodes for the silver cell, which were often around 200 troy ounces in weight. 



> Another thing which we recomended is the use the spoon during pouring process to spread the impact across the mould and soften that initial impact onto the mould.


I'll consider that you are discussing much larger pours than I am, but I would most likely recommend AGAINST anything that interrupts the flow of molten metal. That would be particularly true if you are using any type of metallic item. You *must* keep all manner of unprotected metallic objects away from molten precious metals, as they are strong solvents of other metals, and can easily dissolve them, even when they have much greater melting temperatures. 

I think key to success in your operation will be to pour at the proper temperature, without any unnecessary superheat, and to move the stream from one end to the other while pouring, so you don't concentrate the heat in one place. If you are pouring the metal too hot, you most likely will never overcome the problem of the mold being undercut. Explore that operation, and use a pyrometer to ensure you don't pour too hot. I would also discourage you from pouring fluxed materials. If you must use flux, pour to a cone mold, where you can separate the flux, then remelt. 

My personal choice in melting was to never use anything but a skim coat of borax on my melting dishes---and nothing in a silicon carbide crucible. Pure gold does not require a flux. If your gold requires flux, it isn't pure. Simple as that. 

Don't know if any of this will be useful. 

Harold


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## kurt (Jun 12, 2013)

We use cooking oil in our cast iron mold (olive oil - because its a higher temp cooking oil) (cooking oil doesn't smell as bad as motor oil when it gets hot & starts smoking)

we put about a 1/4' of oil in the mold on a propane burner & bring the mold up to 550 - 600 degrees & let it cook to the point it starts getting black (while metal is in furnce melting) brush oil up onto side of mold from time to time - just before we are ready to pour metal we pour off excess oil & put mold back on propane burner - if you let oil get good & black you should have carbon forming in mold - you want carbon forming on iron but also oily residue in mold when you pour metal -oily residue will ignite when you pour metal & burn on top of molten metal making for a nice even looking surface on top of your bar

Kurt


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## samuel-a (Jun 16, 2013)

Harold_V said:


> Don't know if any of this will be useful.



I think it really does Harold. You are spot on.

I had a similar experience couple of months ego, we had about 3Kg of about 14K (not jewellry though, alloy was Au+Cu+Ni) that i took to a foundry to be cast and assayed.
The foundry coats the inside of the iron molds with some sort of gray cement looking material, pehaps some sort of lampblack paste brand (i do not know exactly).
Anyways, the alloy was poured at around 1200C stright into one spot in the mold, what happend next was something i never saw before.... The metal eat through the mold bottom (.5" thick) down and through the thin SS tabletop and finally spilled onto the cement floor (and squitred all over our shoes of course).

Needless to say, we spent a lot of time collecting all the spilled gold and sweeping the floor to collect all the BB's. The most tedious part was extracting the BB's embedded in our shoes.


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