# Silver Cracking



## Stream_Silver (Jan 2, 2020)

Hi everyone,
This is my first post.
I am a Silver Ornaments manufacturer and have issue with Silver wire cracking.
When i melt 925 (Sterling Silver) and pour it in iron slots to make a rod for making wire, the wire is very tough and i have to remelt the entire thing to make it work.
This happens sometimes only, i have tried using a new crucible and melted pure silver with 1% copper, 7.5% Cadmium, still sometimes i fave the problem.
I melt it in electric furnace
How can i fix the issue.


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## snoman701 (Jan 2, 2020)

I'ts more likely a result of cooling, either too fast from a hot temperature like your initial melt, or too fast from a hot working temperature when drawing it out (meaning you are trying to reduce size too quickly).


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## snoman701 (Jan 2, 2020)

snoman701 said:


> I'ts more likely a result of cooling, either too fast from a hot temperature like your initial melt, or too fast from a hot working temperature when drawing it out (meaning you are trying to reduce size too quickly).



Let me rewrite that.

If you are pouring too cold, it's likely you will have cracking problems do to cooling too fast.

Manipulation of the steriling silver near the melt point will cause cracking issues.

So it's best to pour hotter, cool slowly under a hydrogen flame, until you know that it's good and solid, then quench in water. 

Sterling silver, while appearing molten, can actually be a two phase system of pure silver and silver/cu alloy...and if manipulated at this temperature you can lock the crystal structure in this state.


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## FrugalRefiner (Jan 2, 2020)

I'll add a couple of other possibilities.

If you're remelting sterling silver, the copper in the mix oxidizes more each time the alloy is melted. It results in copper oxide in the alloy, which shows as porosity in the final alloy.

Another possibility is the oxygen absorbed by molten silver. It can absorb large amounts of oxygen when molten, but when it cools, the oxygen is expelled, but it's not necessarily a complete process depending on how quickly the metal freezes. If you're pouring into a thin mold, this can lead to voids in the metal.

Dave


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## snoman701 (Jan 2, 2020)

Should have asked what kind of crucible you are using as well, since you are melting with electric. Should be a graphite crucible, or at least have a gas flame cover.


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## g_axelsson (Jan 2, 2020)

Dave, sterling silver doesn't absorb oxygen, it's a property of pure silver and just a small amount of copper as alloying element prevents oxygen to be absorbed. At least that's my experience and what I've read.

Stream Silver, there is a good site about jewellery manufacturing, Ganoksin https://www.ganoksin.com/ You could probably find quite a lot of help on their forum.

Göran


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## kurtak (Jan 3, 2020)

Could be a "work hardening" problem - so you may need to anneal it at different stages of the drawing out process as you "work" it down from it's original pour size to it's final wire size

each time you run it through the rolls (rolling mill) &/or draw it though a draw plate to work it down in size it changes the crystal structure a bit causing it to get a bit harder with each draw out to a point that after a few draws it becomes "work hardened" & cracks - so it needs to be annealed after a few draws (every 2 or 3 draws 4 at the most) to put the crystal structure back to its original soft state

Working silver (&/or copper &/or gold) by pounding, bending, rolling, or drawing (stretching) will cause it to work harden which will cause it to crack

Annealing it ever so often as you work it will solve the problem

Kurt


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## nickvc (Jan 3, 2020)

Never mind the wire cracking adding cadmium to a melt seems very dangerous and unnecessary, most sterling alloys are just silver and copper, allow the rod to cool slowly when poured even playing a torch over it and don’t try to break it down too quickly without annealing.


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## kurtak (Jan 3, 2020)

nickvc said:


> Never mind the wire cracking adding cadmium to a melt seems very dangerous and unnecessary,



I agree - unless he is using the wire for brazing in which case the cadmium lowers the melt point of the brazing alloy - outside that I certainly see no need to make a silver/cadmium alloy

that said - you can certainly make low melt point braze alloys with out cadmium 

Kurt


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## snoman701 (Jan 3, 2020)

g_axelsson said:


> Dave, sterling silver doesn't absorb oxygen, it's a property of pure silver and just a small amount of copper as alloying element prevents oxygen to be absorbed. At least that's my experience and what I've read.



