Pouring Bars as Perfect as Possible

[buckrogers1]

Sorry for initiating another thread, but no one answered in my other one (advanced questions)
i'm pretty much done refining almost 10kg of pure silver crystals, (< Silver cell < cementing on copper < dissolving in Nitric process )

Now i would like to pour bars as perfect as i possibly can. I'm asking for tips on how to avoid 'layering" , bubbles and other imperfections.

I take as an example the hastily poured bars from cemented silver (the ones i use in the silver cell )
As you can see by the pictures, they have layers (looking laterally) and they do not look shiny and appealing.

Please , i've been scavenging the forum and web for tips , but couldn't find anything really specific.

 

[Martijn]

Some tips that may help: melt the silver with a reducing flame. Silver absorbs a lot of oxygen in molten state which will be expelled when cooling.
Don't wait too long with pooring once its molten.
Heat your mold and let it cool slowly in a furnace or under a flame to let the oxygen out slowly.

 

[FrugalRefiner]

Get your silver hotter, but as Martijn said, try to use a reducing flame and cover it with a black of carbon (charcoal, graphite, etc.) to reduce dissolved oxygen. Preheat your mold till it's very hot. Pour quickly. You can also use a bookmold for very smooth sides all around. Practice.

Dave

 

[buckrogers1]

i'm using an electric furnace , so how do i use a reducing flame? when i pour ?

 

[Martijn]

I think electric is even better. Maybe try to heat it in a mold and just turn off the heat when its molten?

 

[goldshark]

Are you using an electric induction, or resistance furnace? For the best bars, a graphite mold, with cover, is what it will take to get a nice clean finish. I am assuming you are using a resistance wire furnace. Make sure the bottom bricks are level, to insure even thickness of bar. A mold which leaves as little area between metal and cover should be used. Oxygen atmosphere and metal, are the bane of a good finish, especially Silver. Let cool in the furnace with the graphite cover on, till solid. I wish they made a well sealed mold, where you could inject a bit of either Carbon Dioxide, or Nitrogen, to displace the little bit of Oxygen left in the mold. Show some pics of your finished product, if you go this route. Any Silver oxide, in the Silver itself, will off gas until completely purged from the melt. I don't know of a good technique to verify when/if the Silver oxide has been completely purged. Might take some experimenting by you to find the necessary amount of time to purge. This is what causes any pin holes in the finished bullion.

 

[buckrogers1]

Are you using an electric induction, or resistance furnace? For the best bars, a graphite mold, with cover, is what it will take to get a nice clean finish. I am assuming you are using a resistance wire furnace. Make sure the bottom bricks are level, to insure even thickness of bar. A mold which leaves as little area between metal and cover should be used. Oxygen atmosphere and metal, are the bane of a good finish, especially Silver. Let cool in the furnace with the graphite cover on, till solid. I wish they made a well sealed mold, where you could inject a bit of either Carbon Dioxide, or Nitrogen, to displace the little bit of Oxygen left in the mold. Show some pics of your finished product, if you go this route. Any Silver oxide, in the Silver itself, will off gas until completely purged from the melt. I don't know of a good technique to verify when/if the Silver oxide has been completely purged. Might take some experimenting by you to find the necessary amount of time to purge. This is what causes any pin holes in the finished bullion.

unfortunately i'm using one of these electric smelting furnaces that heat the graphite cylinder crucible off amazon. which then i have to pour. I might get one of these tabletop oven type electric furnaces.so that i can leave the silver in a graphite mold as you suggested, and let it cool. Will still not be ideal. Is there any 'affordable" machine anyone can suggest?

 

[richmond_alan]

With practice, good silver bars can be poured from any furnace into a steel mould. This requires a close-fitting steel cover plate placed over the mould immediately after pouring.
The temperatures of the molten silver, mould and cover plate are critical to a good finish as is the purity of the silver. Insufficiently heated silver or mould will result in the layering visible in your photographs as the metal will chill as it is poured. Certain contaminates will taint the upper surface even if the silver is 999. Very commonly, the egress of dissolved oxygen as the metal solidifies causes rough surface texturing and micro-craters.
It takes practice, but very high quality bars can be poured outside of the furnace.
A cover plate should be made from a clean piece of 1/2" steel plate with dimensions closely matching the outside dimensions of the mould. It should sit flat on the mould with no gaps around the perimeter. Attach a handle to the centre of the upper surface so the plate can be handled when hot. Once poured, the silver should fill the mould to within 1/2" of the top.

