HI I am looking for some information on casting and working large ingots of sterling silver. This is more of a large silversmithing project than jewelry making but I’m hoping some of the info will scale up. I am trying to pour and forge very large ingots, 1000gram and larger. I am trying to replicate the process depicted in this video by casting into a ring mold sandwiched between two steel plates.
I am melting the silver in a propane furnace with a graphite crucible. For fluxes I’ve been using a large about of borax, as well as a 5:1 mixture of ground up charcoal and sal ammoniac(ammonium chloride) to prevent and remove oxygen from the liquid silver. I’ve been adding 4 or 5 grams of zinc to the melt as an oxygen scavenger as this isn’t fresh silver. I don’t know the exact temp in pouring at but I am well past the melting point, probably in the 1800-2000F range.
The mold I’ve had the most success with is a 4.5" diameter circle at .375" thick, coated in soot from an acetylene torch. The silver fills it fully and appears to have a relatively clean surface. I’ve had issues with hot tears as the ingot cools too quickly and unevenly, making large cracks in the surface. Getting the steel mold as hot as possible with a torch seems to help.
As I start to forge the ingot it starts to crack. I am forging hot, bringing it up to annealing temp, then letting it cool until its no longer glowing. Hammering gently at the beginning to break down the large crystals formed in the hot pour.
I have melted and poured this ingot many times now, I’m wondering if the cracking is due to impurities that have made their way into the silver while liquid, namely cuprous and cupric oxides from the copper content of sterling. Is there a different flux I could be using to help refine the silver in the melt? Is there a way of assaying what is in the alloy now? At just over a kilo of sterling id really like a way of reconditioning the metal and salvaging it for other projects even if this fails.
Your mistake is in hot forging the silver. It’s unstable at high temps. Let it cool to room temp. Remove any flux or anything you don’t want forged into the metal. Reduce the metal thickness by a good bit but not so much that is starts to crack.
Yes borax is just fine for flux. Just be sure to remove it before forging. To reduce the forming of fire scale I prefer to coat my silver with a 50/50 mix of denatured alcohol and finely powdered boric acid. I then either let the alcohol evaporate or flame it off. This leaves a coating of boric acid to protect the metal. When annealing and melting it’s best to use what we call a reducing flame. This is a flame with less oxygen and therefore less oxidation.
Re heat the metal to a soft glow in a darkened room so as not to over heat it. Then after cooling ad cleaning the metal continue to forge. When it gets too hard to work anneal it again. Annealing too often will also cause tearing at the molecular level. You can quench the metal but with something this size I wouldn’t.
Your silver will need to be refined at this point. Go to a trusted refiner and either have them refine your metal or trade your old metal for new. After about 3 meltings the silver needs to be refreshed by adding more fresh metal.
Good luck with your project and let us all know how it turns out.
Jo Haemer
I’m a retired smith who was trained decades ago to hammer down inch-thick ingots of sterling, using a heavy forging hammer, into sheet that could go through a rolling mill (ie <6mm). Generally, you can’t hot-forge sterling, although some alloys might be more forgiving.
Cleanliness of the source ingot was paramount, any surface impurities could act as stress foci when hammering. You might find it helpful if you have access to a kiln to lock the developing sheet between iron plates and heat it to about (from memory) 300°C for an hour. This is remarkably efficient at removing stresses in the metal, although the iron plates and bolts don’t survive many cycles of use due to accelerated rusting and fatiguing.
I never used denatured (or even pure!) alcohol; I always used methanol, which is a far better solvent for boric acid. It’s significantly more toxic then ethanol, but nothing you can’t cope with in a workshop using reasonable ventilation and being aware of the issues. It’s also very much cheaper.
I operated a small silver foundry for a couple of decades, using ceramic shell casting, which I much preferred to investment casting. However, one had to take significantly more care to ensure that none of the molochite (the refractory mould material) contaminated the casting if you needed to forge it, than with investment casting. Molochite is in the form of both powder and small grains, very hard, ideal for ruining metal if hammered into it!
Thanks for the info. Any input on refreshing the silver after it’s been melted and poured so many times? I’ve read you should use 50% fresh sterling for every melt, but at these quantities that’s just not an option for me.
I am getting a lot of people telling me to forge cold, at least at the beginning, to gently break down the big crystals in the silver. In the video I referenced as a guide they are definitely forging hot right from the start, though professional silversmiths with lots of experience and knowledge in this kind of work probably have better control over the whole process than I.
Our workshop managers at Hatton Garden invariably insisted that silver that misbehaved after much forging (cracking, splitting or spalling) should be returned to the refiners. This was usually, though not always, a cheaper option than sawing out faulty areas and patching in new ones. Even when very expertly fitted and soldered, one could usually see faint evidence of the repair in the form of the very slightly different colour of the solder line. This would show up usually after stoning.
Incidentally, I watched the video, and don’t regard the silver forging as hot forging in the usual sense; silver chills far too fast. It’s difficult keeping bronze hot enough for long enough to be called hot forging, which only really comes into its own with iron (which is very poorly conductive of heat). Sterling is sufficiently soft after annealing that I doubt I would notice much difference between room temperature forging and black heat forging.
In my own small foundry (I use the term loosely, it was a ceramic shell melting and casting setup that could cope with 5kg silver or bronze maximum charge) I would melt and cast all the scrap silver I collected (which amounted to about a kilo a month), drawfile it, and hammer it into sheet for goblets, bowls and boxes. If the sheet misbehaved badly enough, I would send it off to the refiners in return for fresh sheet. I found that adding fresh silver casting grains to a remelt was usually disappointing and rarely economic. Adding fresh silver was good only for satisfactory scrap where you wanted to move the alloy back towards the original composition (since one tends to lose copper, and certainly zinc if present, when remelting).
Wow a kilo of scrap a month is a lot. What type of molds were you pouring these large ingots into?
Also I tried breaking a smaller chunk off the ingot,100 grams or so, remelting and forging it down. Still saw some evidence of cracking and fractures in the metal, which leads me to believe that its the metal itself that has become contaminated more than issues with the mold and pouring. Though it very well may be both.