That's not my understanding at all. 

I was very sternly cautioned against trying to re-use sterling casting scrap without first refining it due to oxide formation and oxygen absorption in the crystal structure. With minimal melt hygiene it's much easier to keep fine silver clean, whereas sterling went to crap if you looked at it wrong. 

It's one of the primary reasons for the popularity of argentinium silver. While argentinium is advertised as "tarnish resistant"...it's popularity is due to the resistance of it to develop firescale due to a passivation effect on the surface. Keeping silver untarnished isn't as much of a concern to the silversmith/artist (that's the end users problem) as is not developing firescale during fabrication.


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## g_axelsson (Jan 5, 2020)

Snoman, I think we are talking about two different things. I'm talking about oxygen gas, not oxides.

Fine silver can absorb oxygen gas that is expelled when it solidify again. Often creating porous silver and bubbles bursting on the surface just before it solidifies. Adding just a little bit of copper stops the effect of absorbing oxygen gas.

I'm not surprised that copper can oxidize and be included in the alloy and I trust you about not remelting alloys too many times.

Göran


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## snoman701 (Jan 5, 2020)

Its not that it stops it, it’s that the oxygen is taken up by the copper as an oxide of copper. 

But yes, we were talking about different things. 


Sent from my iPhone using Tapatalk


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## FrugalRefiner (Jan 5, 2020)

snoman701 said:


> Its not that it stops it, it’s that the oxygen is taken up by the copper as an oxide of copper.
> 
> But yes, we were talking about different things.



From my research, I agree with snoman, though it's really the same thing. Pure silver absorbs up to 10 times its volume in oxygen when molten. When it freezes, it expels it creating the spitting and crabbing often mentioned. 

Sterling silver also absorbs oxygen, but as it freezes, the copper content can react with the oxygen to form oxides instead of the oxygen being expelled as a gas. The copper doesn't prevent the silver from absorbing the oxygen, but instead of it being expelled as gas it can react with the copper to form a couple of different oxides.

But there's a limit to the mitigating value of the copper. Melting freshly refined silver and freshly refined copper under carefully controlled conditions will result in a high quality alloy. There will be some oxidation of the copper content, but it will be at an acceptable level. If the sterling is remelted, more oxygen will be absorbed, the copper content will be further oxidized, and the alloy will become darker and more porous. When jewelers remelt sterling scrap, they typically add a good percentages of freshly refined silver to the melt to dilute these effects. But at a certain point, they must refine any scrap and begin anew to avoid the cumulative effects.

At least, that's my best understanding.

Dav


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## Thipdar (Jan 31, 2020)

Stream_Silver said:


> Hi everyone,
> This is my first post.
> I am a Silver Ornaments manufacturer and have issue with Silver wire cracking.
> When i melt 925 (Sterling Silver) and pour it in iron slots to make a rod for making wire, the wire is very tough and i have to remelt the entire thing to make it work.
> ...



I'm curious to know if you pour your molten alloy into a heated mold, or if the mold is at room temperature. Allowing the pour to cool over a longer time (say, twenty minutes?) would be more akin to annealing it; having it crash down from molten to room temperature would be more akin to tempering it.

-- Thipdar


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## anachronism (Jan 31, 2020)

FrugalRefiner said:


> From my research, I agree with snoman, though it's really the same thing. Pure silver absorbs up to 10 times its volume in oxygen when molten. When it freezes, it expels it creating the spitting and crabbing often mentioned.
> 
> At least, that's my best understanding.
> 
> Dav



Maybe you're melting it with the wrong thing. Ive never seen it absorb ten times its volume in oxygen it behaves itself quite well. The bars are good with no dramas.


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## Thipdar (Feb 1, 2020)

FrugalRefiner said:


> snoman701 said:
> 
> 
> > Its not that it stops it, it’s that the oxygen is taken up by the copper as an oxide of copper.
> ...



In my experience of casting silver and sterling, I've never seen even a 2x increase in volume.

A 10x increase in volume isn't credible. The casting crucibles would need to be proportionately larger than they currently are.