Method:
The pouring station must be close to the furnace and at a comfortable height.
While the silver is melting, preheat the mould and cover plate to a cool black heat.
Inside surfaces of steel or cast iron moulds should be covered by a light carbon layer using a rich acetylene flame.
Allow the silver to sit molten in the furnace to heat it beyond the melt temperature. (This timing takes practice to get right.)
Pour the silver quickly but smoothly along the length of the mould taking care not to let the metal surge or splash.
Immediately, but gently and without knocking the mould, rest the hot cover plate on the mould and let the silver solidify.
The time between removing the molten silver from the furnace to placing the cover plate must be as short as safely possible.

The aim is to delay the formation of a surface skin by keeping the surface of the bar molten long enough to allow the oxygen to egress. This should leave all surfaces of the bar smooth, with the upper surface slightly concave and generally patterned with concentric cooling rings. If you intend to stamp the bar, do so while it is hot and soft. If there is roughness and air bubbles on the bottom of the bar then possibly the mould or the molten metal is too hot. If you are pouring a number of successive bars you may need to briefly quench the mould in water between pours.

Remember, graphite moulds will deteriorate rapidly at elevated temperatures when in the presence of oxygen.

I hope this helps. See how you go. Cheers, Alan

 

[kurtak]

unfortunately i'm using one of these electric smelting furnaces that heat the graphite cylinder crucible off amazon.

If you are talking about a furnace like this that is at least part of the problem



https://www.vevor.com/melting-furna...&v_tag=5be8d190-94cb-11ed-8eb6-d32fa7909027.1
Though they will get the silver to melt they don't get the silver hot enough for a good pour

Is there any 'affordable" machine anyone can suggest?

Get a propane fired furnace like this one ------

https://devil-forge.com/gas-furnaces/91-gas-metal-melting-furnace-fb2mb-10kg-full-set.html
Notice the price is about the same as those totally crappy electric furnaces but will serve you much better as you can get your melt up to temp for better pour

Or you can make your own - there has been a lot of discussion on this forum about building your own - you just need to search

Here are some pics of my small 4 kg (same size as above link) home made furnace

My furnace served me will for 10-12 years & I poured probably well over 1000 kilo bars of silver with it

Kurt

 

[buckrogers1]

With practice, good silver bars can be poured from any furnace into a steel mould. This requires a close-fitting steel cover plate placed over the mould immediately after pouring.
The temperatures of the molten silver, mould and cover plate are critical to a good finish as is the purity of the silver. Insufficiently heated silver or mould will result in the layering visible in your photographs as the metal will chill as it is poured. Certain contaminates will taint the upper surface even if the silver is 999. Very commonly, the egress of dissolved oxygen as the metal solidifies causes rough surface texturing and micro-craters.
It takes practice, but very high quality bars can be poured outside of the furnace.
A cover plate should be made from a clean piece of 1/2" steel plate with dimensions closely matching the outside dimensions of the mould. It should sit flat on the mould with no gaps around the perimeter. Attach a handle to the centre of the upper surface so the plate can be handled when hot. Once poured, the silver should fill the mould to within 1/2" of the top.

Method:
The pouring station must be close to the furnace and at a comfortable height.
While the silver is melting, preheat the mould and cover plate to a cool black heat.
Inside surfaces of steel or cast iron moulds should be covered by a light carbon layer using a rich acetylene flame.
Allow the silver to sit molten in the furnace to heat it beyond the melt temperature. (This timing takes practice to get right.)
Pour the silver quickly but smoothly along the length of the mould taking care not to let the metal surge or splash.
Immediately, but gently and without knocking the mould, rest the hot cover plate on the mould and let the silver solidify.
The time between removing the molten silver from the furnace to placing the cover plate must be as short as safely possible.

The aim is to delay the formation of a surface skin by keeping the surface of the bar molten long enough to allow the oxygen to egress. This should leave all surfaces of the bar smooth, with the upper surface slightly concave and generally patterned with concentric cooling rings. If you intend to stamp the bar, do so while it is hot and soft. If there is roughness and air bubbles on the bottom of the bar then possibly the mould or the molten metal is too hot. If you are pouring a number of successive bars you may need to briefly quench the mould in water between pours.

