Hand pouring sterling silver? Issues with "air bubbles"

Ipv6Ready

Ipv6Ready

Well-Known Member
Silver Stacker
Hi all,

Looking for some help, I've been melting 999 silver with no issues and making my own bars.

I'm using Mapp torch, silca crucible, borax etc etc.

Just recently I've acquired various scrap sterling silver, and decided to melt them in to bars and ran into a little problem.
unlike fine silver, my sterling seems to get air bubbles on the bottom.

The top row are various 999 silver, the bottom row is various sterling (actually higher than 925) as I've been mixing in fine silver to the sterling to make sure it has at least 32gm ASW)

24826_20160705_081201999.jpg


Sorry for the bad quality pix, best i can do using iphone and the dent and scratches are from rolling around in my bag

Note: to make sure each bar has one ounce silver, I mixed in on average 28gm sterling 925 and 7gm 999 fine silver for a bar wieght of 35gm.
Also the sterling is from variety of hallmarked pieces and some are pre decimal coins etc.

Any ideas why or help would be greatly appreciated.
 
I have a feeling it might be due to the non-pure alloy cooling faster than the 999. The air doesn't have time to be pushed out of the way before the metal starts going hard.

Are you heating your mould as you pour?
 
I'm guessing at least part of the issue is that copper has a higher melting point.


Sterling can be tough to pour, from what I've seen/heard..
 
HI Big AD
Yes I have heated the mold for the first pour and after a few it is very hot.

HI GB and AD
Yes I have a feeling sterling behaves differently to both pure copper and silver, sterling seems to cool down rapidly.

I might redo one and heat the mold enough to melt the sterling.
 
Gatito Bandito said:
I'm guessing at least part of the issue is that copper has a higher melting point.
Click to expand...

I was wondering about that also, but the MP of an alloy is often lower than that of its components:-

Silver, Pure 962
Copper 1085
Silver, Sterling 893


Tin (60%) 232
Lead (40%) 327
Solder 190
 
Ok so after searching google I've found this, it seems I'm not heating it enough.

Maybe I just need to heat it longer.


http://www.madsci.org/posts/archives/2001-11/1005200805.Ch.r.html

...about 760oC or so. Above that
temperature, the sterling silver starts to melt. The phase diagram shows
us that the composition of the material that first melts contains 28.1%
copper (and 71.9% silver), while that portion of the lump which hasn't
melted is essentially pure silver. As the mixture is heated to about
800oC, the green line shows that the liquid portion of the mixture now has
a composition of roughly 20% copper, with the solid phase still consisting
of essentially pure silver. At 900oC, the liquid phase contains about 8%
copper (as more and more silver has dissolved into the liquid phase) but
still contains some solid silver. Not until the temperature reaches 930 or
940oC has all of the silver dissolved into the liquid phase.
Click to expand...
 
is it wise to take 999 fine silver and mix with 925 to make bars? you are taking your pure silver and forcing it to be eventually refined again!

isn't it better to just add more 925 silver and make the bar a touch bigger?

sorry, but as a novice who has not done any melting this does not make sense to me. but i do have an interest in trying this one day. does it help with the melt when adding 999 fine?
 
randomname said:
is it wise to take 999 fine silver and mix with 925 to make bars? you are taking your pure silver and forcing it to be eventually refined again!

isn't it better to just add more 925 silver and make the bar a touch bigger?

sorry, but as a novice who has not done any melting this does not make sense to me. but i do have an interest in trying this one day. does it help with the melt when adding 999 fine?
Click to expand...


First stay away from sterling when hand pouring, it is trickier as I have found, though I think it was just impatience and pouring too early before the ally fully melted. 999 is so much easier.

Why did I do it? Logically you wouldn't melt easily recognisable silver bullion or jewellery with hallmarks. But the items I melted were cheap scrap, selected worn down pre decimals from bulk buys, badly milk spotted or otherwise damaged items and I am unlikely to ever sell these unless at at scrap value so no real harm done.

it was just a bit of fun and silver is silver. My next step is to do some lost wax casting.
 
Thanks Ipv6Ready, keep us posted. Great to see what the mad scientists are getting up to in their labs :)
 
have you considered trying to refine the sterling into 999? what would be the process?

can you heat it to ~760C like your phase diagram link suggested to turtle off the copper?
 
Ipv6Ready said:
Why did I do it? Logically you wouldn't melt easily recognisable silver bullion or jewellery with hallmarks. But the items I melted were cheap scrap, selected worn down pre decimals from bulk buys, badly milk spotted or otherwise damaged items and I am unlikely to ever sell these unless at at scrap value so no real harm done.
Click to expand...

ah, ok. makes sense.

Ipv6Ready said:
it was just a bit of fun and silver is silver. My next step is to do some lost wax casting.
Click to expand...

oh yeah, that is awesome. my parents had the equipment but it has all been left idle and not used for years. one day i'd like to melt a massive chunk of silver and make.... ????.... anything really! one day...
 