Jewelry casting (at least for pure silver) would have to use different equipment, possibly including an oxygen-free environment, otherwise the results would be a silver sponge. 


FrugalRefiner said:


> When it freezes, it expels it creating the spitting and crabbing often mentioned.


Even if the oxygen were expelled as the molten silver freezes, it would leave casting porosity within the cast product.

From what I've learned about the chemistry involved, it takes more than heat to produce silver oxide, and it takes more than cooling to reduce silver oxide.



FrugalRefiner said:


> Sterling silver also absorbs oxygen, but as it freezes, the copper content can react with the oxygen to form oxides instead of the oxygen being expelled as a gas. The copper doesn't prevent the silver from absorbing the oxygen, but instead of it being expelled as gas it can react with the copper to form a couple of different oxides.


That would make freshly-cast sterling jewelry have a green tinge to it, but it doesn't. It generally has a black tinge to it, more like silver sulfide.



FrugalRefiner said:


> But there's a limit to the mitigating value of the copper. Melting freshly refined silver and freshly refined copper under carefully controlled conditions will result in a high quality alloy. There will be some oxidation of the copper content, but it will be at an acceptable level.
> 
> If the sterling is remelted, more oxygen will be absorbed, the copper content will be further oxidized, and the alloy will become darker and more porous. When jewelers remelt sterling scrap, they typically add a good percentages of freshly refined silver to the melt to dilute these effects.


The addition of 'fresh' silver is usually to offset the surface contamination of the re-used sterling. Once cast, the re-used sterling will almost always have a surface coating of investment (think "plaster of paris") that hasn't been adequately cleaned off. That investment should be drawn off by use of flux once the metal is molten, so the investment goes to the slag (and usually contaminates the crucible or melting dish), but that marginal amount needs to be compensated for so that the casting has sufficient metal for the volume being cast.

Also, some casters that re-use sterling may try to compensate for the presence of silver solder by adding a bit of pure silver into their 'sterling', in order to make sure the silver content doesn't drop below the legal limit.

So far as i know, 'fresh silver' is not added to offset any form of an oxygen build-up.


FrugalRefiner said:


> But at a certain point, they must refine any scrap and begin anew to avoid the cumulative effects.


I'm not so sure that it "must" be done, but I agree that it is a good idea. I haven't seen any hobbyist jewelers refine their silver and copper between castings. The hobbyist jewelers that are more concerned will use fresh silver and copper, or fresh sterling casting grain, and scrap out their residual sprues and buttons, etc. After all, quality products require quality ingredients. It's hard enough to produce an economical piece of jewelry without having to add a refining circuit into the process (but at some point, it becomes more economical to "do it yourself", rather than pay the refiner to do it for you - the 'break-even point' happens somewhere after you've tooled-up and have refined an adequate volume of your scraps).



FrugalRefiner said:


> At least, that's my best understanding.
> 
> Dav



-- Thipdar


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## snoman701 (Feb 1, 2020)

I'm not following the last couple posts on this thread. 

Get yourself a copy of Silver: Economics, metallurgy and Use. You'll find that in the chapter dedicated to oxygen content in silver, everything written in by Dave and myself is quite consistent with their reviews.

For what it's worth, other experiments have found the volume of oxygen absorbed by molten silver to be as high as 20x. 

https://royalsocietypublishing.org/doi/pdf/10.1098/rspa.1926.0128

It's quite easily avoided, by maintaining a reducing environment on the surface of molten silver with either a cover flame, or melting with a reducing furnace. Put your silver in a retort, or even just try to cupel pure silver, and you'll see it readily enough. It's a very well documented phenomena, that is very easy to reproduce.


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## anachronism (Feb 1, 2020)

I think the misunderstanding is the clarity of the terminology. 

Its easy to assume that "absorbing xxx times the volume" means the silver grows by that amount. Effectively if it absorbs 20 x the volume of the metal in oxygen then the effect isn't even visible.


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## FrugalRefiner (Feb 1, 2020)

Think about a sponge. When we put it in a bowl of water, it absorbs water. When we squeeze it out, the water is expelled. The sponge doesn't change in size. The absorbed water simply fills the voids in the sponge.

