ARGENTIUM - Workflow / hardening

[Nick martin]

Sorry everyone, got another Argentium question!

Currently working on three pieces simutaneously, all made from Argentium, although one of them has elements of Sterling Silver as part of the construction ( tube ). It also has Argentium easy and hard solder joins.

What I'm trying to establish is... if I place the finished Argentium in an oven to harden it and bring the Germanium to the surface more, will pickling it afterwards have any detrimental effect or not?

I haven't attempted to heat-harden any Argentium yet, and maybe I should have experimented a bit beforehand, but I'd like to ask the question in case I ruin any finished pieces.

Presumably the sterling silver tube will tarnish due to the heat and oxidisation in the oven, so I'd need to pickle it to remove the copper oxides, but will that then undo the oven treatment on the Argentium in any way?

So far I'm enjoying working with Argentium, and if these three pieces turn out well, then I think I'll use it wherever possible when making silver items as it fuses / solders beautifully, not to mention the shine it polishes up to!

Thanks all,

Nick

[ajda]

You've identified the main problem with mixing argentium and traditional sterling elements in one piece. (Argentium is technically "sterling" so forgive me if I always use the word "traditional" to qualify when talking about both.)

And forgive me for length here, but it's easiest to copy/paste from an info sheet that I send out to beadmakers who buy my Argentium tubing for coring beads:

...if you wish, it can be easily be finished to a significantly greater hardness - so it will take a higher polish and will better resist wear and tear.

This can be done in a kiln or domestic oven. Ideally, the finished piece should be cleaned, then heated at 280-300°C for 1-2 hours, or at a lower temperature for longer (220-240°C for 2-4 hours), followed by air cooling - do not quench at this stage. Standard kilns used for glass annealing may need careful monitoring when ramping up to these relatively low temperatures - mine has a tendency to overshoot the target so I always wait until it has stabilised at the right temperature before putting my pieces in. If you put them straight into a hot kiln, there's a risk of cracking some glass items due to thermal shock, so you might prefer to put them in cold and ramp up slowly - but monitor carefully until you know how your own kiln behaves at these temperatures.

Heat treating for hardness is called "precipitation hardening" - an article explaining how this works can be found here: http://argentiumguild.blogspot.co.uk/2011/07/precipitation-hardening-of-argentium.html Precipitation hardening may lead to some surface oxidation which might need light pickling and/or rinsing in soapy water.

Argentium's special capacity to resist tarnishing is due to the formation of an invisible "passive" layer of germanium oxide on the surface which shields the layers beneath from contact with oxygen and sulphur compounds in the air. (Stainless steel works in the same way, by forming a passive layer of chromium oxide.) It occurs naturally over time and if the item is scratched or abraded it will re-form. However, passivation can be encouraged at the outset by heating at lower temperatures again - 100-120°C for up to 16 hours* (at least 3-4 hours recommended). Incidentally, this passive layer also shields the wearer from direct contact with the other metals in the alloy, particularly copper and its oxides which may cause irritation to sensitive skin - hence Argentium is sometimes described as hypoallergenic and can be worn by people who have cannot tolerate traditional sterling silver.

To develop maximum tarnish resistance, it is important to clean and degrease carefully between each stage of finishing, so before heating for precipitation hardening and before heating for passivation. The makers of Argentium recommend an ultrasonic bath at 50°C for 2 minutes using a neutral (pH 7-9) aqueous soap solution. They go on to say: "Strong alkaline solutions should be avoided as these have a detrimental effect on the tarnish resistant properties. After degreasing, pieces should be rinsed with water at room temperature and dried (if using hot air this should ideally be below 70°C). It is important not to use deionised/reverse osmosis water as this can damage the protective germanium oxide and reduce tarnish resistance."

*In a recent email discussion with Peter Johns, the inventor of Argentium, he suggested that only 2 hours was necessary for passivation. I still err on the long side to be sure.

So, if you heat treat for hardness first, you can then pickle - the pickle may have some small effect on the germanium oxide layer, but I don't think it will actually strip it off. I sometimes pickle briefly after the hardening stage and/or burnish with stainless steel shot in a tumbler. If you then heat treat again at the lower temperature, I don't know how much the traditional sterling will be affected - if you can get away without pickling again it is probably better, but may be OK.

Alan

[Nick martin]

Thanks for all that info Alan, I was aware of most of it from reading but thanks all the same.

Your last paragraph answers my question in a nutshell, so I'll follow that path and see what happens.

Cheers,

Nick

[ajda]

I'm still gathering information on the best procedures to follow - I'll see if I can get a definitive answer from Peter Johns on pickling after heat treating.