Remember, graphite moulds will deteriorate rapidly at elevated temperatures when in the presence of oxygen.

I hope this helps. See how you go. Cheers, Alan

interesting, using steel molds, haven't seen these. any place u recommend getting these? Do they even exist with cover? Have to fashion one myself i guess?
i'm not clear. on what you mean by covering with a light carbon layer. shall i sprinkle some graphite powder on the bottom of the mold and burn it w the flame? Sorry for the questions. Thanks for the help!

 

[buckrogers1]

If you are talking about a furnace like this that is at least part of the problem



https://www.vevor.com/melting-furna...&v_tag=5be8d190-94cb-11ed-8eb6-d32fa7909027.1
Though they will get the silver to melt they don't get the silver hot enough for a good pour


Get a propane fired furnace like this one ------

https://devil-forge.com/gas-furnaces/91-gas-metal-melting-furnace-fb2mb-10kg-full-set.html
Notice the price is about the same as those totally crappy electric furnaces but will serve you much better as you can get your melt up to temp for better pour

Or you can make your own - there has been a lot of discussion on this forum about building your own - you just need to search

Here are some pics of my small 4 kg (same size as above link) home made furnace

My furnace served me will for 10-12 years & I poured probably well over 1000 kilo bars of silver with it

Kurt

I see.. i have to have higher temperatures than this electric furnace allows, yes thats the one i have, bit crappy IMO.
What do you think of the Rapidfire Front loading one? I could leave the silver to melt inside the cast with this one, no pouring at all:

Electric Furnace (see this one on Amazon)

Else if you recommend going the gas route.. i'll get one of these ( melt and return , lol, taking advantage of amazon return policy )

 

[kurtak]

I might get one of these tabletop oven type electric furnaces.so that i can leave the silver in a graphite mold

If you are making pours of 10 ozt to 1 kilo get a cast iron mold like this - they last forever

https://www.ebay.com/itm/3807020429...lid=20006&customid=4581802406898353_377867585
They make them in different sized

Here are some pics of some of my kilo bars poured using a cast iron mold

Kurt

 

[buckrogers1]

If you are making pours of 10 ozt to 1 kilo get a cast iron mold like this - they last forever

https://www.ebay.com/itm/3807020429...lid=20006&customid=4581802406898353_377867585
They make them in different sized

Here are some pics of some of my kilo bars poured using a cast iron mold

Kurt

I see.
Nice bars.

Which brings me to the next question. To have the bars cast instead of poured. Machines for that are prohibitive for the hobbyst, but is there maybe a foundry or a service that can cast perfect mirror like ingots , sending them my silver crystals ?
I googled various and some emailed me back but no one so far does this service.
If the price isn't too high to be cost effective for 10kg of silver, might be worth a shot.

 

[kurtak]

Do they even exist with cover?

The cover richmond_alan is talking about is just a piece of steel plate he sets on top of the mold right after making the pour - it helps to hold the heat in while the silver cools so the silver better levels during cooling

I never used a cover - IMO its more about having you mold hot enough - having the molten silver hot enough (you can get it to hot) & how fast you make the pour

That said - a cover on top the mold after making the pour may well give you a better leveled top to the bar - I don't know as I never used one

Instead - what I did is right after making the pour I would just hit the top of the poured silver with my MAPP gas torch & play the flame over the top of the poured silver for a little bit

I heated my mold by setting it in a "single burner" propane camp stove like this

Robot or human?

i'm not clear. on what you mean by covering with a light carbon layer.

Alan is talking about coating the inside of the mold with carbon - if you don't do that the silver will stick to the mold & you wont be able knock the bar out of the mold

I use cooking oil to do this - will post about that IF/when I get more time

Kurt

 

[rabbit60]

I regularly pour Silver bars of about 400g using a Vevor Furnace without too many problems. I set the Furnace temperature at about 100 degrees above the metal melting point. I use a Graphite Mould and have a propane torch fixed in a Lab Stand heating the Mould I turn the torch off just as i start to fill the mould.