Oxygen being absorbed into silver doesn't simply fill the voids between the silver atoms, but we can think of it in similar fashion to understand that absorbing x times its volume of oxygen doesn't mean the volume of silver significantly increases.

Consider that silver has an atomic mass of 107.868 grams per mole. It has a density of 10.49 grams per cubic centimeter. So one mole of silver occupies 10.28 cubic centimeters. Two moles would occupy 20.56 cubic centimeters. That doesn't change much whether the silver is a solid or a melted liquid. Oxygen has an atomic mass of 15.99 grams per mole. But a mole of oxygen gas occupies a volume of 22.4 liters. Silver oxide (Ag2O) has a molar mass of 231.735 and a density of 7.14 grams per cubic centimeter, so a mole of silver oxide has a theoretical volume of only 32.45 cubic centimeters, not 20.56 cubic centimeters plus 22.4 liters.

So when silver absorbs x times its volume of oxygen, the volume doesn't increase by a factor of x because the oxygen is no longer in the form of a gas.

I hope that helps.

Dave


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## Thipdar (Feb 1, 2020)

FrugalRefiner said:


> Think about a sponge. When we put it in a bowl of water, it absorbs water. When we squeeze it out, the water is expelled. The sponge doesn't change in size. The absorbed water simply fills the voids in the sponge.
> 
> Oxygen being absorbed into silver doesn't simply fill the voids between the silver atoms, but we can think of it in similar fashion to understand that absorbing x times its volume of oxygen doesn't mean the volume of silver significantly increases.
> 
> ...



That all seems reasonable until I think about the time I cast a three-troy-ounce pour.
Did that 93 grams of silver release roughly 20 litres of O2 when it solidified in the mold?
How did that happen without causing a catastrophic event in the centrifuge?
That much O2 is a fire hazard, and a sudden emission of O2 should have shattered the mold and sent shrapnell around the shop...

-- Thipdar


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## Thipdar (Feb 1, 2020)

snoman701 said:


> I'm not following the last couple posts on this thread.
> 
> Get yourself a copy of Silver: Economics, metallurgy and Use. You'll find that in the chapter dedicated to oxygen content in silver, everything written in by Dave and myself is quite consistent with their reviews.
> 
> ...



Very interesting reading.

Postulating that the O2 can be "embedded" into silver without being chemically "absorbed" by it, there are still some problems with what has been described so far.

Granted, this is a bit of apples vs. oranges, since that was talking about silver, and this is talking about sterling...



FrugalRefiner said:


> But there's a limit to the mitigating value of the copper. Melting freshly refined silver and freshly refined copper under carefully controlled conditions will result in a high quality alloy. There will be some oxidation of the copper content, but it will be at an acceptable level.
> 
> If the sterling is remelted, more oxygen will be absorbed, the copper content will be further oxidized, and the alloy will become darker and more porous.



What justifies the assumption that the copper oxides will remain with (and thus build up in) the sterling (and not be taken off by flux to the slag)?

It seems that the cycle of remelting would produce the roughly same amount of copper oxides each time the metal becomes molten and retain roughly the same amount of copper oxides each time the sterling solidifies. That would incrementally reduce the copper content.

Further, if the *copper* is continually oxidizing due to re-heating, why would that require the addition of 'fresh' *silver*, as has been stated previously?

Back to the original question. I suspect that snoman's original response is more on-target. 



snoman701 said:


> It's more likely a result of cooling, either too fast from a hot temperature like your initial melt, or too fast from a hot working temperature when drawing it out (meaning you are trying to reduce size too quickly).



I think the cracking is a thermal issue rather than a chemical issue. I'm still interested in knowing whether Stream_Silver is casting into a heated mold or a mold that's at ambient temperature.

-- Thipdar


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## butcher (Feb 1, 2020)

Carbon, hydrogen, oxygen can become a part of the crystalline structure of an alloy of metal.
temperature and time, as well as cooling, or annealing can affect this crystal structure...

In the case of pure silver, the oxygen will form silver oxides at high temperatures but will spit out oxygen upon cooling.

sterling the oxygen forms fire scale on outer layers and copper oxide within the crystal matrix making it brittle...