I recently asked him how important it was to anneal the whole piece evenly before heating for hardness - my question and his answer below:

Q - Mark Grimwade's book on metallurgy for jewellers talks about a "homogenizing anneal" before precipitation hardening, but how critical is it to do this evenly and thoroughly? When I make a piece in several stages, some parts may be more annealed/more work-hardened than other parts. Or if I use loose chain from Allied, for example, which I assume has been hardened already, and I then fuse/solder new links, clasps or other fittings, I affect some of the chain links by direct or close exposure to the flame. How important is it to then re-anneal the whole before heating in the kiln for hardening?

A - Mark is describing the only way you can precipitation harden sterling silver by first heating it for 30 mins at 750°C followed by a rapid cooling by plunging into cold water. Then you reheat at about 300°C. Argentium does not need the long anneal or the rapid cooling to harden. It will harden even if it has been slow cooled after annealing or soldering. It is probably better to make sure the piece is evenly annealed. You can anneal and then quench Argentium to harden it and you will get a better hardness. However it does not need a long soak, just normal anneal - allow to cool below red heat and quench.

If you've used solder, you'd obviously need to be very careful if you were going to try to re-anneal the whole piece - I think easy solder melts at just over 700°C - so may be safer not to try that.

Alan

[metalsmith]

Sorry everyone, got another Argentium question!

Currently working on three pieces simutaneously, all made from Argentium, although one of them has elements of Sterling Silver as part of the construction ( tube ). It also has Argentium easy and hard solder joins.

What I'm trying to establish is... if I place the finished Argentium in an oven to harden it and bring the Germanium to the surface more, will pickling it afterwards have any detrimental effect or not?

Thanks all,

Nick

From the chemistry, "Germanium is insoluble in dilute acids and alkalis but dissolves slowly in hot concentrated sulfuric and nitric acids" (wiki - source not referenced), then pickling should not affect the Ge.

I don't believe that pickling would "bring the Ge to the surface", tho of course this is other than the pickling removing Cu and therefore relatively increasing the peripheral representation of both Ag & Ge.

Hope this helps

[ajda]

Thanks metalsmith - that's useful. It could be that heating does actually bring it to the surface. Since Ge atoms are smaller than Cu they are more mobile and can migrate within the alloy more easily. Johns reckons that may be the reason why it hardens so easily with heating, though he hasn't investigated fully - I suspect it may also mean that they migrate more easily to the surface where they can react with oxygen to form the passive layer.

Alan

[metalsmith]

It could be that heating does actually bring it to the surface....
Since Ge atoms are smaller than Cu they are more mobile and can migrate within the alloy more easily... they <may> migrate more easily to the surface where they can react with oxygen to form the passive layer.

Alan

Hi Alan, by smaller, what do you mean, for eg.

Element Cu Ag Ge
Atomic radius 29 47 32 A

Perhaps, at nearly twice the size, Ag forms the framework in which the similar sized Cu and Ge can migrate, but with Cu being dissolved out to solution in pickle, then Ag and Ge remain, with Ge continually filling any gaps in the lattice. Yes, heating would help with migration as well as oxidation of Ge.

[ajda]

Ah - looks like my error in talking about "size" of atoms... should have listened more in Chemistry lessons instead of messing with the bunsen burners...

[Lucie]

Argentium's special capacity to resist tarnishing is due to the formation of an invisible "passive" layer of germanium oxide on the surface which shields the layers beneath from contact with oxygen and sulphur compounds in the air.

Quick question - Does the germanium oxide layer appear as a white or lighter silver patch / section by any chance?

[ajda]

After heating with a torch, for annealing or any other purpose, the silver does look matt and white, but whether you're seeing germanium oxide I don't know. After polishing/burnishing then heat treating properly for tarnish resistance, it looks like bright new silver and shouldn't be at all patchy.
Alan

[Lucie]

I get the odd argentium casting that comes back with a white / light mark across them , usually only on one side. It seems to polish off fine. Pickling doesn't affect it.
It's a bit of a pain for sections of casts that can't be polished. I still prefer argentium over sterling though - much nicer colour & shine.
It would be interesting to know if the marks are germanium oxide :)

[ajda]

I heard back from Peter Johns re what happens to a piece of mixed sterling/Argentium when heating and pickling:

Nothing will happen to the sterling at 300°C that will need more pickling time than the Argentium. I am fairly certain that the sterling will stay clean at the passivation temperature.

Pickle in theory will remove the GeO and will probably make the passivation a waste of time. We have conflicting evidence on this so I am not absolutely certain if the GeO is removed. It may also be dependant on other factors like how many times the piece was annealed or soldered.

I think more of a problem when mixing other metals with Argentium is carry over when the piece is polished. The carry over can work both ways so it will be interesting to see what happens.

Alan