 

[richmond_alan]

interesting, using steel molds, haven't seen these. any place u recommend getting these? Do they even exist with cover? Have to fashion one myself i guess?
i'm not clear. on what you mean by covering with a light carbon layer. shall i sprinkle some graphite powder on the bottom of the mold and burn it w the flame? Sorry for the questions. Thanks for the help!

For both silver and gold ingot casting we had steel moulds machined with polished internal surfaces which gave excellent quality bars. The cover plate was the most reliable and simplest method for large silver production runs using LPG furnaces. Cast iron moulds work but leave a rougher surface on the bars. (For gold we covered the freshly poured ingot with a "fluffy" oxy/acetylene flame as it chilled.)

The cover plate is simply a rectangle of 1/2" steel plate to keep the heat in, it should not be allowed to contact the molten silver!

As Kurt commented, a thin coating of oil can be used instead of a coating of soot, (don't use graphite powder), on the internal surfaces and getting the right temperature into both the molten silver and the mould is crucial. Cheers, Alan

 

[buckrogers1]

For both silver and gold ingot casting we had steel moulds machined with polished internal surfaces which gave excellent quality bars. The cover plate was the most reliable and simplest method for large silver production runs using LPG furnaces. Cast iron moulds work but leave a rougher surface on the bars. (For gold we covered the freshly poured ingot with a "fluffy" oxy/acetylene flame as it chilled.)

The cover plate is simply a rectangle of 1/2" steel plate to keep the heat in, it should not be allowed to contact the molten silver!

As Kurt commented, a thin coating of oil can be used instead of a coating of soot, (don't use graphite powder), on the internal surfaces and getting the right temperature into both the molten silver and the mould is crucial. Cheers, Alan

Yeah i imagine that steel would make for very polished surfaces
Unfortunately i can't find any for sale
Might have to stick w graphite ones, buy one per pour so they're spotless
And cover it.. got it.
Will have to get a propane furnace. And set up a propane torch to blow onto the mold .
Borax in the crucible ? Is it necessary for pure silver crystals?

 

[FrugalRefiner]

Borax in the crucible? Who said that?

Dave

 

[goldshark]

You need

interesting, using steel molds, haven't seen these. any place u recommend getting these? Do they even exist with cover? Have to fashion one myself i guess?
i'm not clear. on what you mean by covering with a light carbon layer. shall i sprinkle some graphite powder on the bottom of the mold and burn it w the flame? Sorry for the questions. Thanks for the help!

You need to coat the inside of the mold with a release agent. Oil or soot will work. The oil should be a very thin layer of just about any oil, but not WD-40. I think it is too light, and may evaporate off with the mold preheat. Sooting with an Acetylene torch, is accomplished by turning on the torch just enough so that it smokes heavily. Turn the mold upside down, and allow the smoke to coat every part of the mold. This allows a few thousands of an inch of high temperature release agent. Don't heat the mold to over 550 F, or you will start to react the carbon layer coating with Oxygen, rendering it useless. An infrared temperature point and shoot thermometer works well, if it is within 3% of calibration .
Steel molds will last a long time. Any machine shop can make one with a milling machine. Get several made in different sizes, all at once. This is because once they jig up whatever tooling they need, the job goes very fast. Unless you are doing kilogram molds. Any piece of flat plate closely fitted to the outside mold dimension will work. Much larger pieces of plate will have a tendency to tilt to the heavy side, if not centered properly. This may lead to a spill. Too small, well I will let you think that one over. As Richmond said, something to grab and manipulate the cover is essential. Tell the machine shop the need to polish the inside, for a better finish. A slight obtuse bevel on the sides (1 degree) will help with the release. I think steel gives a better finish than cast iron. Coat with oil when finished pouring, to maintain a smooth, unoxidized surface within the mold. You don't need to coat the lid, and you can preheat this to red, if you want, but not necessary to heat beyond 600 F.

 

[richmond_alan]

Yeah i imagine that steel would make for very polished surfaces
Unfortunately i can't find any for sale
Might have to stick w graphite ones, buy one per pour so they're spotless
And cover it.. got it.
Will have to get a propane furnace. And set up a propane torch to blow onto the mold .
Borax in the crucible ? Is it necessary for pure silver crystals?

No, don't use Borax or any other flux, use clean metal in a clean crucible.
You'll be able to reuse the graphite moulds quite a few times and they release the ingot without the need for soot or oil. If there are machining marks in the graphite they will transfer to the bars.