Silver casting and alloying

Hi, Tim --

Sterling silver is 925 parts silver and 75 parts alloy -- generally copper with a trace of deoxidizers such as zinc or silicon. Sterling with just fine silver and copper casts very well, however. If you are melting in a gas furnace, flux with 50/50 borax and boric acid to keep the melt clean. Electric furnaces don't like borax so much, but you could use crushed charcoal.

What sort of mold are you going to use -- sand or lost wax?

Ok, so my proposition will work just fine?

I don't like the idea of such oxygen-sensitive metals, last pure copper I poured (even with degassing innoculant) ended up literally full of bubbles, presumably oxygen.

- I'll be using a clay/graphite #4 in my crucible furnace (check my website).

Ok, I have both on hand. Why is boric acid specifically necessary? More absorbent? I always use a silica and borax slag with brass and it works great. Should I use any charcoal with flux? Or maybe just charcoal? (With zinc I'm guessing a cover flux would be preferred...) Oh, and pouring temp - about 2000°F?

Sand. Hmm, I ought to look into a good and fine facing sand, if for nothing else to save on finishing loss and time.

Tim

-- "California is the breakfast state: fruits, nuts and flakes." Website:

As I recall, it's a reducer, so you return the oxides as clean metal to the heat. You can accomplish the same thing with soda ash, but it's very aggressive towards your crucible. Very dirty borax from crucibles readily gives up metal when melted with soda ash. It was the way I used to clean casting crucibles for my customers, one of the services I provided as their refiner.

I'm not convinced they go together. A charcoal cover excludes oxygen, which is a serious problem with molten silver. It has the capacity to absorb something like 9 times its volume of oxygen, but give it us as it solidifies. The addition of copper to silver more or less eliminates that problem, but you then must deal with the oxidized copper. It is for that reason that you should use boric acid along with borax. Borax alone removes copper, whereas the addition of boric acid returns the vast majority of it to the heat, so you don't alter the alloy.

Or maybe just charcoal? (With zinc

If you choose to use flux or any type of cover, make certain you skim it totally before pouring. Yeah, I know, you already know that, but a word to the wise------

That's well above the melting point of sterling, but the object being poured will dictate pouring temperature. I'm inclined to think it's a bit too hot for heavy objects. Could be pretty close for thin ones, though.

Why don't you let us know how it goes and what worked for you?

One other thing. If you think you can melt off a corner of your ingot, you're in for a good surprise. Yeah, it can work, but the conductivity of silver makes it hard. You'll need a rose bud. The entire bar will be near melting point, and there's not a chance you can put the whole bar in a #4. If you have to saw it, save the chips and other waste you generate in handling. I can tell you how to refine the silver. No big deal for a guy like you that understands electricity and chemistry as you do.

Harold

. It has the capacity to absorb

Sigh! The above should say "gives it *up*-------not us.

Hmm odd. To (chemically) reduce, it would have to be a pure material, like boron (which would partially dissolve in the melt, probably acting as a grain refiner, but in excess, embrittling it), so that it oxidizes sacrificially to the metal, same as carbon's purpose. But boric oxide, at best it would form copper borate directly. Hmm..

Hm. And borax is a combination of boric oxide and soda ash, but it doesn't work the same I take it? Is silica useful at all? I'd think it would be a somewhat less active boric oxide, but serve the same purpose.

For which I figure an addition of zinc will go well. Then I have to deal with a little zinc and oxide, but that's not much of a problem. (Nowhere near fuming bronze at 2200°F here... fun stuff!)

Ok.

Heh... actually... I never bother skimming. Because I formulated my slag to be just thick enough at pouring temperature that it stays in the crucible. This one time, I added too much lime and magnesia and it was literally watery! Lost a bit of that casting, too. :oP

I haven't had any problems with pouring too hot. Too cold is trouble though.

It's going to be an 8 x 13 x 1/4" thick rectangle with lots of fancy cutouts (hopefully the thickness and width total under 100 t.oz.!). That's a big mold so I'm going to do a pop sprue somewhere in the center, which leaves no room for error if the melt is covered in fluid slag.

Just suck it and see, eh? I'm just hoping I don't royally screw up the entire melt... oops, there goes $350!

Heh heh. Yeah, copper has familiarized me with its conductivity as well.