I watched a show on sbs once about metal working and sculpture in Ancient Rome and a fair part of it and Roman metalwork featured lost wax casting. It looks really interesting.
 
A little update, I have given up "hand pouring" sterling, well at least wth my setup.

I just don't seem to be able to "maintain" the required temperture to keep the sterling in liquid state long enough to pour, using Mapp gas (yellow cylinder), even with heat resistant bricks keeping the temperature confined, and super heating the mold.

I know my setup can reach 1085c as I have melted and poured pure copper. The super fast solidifying of sterling seems to be normal.

My first pours was with pure silver and i easily heated 999 silver in the crucible until the silver swirls madly (what YouTube vids calls Mercury effect) and the pouring of the silver limits air being trapped in the mold.

However with sterling silver, using the same technic to pour sterling into the graphite mold the sterling basically solidified too fast and lumps would form. So to overcome this I heated the sterling directly in the graphite mold.

My assumption to overcome this trapped air was to heat the sterling in mold even longer thinking the air bubble will escape. However over countless try it dawned on me that the longer I was heating the sterling in the mold, the more bubles were forming. So on a different attempt I stopped the heat immediately after the sterling liquified in the mold and success.

My theory as the sterling is heated / melted in the mold just like pure silver swirling in the crucible the sterling silver "swirls/runs" from one side of the mold to another due to slight differnce in the temperture and this swirling action in the mold was sucking in air and trapping the air in the bottom of the mold, hence the longer I heated in molten state the bigger and more numerous bubbles.

Well I just bought 8 ounces of junk 925 coins, so this weekend Im sorting the coins and going to melt badly damaged coins and experimenting a bit more.
 
neonuke said:
have you considered trying to refine the sterling into 999? what would be the process?

can you heat it to ~760C like your phase diagram link suggested to turtle off the copper?
Click to expand...


I haven't considered refining the sterling to 999. I don't have enought sterling etc to make it worthwhile. Just the expense of getting all the chemicals and equipment seem like over kill for people like me.

The most common method seems to generate toxic fumes, and in a high rise probably best I didn't do on my balcony.
 
See if you can get in contact with Craig of silverfoxrefining.com
He may know a tip to start with !
:)
 
Great post, I've built my own small furnace ( it weighs in around 160 lbs and uses a 500 000 btu propane torch, this one is noisy though :) )

I see some possible issues with pm.

Melting alloys are a pain, sometimes you get surprises. I've got a few stories, none the best than when my cast produced a couple of lumen worth for a dozen minutes when I casted and a chunk came out. I bet I found lithium. anyways.

When melting known alloys, use the phase diagram, not the boiling point of one and the other.
1-Use another mold than a silicone carbide. Maybe clay.
2-Use something different than borax

Limit the amount of oxygen reaction / oxidation and cooling.

my 2 cents.
 
You have moisture in your mold, gasses are expanding after the pour, this is why heating in the mold makes it worse, your mold is not thoroughly dry, I am surprised it has not blown up in your face
 
BenKenobi said:
You have moisture in your mold, gasses are expanding after the pour, this is why heating in the mold makes it worse, your mold is not thoroughly dry, I am surprised it has not blown up in your face
Click to expand...


Hi thanks, but is this a general warning or something you know about sterling silver?

I do keep the molds in a zip lock bag after it cools, don't you think that is enough?

Plus I heat the mold very high as I have it near the back of the makeshift firebrick enclosure for the first use, than I might use the mold five to ten times, so I suspect all moisture have been heated out.
 
No it is not a general warning it's highly likely possibility. If you want my cv I have been involved with precious metals since I was 14 yr old,plenty of casting experience from bars to jewellery, lost wax, kiln firing, mold making and home refining and pouring. I my experience I have seen what you are showing in lost wax castings and bars, due to the mold not being dry, and also from over heating, mostly from moisture. I would be seriously looking at your casting environment, eg cold area and hot metal pours create moisture and gas explosions from this are a hazard, bake your molds 150- 200c in a kiln or oven for a couple of hours before pours, zip locks won't work, plastic bag molecular structure is not resistant to atmospheric water/ humidity working in the temp range you are working. Just how hot are you getting your metal before pouring, as in temp, I would not recommend above, 1100c for sterling, you alloy composition should not matter to the extent that your issue exists. You must note all changes in your procedure and note the effect of those changes, you must strive for consistency. There are a few of your pours that look fine, you must note whatever change. I will pm when o get home from work and give you some reference books to chase up at a library or online, these are crucial for your future if you want to do this correctly and teach a lot about method, alloy temps etc for casting, lost wax etc. you do not mention your type of mold? Carbon graphite/ silica/ home made or cast iron?
 
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