Using a carbon cover or a carbonous flux or adding metals like steel to the melt as a deoxidizer for remelting the sterling, as well as other metals or deoxidizers including cadmium have been used with some success, or of alloys of silver with germanium...


https://scholarworks.rit.edu/cgi/viewcontent.cgi?article=2328&context=theses
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5050507/


https://www.google.com/search?bih=705&biw=1024&hl=en-GB&sxsrf=ACYBGNSLaos7Sf3vjWo55vQoaxBqiCKRxQ%3A1580601932531&ei=TBI2Xs6BIJLa-gT45bPYCA&q=deoxidation+copper+remelting+sterling+silver&oq=deoxidation+copper+remelting+sterling+silver&gs_l=psy-ab.12...57762.58110..62107...0.3..0.155.258.0j2......0....1..gws-wiz.......0i71.aZNcFvXyfBc&ved=0ahUKEwjO1-CFybHnAhUSrZ4KHfjyDIsQ4dUDCAs

https://www.sciencedirect.com/science/article/pii/S2211379718315730
https://spiral.imperial.ac.uk/bitstream/10044/1/16107/2/Talukdar-MI-1968-PhD-Thesis.pdf
https://www.google.com/search?hl=en-GB&biw=1024&bih=705&sxsrf=ACYBGNT4LP9obeA4O_FR2_i4x0AIFSOwPQ%3A1580605452115&ei=DCA2XsvTBsXV-gTu5YyICQ&q=silver+oxygen+phase+diagram&oq=+silver+oxygen+&gs_l=psy-ab.1.1.0l4j0i22i30l6.24880.44624..47852...0.1..0.287.1873.0j5j5......0....1..gws-wiz.......0i71j35i39j0i67j0i273j0i131i20i263j0i20i263j35i305i39j0i10.nZPiDrZLxkw


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## g_axelsson (Feb 2, 2020)

Thipdar said:


> FrugalRefiner said:
> 
> 
> > Think about a sponge. When we put it in a bowl of water, it absorbs water. When we squeeze it out, the water is expelled. The sponge doesn't change in size. The absorbed water simply fills the voids in the sponge.
> ...



You got your math wrong.

93 g silver is about 10 ml of volume or even a bit less. 20 times 10 ml is not 20 liters, it is 200 ml or 0.2 liter. You are off by a factor of 100.

And that is the maximum amount of oxygen dissolved in silver. If you have a reducing atmosphere then the effect becomes smaller. Melting in a graphite crucible creates a reducing environment or melting with a rich flame.

Oxygen in silver behaves like carbon dioxide in a fizzy drink. Ever seen the diet coke and menthos? That's what happen when a large amount of gas is dissolved in a liquid. The liquid in it self doesn't increase a lot from the dissolved gas, but once released the volume of the gas expands.

Palladium is behaving in the same way as silver, it dissolves oxygen. This is an example when they didn't know about this effect and melted palladium powder to make a bar. When the palladium solidified it started on the outside, then the gas pressure became too high and vented straight up, creating a hollow pipe.
https://www.youtube.com/watch?v=r9sTVx_YV5A

Another example is acetylene in acetone, about 400 times it's volume can be absorbed under pressure, when you release the pressure the gas is released. An effect used in virtually every acetylene tank in the world.

Göran


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## Thipdar (Feb 2, 2020)

g_axelsson said:


> Thipdar said:
> 
> 
> > FrugalRefiner said:
> ...



Centimeters^3 vs. liters - got it (and thanks for pointing that out!). I'm a product of my upbringing, and 'metric' is not my first language.

This aspect (while interesting) still seems off-topic to the original post.

Is it possible that this oxidation issue is relevant to the original alloy (1% Copper, 7.5% Cadmium and 91.5% silver)?

-- Thipdar


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## g_axelsson (Feb 3, 2020)

No, the original alloy contains copper, a small amount but to my knowledge it's enough to stop the effect of dissolved oxygen. My guess is that the addition of zinc also keeps the oxygen out.

But then, it seems like Dave and me have differing opinions in this matter.  

If I ever get my electron microscope up and running I could actually do some tests to see if there are any oxides forming inside an alloy. Nothing I can do for the moment though.

