Casting

Now after all that work, it's time for the pour. There will be much more work to follow as we assemble the parts, finish the metal and patina. Many people think the pour is the big thing in making a sculpture. Granted it is exciting and for me, nerve racking. All that work can be shot in about 5 seconds as molten bronze makes it's way through the mold. Now maybe you can see it is the apex, but really only a very small and short part of the process.

It's pour day. First we plan how much metal we need from the weight of the wax forms plus gating. It's about 9 times the wax weight, bronze to wax. I use 10 as a fudge factor. It's better to have too much bronze than too little, we can always pour ingot and reuse it. Whoops, short pour, not at all good form! Next we plan which molds will be poured and in what order. We want to have enough metal for all the molds in the pour, and since we are going to do two heats, we want to leave some metal, called a "heel", in the pot so the next batch will melt faster. The pot we are using has a capacity of 300 pounds. It is best to melt a full pot to minimize gas inclusion. We also vacuum out the furnace, crucible and surrounding area so there won't be any dirt in our metal. Foundry work is really pretty clean if you want good results.

A pour is really a dance between the participants. And as in any dance, someone has to lead. If the order is broken, someone stumbles and falls. It's too dangerous for matters of opinion to erupt at pour time. The guy I pour with, Charlie, and I have done this same routine for 7 years. We each know what has to be done and when. We don't talk, we just do. We have never had an accident and we don't plan on having one. Let's rock and roll.

There are some specific pieces of equipment used in pouring and I want to review them here. First is the furnace. Here we have it up to heat and the bronze is melting. Sitting inside the furnace is the crucible. It is made of silicon carbide crucible. This is the vessel where the bronze is melted. We use Everdur, a silicon bronze. It is about 97% copper and 3 % silicon. It is very corrosion resistant and has good pouring characteristics. This is the alloy of choice for most art foundries. Sitting on the furnace is some gating we are re-melting. It's heated first to dry it out so it won't cause rapid boiling when added to the melt. With molten metal, any water vapor is dangerous.

This is called a tong. It is hooked to the crane and is used to lift the crucible from the furnace. The bottom part fits around the crucible, the top part are a scissors. As the thong is lifted by the chain, the scissors close, causing the bottom to close around the crucible and grip it tightly.

This is called the pouring shank. The crucible is deposited into the shank by the thongs. The tongs are removed and the crucible is locked into the shank. One man is on the opposite end to control direction, called the shank man. The pour man is on the side with the double handle. The top of the melt is skimmed of junk ("dross") and away we go to the molds. From removing the crucible to pouring takes about a minute. It's all over in two or three minutes.

About 30 minutes before the metal is ready we heat up the burnout furnace and preheat our molds to about 1400F. This will assure us there is no moisture in the molds and help metal flow and cooling rates. Water is very bad as it will create steam and literally blow the metal out of the mold and maybe on to you.

The first pour went well. The crucible is back in the furnace heating up and here are the molds, still glowing red. For good luck, we spit on the cup metal ("button" in the business). I think this is an old custom, but who knows. I learned it many years ago and in this line of work not much changes. At one pour, we had a Shinto priest to bless us at the artist's request. The pour went well. Any help we can get. Metal casting is rife with superstition and ritual going back to the ancient Chinese.

On the second pour, we had this mold filled and were pouring the next mold. Pop; it cracked and molten bronze came flowing out. This is about the worst thing that can happen. Even the most carefully planned and executed pour can go awry. Now you get to see how we will go about recasting this section. Good journalism, bad for me. To find out how we recast a section click HERE.

As the metal cools, it contracts and the shell starts to crack. By beating on the button with a hammer even more shell comes off. Remaining shell is taken off with a bronze hammer to avoid scratching the cast, an air hammer for the core and then the casting is bead blasted to remove any remaining shell.

Here is how they come out of the mold after breakout. A little blasting and we'll remove the gating. We use a plasma cutter to remove gating. Once the gating is removed, it's on to chasing. We'll cover that in the next section. In addition I'll show how we are going to make a new wax to recast the blown out mold.