Well, this is from the Royal Canadian Mint, it's a thicker bar that would fit, although it'd probably wedge itself in nicely and crack the crucible.

Just remember I don't have any nitric acid. At least until I make a distillation apparatus and get some nitrates.

Tim

-- "California is the breakfast state: fruits, nuts and flakes." Website:

Sterling is completely molten at 1650F, usually for casting you want a couple hundred degrees superheat. So I'd cast at about 1850F or thereabouts. The higher you go over the necessary temperature, the more problems you will have with oxidation and porosity due to dissolved gasses coming back out of solution when it solidifies. 2000F is pretty hot, IMHO.

I have alloyed a lot of silver and copper alloys over the years.

The tricks I learned are these.

#1 Melt the metal with the higher melting temp first, then add the others in order. If you try to mix silver and copper cold and melt, you will wind up with blobs of unalloyed copper in the melt. For sterling, since the amount of copper is relatively small, you would do well to first mix up a small batch of 50/50 ag cu and then add that to your fine silver melt.

#2 Use the borax and boric acid mix, it's cheap and it works great.

#3 Do not add charcoal to the borax and boric acid mix. It turns into a viscous goo that does not pickle off and screws up your crucible.

#4 Zinc is not necessary, but it doesn't hurt, It doesn't take much. A half of 1% will probably be enough for your melt. Throw it in just and stir before your pour. Most of it will burn off. Don't use pennies or you'll wind up with the skins.

#5 It is ok to go a little light on the copper. There is no law that says .930 can't be sold as sterling.

#6 use a clean crucible. Preferably new or only used for silver.

#7 It is easier to fill your crucible with shot than ingot.

Paul K. Dickman

You're allowed as many typos as you like Harold! I didn't notice untill you pointed it out.

I'm far from able to answer your question in the way of education, but in the school of hard knocks, I know what worked. I get the idea that the excess of soda ash alters the chemical balance, which has a profound affect in practice. As the oxides are reduced the color of the flux changes and you start finding beads of metal in what was a bead free mix, so something is happening that is of benefit, at least with precious metals.

I took no chemistry classes in school, so I'm unable to explain the simplest of reactions. The processes I discuss were all learned from old books on processing metals. I was self taught as a refiner, something about which I was rather proud, considering it turned into a very successful business, although that was not my intention.

I used silica in melting very high grade waste materials, but I used it along with borax, soda ash and fluorspar. Because of its high melting temperature, it may work, but isn't a good idea if introduced as silica sand. Broken glass (not borosilicate) can be added, which melts at a much lower temperature. My purpose in adding the silica was a result of reading books that suggested it would help lower the erosion of the furnace lining, all the while providing the needed flux. I used that combination of fluxing when I'd reduce the waste, which was high in silver chloride (which reduced to silver and acted as a collector of other values) and contained a lot of gold and platinum and palladium. The addition of fluorspar made my slag quite fluid, a good feature in my circumstance, considering it permitted the small beads of values to collect by gravity at the bottom of the furnace.

In order to prevent the loss of the fine materials, I'd mix the entire mess and add water, then pour it into cakes, which would set up hard almost instantly, and dry in a few days. The cakes were introduced to the tilting reverb furnace I built, with no losses due to dusting. It was one of the things I came up with on my own and it worked to perfection. An assay of my slag showed almost perfect extraction of values, leaving behind so little value that the slag was not worth further processing.

In favor of crucible life and cleaner castings, it's not a bad idea to cast without the use of flux when you're able. If your slag is thick enough to stay in the crucible, you're costing crucible life by contamination. If it's too fluid, it's likely too aggressive, so you lose crucible life to aggression. There's no easy answer except for ignoring flux when possible. However, having worked with precious metals for years, I fully understand the value of fluxing.

Not today. Have you checked the price of silver lately? It's over $7/ounce.

I think you missed the point. A #4 should be capable of holding 4 pounds (av.) of brass. 100 ounces of silver weighs 6.85 lbs. av., plus you'll have the copper to deal with. The volume of the crucible is way too small unless I'm missing something. I'm not convinced the difference in specific gravity will offset the volume enough to help. Could be I'm wrong.

Understood, and you do need nitric. Save the waste, you never know what the future will bring.