Göran


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## FrugalRefiner (Feb 3, 2020)

This is some of the information I found.



> *Melting & Casting Problems*
> Sterling silver is an alloy consisting of at least 92.5% silver. The remainder is usually copper, but occasionally small additions of nickel, tin or zinc are used. Wrought sterling (sheet, wire, tubing) usually is deoxidized with phosphorus. But, phosphorus has a tendency to cause hot-shortness (cracking at elevated temperatures) in some castings. Silver has the ability to absorb a tremendous amount of oxygen in the molten state. Figure 5 shows the silver-oxygen phase diagram. This diagram illustrates that at room temperature, silver is just about incapable of absorbing any oxygen. The solubility increases very slightly until the melting point is reached. At this point, the silver can absorb 10 times its volume of oxygen} the silver itself does not oxidize, but the copper does. Two different oxides of copper form. The first is cuprous oxide (CU20), which is a reddish color, and when viewed under polarized light, is a bright blood- red color. The second is cupric oxide (CuO), which forms when oxygen continues to be supplied to Cu20. Cupric oxide is stable. Cuprous oxide can be reduced back to copper with carbon or hydrogen, or it will form cupric oxide in an oxidizing atmosphere. Thin layers of cupric oxide are easily removed by pickling in a hot 7% solution of sulfuric acid.


Hoover & Strong - Improving Your Silver Casting



> Silver has a known affinity for oxygen, which affinity increases with temperature. When exposed to air, molten silver will absorb about twenty-two times its volume of oxygen. Like silver, copper also has a great affinity for oxygen, typically forming copper oxide. This may be of the cupric or cuprous variety, or both. Hence, unless air is excluded during the casting process, the cast article may be porous and characterized by the presence of internal voids. Thus, in melting sterling silver and other silver-copper alloys, care must be taken to prevent oxidation.


 Total Materia - Silver and Silver Alloys



> It required but a very simple experiment to prove that not only does Sterling silver absorb oxygen from the air but that this absorption commenced at a temperature below 500°. Of Sterling silver in the form of machine turnings, 10.2 grams heated in a glass tube in an atmosphere of oxygen, absorbed 35 cubic centimeters of the gas.


Journal of the American Chemical Society, Volume 29 1907



> The silver and copper that are used to produce the alloy are kept in an oxygen-reduced atmosphere during the melting and pouring process to ensure optimum deoxidation.


Rio Grande - .925 Sterling Silver Casting Grain

Dave


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## g_axelsson (Feb 3, 2020)

That's some digging you've done there, Dave. Pure gold!

I'm hitting the bookmark button for this one.  

It seems like there are varying views on how silver and sterling behaves over the last 100+ years. But the general advice seems to be to avoid oxygen and oxidizing environment when casting silver and sterling.

Göran


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## Thipdar (Feb 4, 2020)

g_axelsson said:


> That's some digging you've done there, Dave. Pure gold!
> 
> I'm hitting the bookmark button for this one.
> 
> ...



All of the lost wax casting I've done (silver, sterling, bronze and gold) has been melted with with either an acetylene or an oxy-acetylene torch. That seems like it would be an oxidizing environment.

Whatever fire scale occurs is dealt with afterwards by putting the cast items into heated sulfuric acid (what's referred to as a "pickle pot").

I do know that some casting systems use induction heating, but I haven't been able to afford that kind of equipment.

-- Thipdar


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## rickbb (Feb 4, 2020)

Thipdar said:


> g_axelsson said:
> 
> 
> > That's some digging you've done there, Dave. Pure gold!
> ...



It can be, or not, depends on how you have the flame balanced with the oxy/acetylene. You can adjust the oxy to be rich or lean, one way makes it an oxidizing flame, the other a reducing flame.


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## g_axelsson (Feb 5, 2020)

Thipdar said:


> g_axelsson said:
> 
> 
> > That's some digging you've done there, Dave. Pure gold!
> ...



Firescale? Then you aren't casting pure silver. Pure silver doesn't oxidize. The oxygen absorption is in pure silver.

And as Rick pointed out, depending on how you adjust your gases you could have an oxidizing (lean), neural or reducing (rich) flame.

Göran


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