Monday, 1 March 2010

Lost-Wax Silver Casting

I'm playing with lost-wax silver casting at home, and although you might judge the results to be less than impressive, in fact I'm pretty pleased with the progress I've made and I think ultimately I will succeed.

Summary of the lost wax process: forgive me if you already know this. Make an original item, like a ring, for example, out of wax. Cast 'investment' (a heat-resistant variant on Plaster of Paris) around this. Heat the investment to melt out the wax and pour molten silver into the newly vacated ring-shaped mould. Remove the investment and hey presto! one silver ring. Or whatever.

In my case it was a hammerhead shark. When we were on the Galápagos cruise, one of the naturalists, Socrates by name, wore a silver hammerhead shark pendant on a leather thong around his neck. I coveted that, from the moment I saw it. Sadly, when we called at Santa Cruz and had a chance to walk through the town, I'd stupidly chosen just to bring a few dollars and leave my credit cards on the boat, so didn't even bother looking for somewhere to buy one.

Now when we did silver casting the last time we attended the lovely George Grant's class many years ago, he had a proper oven for burning out the wax and a proper centrifugal caster to make sure the silver shot firmly into the mould and filled all the little nooks and crannies. I don't have either of those, and am trying to find ways to make it work anyway. It has to be possible. People have been doing lost-wax casting for centuries, not least the Benin, of 15th century bronze heads fame, all long before the advent of centrifugal or vacuum casters, the other common way of forcing the hot metal into the mould.

Since I lack any form of mechanical caster, I had to make sure the air in the mould could be pushed out of the way by the advancing hot metal. In cuttlebone casting (slice cuttlebone in half lengthways, scoop out your design, stickem back together and secure with wire, then pour in the silver.) you allow for this by cutting straight grooves to form air-escape channels, and I figured that would work for lost-wax, too, so attached thin wax rods to the tips of the fins. I hope this is clear!

First Attempt
For my first attempt, having invested the wax, I turned the family oven on flat out (250°C) and stuck the mould in, resting on an aluminium foil (ex-readymeal-moussaka) dish. And, in the words of Isiah 6:4 'the house was filled with smoke!' Stinking, acrid wax fumes, setting off the blasted smoke detectors and forcing us to open the kitchen doors to the outside world, despite the arctic temperatures out there!

Once the mould was ready, I melted the silver, then shot upstairs (silversmithing takes place in the cellar) and brought the mould down. Of course, the silver had chilled and I had to re-melt it, which took ages because it was now in a single lump, so had a much reduced surface-to-volume ratio. And of course, that made me rush, so I poured before it was really fully liquid, and a solid lump blocked the entrance almost immediately.

I was encouraged despite the disaster, to see that the initial few drops of silver had made their way right to the tail fin, so the mould was hot enough, the air was able to escape and in theory it should all work. In theory.

Second Attempt
This is an early wax model I made. In truth, I made a wax, cast silicone rubber around that because I knew I'd need several waxes before I succeeded, and now I use green wax which is harder and less easy to damage when you handle it. You can also get blue, carving wax, which is much harder, but which melts to a rather glutinous liquid that doesn't fill moulds well.

Searching the web for further information I came across some YouTube videos (this is video 5 of 9) demonstrating the process. I'd not thought of steaming the wax out of the mould, but it works perfectly well and doesn't fill the house with smoke!

Once the wax had gone, I stuck the mould in the oven to get it as hot as possible. The real wax burnout oven heats the mould to 900°C, then cools it to 600°C ready for casting. I don't know why it has to go so high, but our Neff will only go to 250°C on conventional and 200°C on fan, so it got 200°C fan. No more smoke, hoorah!

This time, once the mould was ready and I'd melted the silver, I asked Jenny to keep the silver hot while I brought the mould downstairs. That was a good move.

Then I blew it.

Somehow I couldn't seem to keep the silver completely molten, I don't know why. It looked OK, but as soon as I poured it, I knew it wasn't right. Quite a bit of metal went into the mould, but then it chilled much too quickly, and as you can see, it didn't really fill the mould properly. The big, knobbly lump on its head is silver I kept pouring into the opening after it had frozen right at the entrance. Usually it forms a small, neat puddle. Clue there! The tips of the fins are not sharp and it should have flowed into the air-escape channels if it was all working according to plan.

The body right behind the hammerhead (lost in a mass of silver where I poured the metal in) is much narrower than the original wax, and I think we just didn't get quite enough metal into the mould before the funnel section froze, so as gravity pulled it all downwards, that part of the body just got stretched a bit.
I'll keep you posted on the third attempt.

Oh, and don't be mislead by the colour of the metal casting; that's just the tungsten light I used to illuminate it. It really is just sterling silver.

So there's more work to do, but I think it will work.


MIke said...

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castech said...

Lost Wax Casting which is widely known as Lost Wax Investment Casting is one of the most effective casting process used in Metal casting.

Anonymous said...

Hey I used to work in a foundry and we used sand molds for our castings. The model for the mold was, of course, made in halves, top and bottom, and each half aligned with the other on opposite sides of a piece of plywood. The plywood was positioned between a pair of forms which would be filled under pressure with the sand, dampened with a binder. then the forms were lifted apart and the model removed and the forms were put back together. The model always contained the model of the part to be made and a sprue and on the opposite end of the sprue was an air hole. I would heat the ladle which in this case held a huge amount of iron compared to your little thimble sized cup but still the same principle. The ladle was filled and I would proceed to the pour deck. I would hook the ladle into the mold and pour into the sprue until the iron showed at the air hole. That meant that the whole mold was full and the air got out. Now considering we kept the iron well above the melting temp and the ladle lined with a sillicate about 4 inches thick was heated red hot and glowing almost orange yellow we had no trouble keeping the iron hot to pour. I suggest heating the mold if possible and raising the pour temp of the metal. Adding an air hole can't hurt can it?