Harold

Ok. Redneck way, don't know too much behind it, but you know the practice damned well. I can respect that :)

My bronze slag mix is just glass blended from raw materials - it could use cullet and borax just as well.

As a matter of fact, borosilicate is mostly silica with 5-10% boric oxide and a few points of sodium or potassium, so it would melt in the midst of the other fluxes (boron, lime, soda etc.) just fine. A little more expensive though.

Yep, sounds good. Most of the slag I skim ends up with beads of copper somewhere in it, a little carbon and lime, or sure, fluorospar if you want to nuke viscosity would free that up good. Not that copper is worth quite that much!

Hmm interesting, I always thought I was the first on the internet to make a furnace and call it "reverb"! Got any drawings or pics?

Howso? I'm not afraid of contamination in normal use, since I always pour a bastardized alloy primarily of copper, with maybe 25% zinc and 5-10% total of lead and tin.

No agrument there!

I like it because it keeps the zinc in and cleans oxides out, not to mention keeping them from forming in the first place.

Oops, you're right - missed a factor of two there!

Aluminum.

What standards are you on, exactly?

I think the AFS standard is pounds (#) of aluminum. Some companies might deviate, which *would* explain how a #12 could cost a mere $20.

Lemme see, SG = 10.45 let's say, aluminum = 2.7 so I can hold 15.5lbs of silver.

If it were 4# of Cu however, I would certainly be in trouble.

Tim

-- "California is the breakfast state: fruits, nuts and flakes." Website:

Yep, BTDT, things dissolve much better at the highest melting point!

Gotcha. For quicker dispersion I could even mix a eutectic, looks to be around 30% Cu and melts at 1440°F. *Shrug* 50/50 alloys are easier to use...

Okay.

No doubt mostly from dissolved oxygen, the rest from there simply not being much present.

Any idea what zinc does in larger quantity, say up to oh, 2%? I'd say as much as 5%, but that leaves 2.5% Cu to keep it in the Sterling range...

Hm, how about aluminum to degas it? Probably a bad idea without precise control...

Odd, I don't have any trouble with skins when just melting them for zinc below 1000°F, as long as I give a few minutes for them to dissolve. Copper is molten at 1984°F, right near my pouring temp after all. Come to think of it, I have some lumps of already melted pennies that'll do.

Yep, that's my understanding!

Hm, that could add another $40 to the project... On the other hand, more tools!

Yep, I don't like thick stuff either. I'll cut down the ingot before melting.

Tim

-- "California is the breakfast state: fruits, nuts and flakes." Website:

Thanks, Tom. I hate showing what a dope I am!

Harold

Unlike (proper) machining, which requires considerable skill and knowledge, if you stick to the "recipe", you can expect and get good results each and every time. The only thing left, really, is your honesty. Without that, you're finished as a refiner.

Drawings? Naw, I do things like that in my head. Pics? Yep, quite a few, even a couple already in the computer that I scanned from old photos. If you'd like me to send them to you, just ask. Combined they're something like 120 kb. The furnace was fed through one of the trunnions, and had electric ignition. Natural gas fired, with a capacity of a couple gallons. I could fill the two large cone molds in the one photo. The furnace had to be mobile, so it was built on a channel iron frame on casters. Took something like 500 lbs of high alumina refractory. The large hand wheel that powered the chain driven tilt mechanism, obviously off an old coffee grinder, was found down the street from the castle, while they were widening the road. An old model T jack and a couple other artifacts were found with it. It was a natural for the furnace, all I had to do was build the handle, which was full floating. Took three turns of the handle to tilt the furnace to pouring position, and it was fairly well balanced, although bottom heavy so it couldn't go the wrong way involuntarily.

Buildup of flux. It's what usually ruined the casting crucibles I reconditioned. Unless you can skim it occasionally, as it gets dirtier it gets all the stiffer until it won't come out of the crucible without the use of something to dissolve it. That usually equates to a ruined crucible. When I was using my #8 crucible furnace for reducing the waste material (eventually melted in the tilting furnace), it wasn't uncommon to get only three heats from a crucible before it perforated at the flux line. Try that on your #4 silicon carbide crucibles and tell me how much you like fluxing------it gets damned expensive.

