We're Learning! (Bronze Casting No. 6)

Our most recent session was the culmination of the whole process involved in casting a new object: new rubber silicon molds, wax patterns, plaster molds, wax burnout, bronze melting and finally, casting. And we have the wonderful feeling that we're learning.

We wanted to cast a new pinecone and some acorns, too, and also a small medallion with the symbol of the School on it. These were the objects for which we needed entirely new rubber molds. The medallion was a real challenge because the engraving is very low-relief - we weren't at all sure if we could capture it first in rubber, then in wax, then in plaster, and finally in bronze.

We made up our molds - medallion, pinecone, acorns, and a new salamander, too, because we had some extra material. And then we cast them using our newly made wax. We put two acorns together to improve our casting odds - maybe at least one would turn out.

(image is there, but barely visible in wax)

Next we made up the plaster molds, mixing sand and plaster as usual.

We decided to put one of the medallions beside a salamander, and just pour the plaster over top of them both. 

These two, we decided to drop onto the liquid plaster that was in fact sitting on top of this other salamander (photo below). We were keen to see which method would give us a better imprint - pouring plaster directly onto the pattern, or placing the pattern on the wet plaster.

It seemed like a good idea at the time, using the bottom/top of one mold as a space for these other wax medallions. But of course later we realized that it was quite likely the mold would crack once we poured molten bronze into one side of it. 

 

Here you can see, after the burnout, the two medallions and the other molds, which all seemed to come out well. Clockwise, salamander and medallion, two acorns, salamander (and on underside, two medallions), and pinecone.

Then, finally, melt day arrived. We started a fire in the furnace at 2:10pm and decided, this time, to put the bronze-filled crucible quite low down so it would be sitting pretty close to the floor, but be totally covered above with charcoal. In previous trials, we'd been putting it much more in the middle of the furnace, but it was getting harder to reach melting point.

Placement seems to have made a difference! Like in our earliest attempts, the bronze melted very quickly, in about an hour, and with no need for the bellows.

    

We tried cleaning out some of the dross but it wasn't easy, and then decided to just go ahead and pour. Sigrid did the honours this time.

She managed to fill two molds, and so to melt the remaining bronze, we brought out the bellows to quickly get back up to meltpoint. We also remembered to use borax to clean out the dross!

And we also, finally, added some copper-phosphorus to the melt. I'd read somewhere that adding phosphorus - even in the form of these welding rods - acted as a flux and improved pouring, and perhaps it also aided in clearing out dross. So we dropped it in, it melted right away, but I'm not sure it helped at all. We'll have to try again.

None of the three medallions came out well, although we did get details in parts. Where did it go wrong? In the final bronze pouring, or in the plaster molds? To be determined, after trying again.

The pinecone and acorns came out quite well, although they are marked with little specks of black - I'm guessing charcoal powder in the molten metal? 

We only had enough metal for one salamander, which came out missing its extremities. But we did fill an empty plaster mold of an Egyptian cat from a previous session, and it came out pretty well, just  a little defect in a front leg.

It was very satisfying to do the whole process once again, from new object to pattern and all the way through to bronze copy. The phosphorus flux is a question mark to be determined, but it looks like we've figured out the optimal placement of the crucible, and the mystery of why we couldn't melt bronze with the natural draught of the furnace alone, like we had last year, appears to be settled. If it works again next time, we'll say it's settled for sure. And just like that, we've learned something!