Wednesday, May 28, 2014

lost wax casting

I recently took a class in lost wax casting. The basic technique has been around for thousands of years (!) but I expect it is a bit different now compared to then. You make pieces in wax that are to be cast afterwards. You build a tree of the wax pieces (this is called sprueing) and fasten it on the lid of a metal cylinder. When that is done, you fill the cylinder with some kind of plaster and let it harden. You melt away the wax in a kiln, and then fill the empty space with molten metal.

Note that there are different methods and varying equipment that can be used for casting. This is just one way, and not a comprehensive guide!

Working with wax
Wax work has so many possibilities that it is a bit overwhelming. There are different types of wax, some are stiff and hard, some of soft and easy to shape with your fingers, but attract dust and other particles because of their stickiness. You can melt, carve, saw and file wax. You can melt it and pour it into silicone molds. In the picture below there are some blue wax pieces, for instance the flowers and the small cedar branches, that have been made with wax that is molten and then poured into a mold. When set, this wax is very bendy and plasticky. The smooth looking blue rings are the blue type of wax  used for making rings. They are sold as tubes; I sawed a number of rings from the tube and then carefully melted them with a torch. If you heat them too long, everything will melt into a puddle, so take a break sometimes and let everything cool down.

Wax shapes: the red in the back is soft, sticky wax, the blue is harder.
Notice the sticks that make all the wax shapes look like weird lollipops. This is a different type of wax that is used to build the tree (aka sprueing). Below you can see the start of a tree and the finished tree on the lid of the cylinder. Building a tree is tricky. I had many pieces, and two trees to build, so I wanted my trees to be as tight as possible. If you look at the very first picture again, you see I have some of my smallest pieces on their own little tree, which can then be set on the top of the big tree. You need about 1 mm of space between pieces, so it can be built pretty tightly. But if pieces fall off during handling, it can be difficult to repair a tight tree.

Note that I cut down all the pink wax sticks as far as possible so save both space and material. Every sprue you add is a piece of silver, so it adds to the weight and cost. Cutting down sticks of wax is always best done with a heated scalpel. Keep a small candle on your work surface to heat the knife, and paper to dry off the wax that accumulates on the blade.
This is precision work. It's helpful to hold
the tree in tweezers and fix it as well as possible while working
Before fastening the tree on the lid of the cylinder, you have to cut it to its final length (again with a heated scalpel). Then lift it carefully and weigh it to determine how much silver you need for casting. A rule of thumb is the weight of silver = 10x the weight of wax.

Once you have weighed your tree you can fasten it to the lid, again by melting the areas that are going to be in contact, both on the wax that is on the lid of the cylinder and on the bottom of the tree. Hold the tree steady while it hardens, and then it is done. If you're experienced, you might want to put the tree at a slight angle if you have bulky items that come a little too close to the wall of the cylinder. This way you can cheat a little and gain a little extra space.

A finished tree. Note that the trunk is
very short now. You only have as much
room as your cylinder is high (a little less actually)

Finished tree seen from above
Tree inside the cylinder
After fastening the tree, the cylinder is put in place and filled with plaster. We used something gypsum-based, but it was especially meant for casting - meaning it can withstand extreme heat without breaking. The plaster had to dry for about 15 minutes.

Removal of wax and preparation for casting
The wax is removed (burnt out) from the cylinder in a kiln at about 600C for 3-6 hours. My teacher had the cylinders standing at 600C just before casting, so they are hot and the heat will help the silver stay molten.

Sterling silver scrap is cut into approximately equal parts, to help with melting. If a piece is not molten it  will stay in the crucible, so it's not the end of the world, but you don't want that to happen to too many pieces.

The crucible is filled with the silver scrap and the cylinder is put in place in the centrifugal casting machine. And then the hot phase starts!
A big-ass torch, so loud that we needed ear protection
Heating the crucible; the cylinder is directly behind it

Getting hot. The silver is probed from time to time to check if
everything has melted.
Almost ready to let the centrifuge force the metal into the mold.
When the silver is completely liquid, the security pin holding the rotational device of the centrifugal caster is dropped down and it starts to spin. At that moment the flame is removed.

After casting
The rotation stops and now we can remove the cylinder with big tweezers. It is placed on a heat-tolerant surface for about 10 minutes to let it cool off slightly. At this moment the silver is still hot and orange in colour. After 10 minutes the cylinder is lifted from the surface, again with tweezers and gloves (still hot!) and quenched in a big bucket of water. The plaster will crumble and dissolve in the water, and your silver tree can be found in a now almost empty cylinder.
The hot cylinders cooling off before a water bath
The silver tree with all your items on it can now be inspected. Did you melt enough silver? In my case I had one tree where we had slightly too much silver, and the tree had a big foot of silver, and the other tree had slightly too little silver, and some of my pieces were not attached to the tree. With too little silver, there is a risk that not all cavities get filled, and the pieces lowest on the tree might end up being incomplete.

The plaster sits in all the nooks and crannies of your tree. You can try to remove it as well as possible with tweezers, brushes and water, before submerging the whole tree in pickle. As you can see in the pictures, there is a lot of oxidation that needs to be removed.
The silver tree right after casting and quenching. It's black! 
The oxidised tree, some loose pieces (too little silver, so they are not
attached to the tree, and some pickled pieces in the background)
This is how it looks after pickling:
You can see that I have started finishing the pieces in the background. First you saw off all the pieces from the tree. After that I had to do quite a bit of finishing work, like sanding, filing and polishing. As soon as I have finished some pieces entirely I will show the result.