Woops! You caught me napping there. I hate it when that happens. I had the formula reversed in my mind. Of course your crucible is more than adequate, even with 7-1/2% copper added (proper for sterling). I should have been more than aware that you were right, for I used to cast silver anodes that weighed near 200 ounces with my #8 crucibles, and it didn't fill them.

Skim your heat, and be sure to let us know how the casting turns out.

Harold

Heheh!

Brings to mind, for some reason:

(Homer was just voted on-the-spot as union leader.) Homer: "Woohoo! What's it pay?" Carl: "Nuthin'" "D'OH!" "Unless you're crooked..." "WOOHOO!!"

Please do! :)

If you'd like, I could put them on ABYMC.com too.

Ouch, must've taken a lot of pilfered gold to pay for that...(J/K)

Ah, true. I always assumed the remainder left was pushed out by the heavy bronze (and stirring action), and most of the time it does come up to the surface and cover it.

Hey, if it's getting more viscous (from dissolving more thick stuff like silica, alumina and carbides the crucible is made of), it also is getting less active.

Actually my #4 is a clay graphite - probably worse for fluxing action, since SiC (I have a #6) is non-oxide. Or do they both get fluxed about the same in your experience?

Which reminds me, I've done several salt and aluminum melts in the SiC, it seems to be doing okay so far...

Ok!

Tim

-- "California is the breakfast state: fruits, nuts and flakes." Website:

snip---

Consider it done.

I don't have a clue what that is, but feel free to show them if you feel they'll benefit anyone. The furnace worked beyond my wildest expectation, as I alluded previously.

If you happen to put them on ABYMC.com , why don't you let me know so I can see what you have to say about it. A link would be nice.

Actually, the remnants of over 20 years of refining, and yes, I was well paid for my efforts, but only because of the shear volume of gold I processed. Considerable platinum and palladium accumulated from old dental gold. For the most part, it is lost in the first operation (inquartation), to be recovered only when the silver used for the process was finally parted in a silver cell, and the sludge refined. Because the amount of these metals came in in very small amounts, there was no accounting for the customer. The cost of assay would have been far greater than the value in almost all instances.

There is always a small amount for which there was no accounting due to the method I chose to use. I'm sure you can understand that there is drag-out from filtering, for example. Even though you wash down the filter, there's a small amount that remains behind, especially if there's a lot of silver chloride. The customer gave up a minor amount of value, but avoided higher costs of refining. Either way, the customer would have taken the loss, but by choosing my method, I could turn a batch of gold in just a couple days unless it was quite large. Even then, I always turned a lot in a week or less.

The chief reason I succeeded in refining was because of quick, honest service. Major refiners have a way of dragging you out for at least 30 days. Many of my customers were small operations that couldn't tie up their gold that long. Many of them asked for gold in advance, while I did their refining. Service like that doesn't happen with the big boys.

True, less active, and if used strictly as a shield, that's not a bad thing. My problems as a refiner were considerably different from those of a guy in a foundry. My goal was 99.95% purity, or better. Everything had to be handled carefully, or the refining was of no value. You likely understand that certain metals combined with others destroy their qualities (ductility). Lead in gold raises all kinds of hell, making it extremely brittle, as does silver in platinum. Cleanliness of operation is of chief concern in refining precious metals. Melting in dirty crucibles had a tendency to work against me. When it was necessary to melt with flux, I usually used it copiously, then poured into a well blackened or otherwise coated cone mold. I had two sizes, the large ones in the pic I've forwarded to you, and a smaller one, maybe half that size. They're excellent for separating flux from the button because the metal concentrates on the bottom with a minimum surface area, while the flux is on top, and usually fairly thick so it breaks away nicely.

I don't recall a very big difference in performance, and I used both about equally. I favored the silicon carbide because they didn't require seasoning, although I never had any problems with cracking a graphite/clay crucible. Keep in mind that when I used flux, I used plenty, otherwise I sustained a loss to prills in the flux. It was very important for the flux to fully dissolve all the dirt possible, and to stay fluid. A viscous flux was death for me.

I've never done any melting with salt, so I don't have a clue. I'm not very interested in aluminum casting, but I fully intend to cast ductile iron, along with steel and stainless. I'll also set up for brass and bronze. I hope to build a model steam locomotive, so aluminum doesn't really lend itself well to my objective. Needless to say, it would be no problem to cast it once I'm set up for brass.

Harold