Two Letters Re: The Home Foundry, Metal Casting

Mr. Rawles,

First thanks for a great blog. I’ve been a regular reader for a year or so now.

I’ve read with interest the entries by Dan in Oklahoma on the home foundry. I learned the foundry trade from my father by helping him in a small bronze foundry. I’ve learned the pattern making trade from my father and by jumping in with both feet and making patterns. I’ve since moved on to my own business casting aluminum parts for the WW2 aircraft restoration industry and hope to add magnesium castings to my capabilities.

I have one bone to pick with Dan in Oklahoma about terminology: A forge is what a blacksmith uses to HEAT metal. A furnace is what a foundry uses to MELT metal.

I am in west central Ohio and use the Keener Sand Company in Columbus, Ohio for any sand supplies I need. They can supply the western and southern bentonite (the clays) and the olivine (the sand). One should be aware that there are different grades of fineness of olivine. Some foundries use a mix of grades depending on what type of metal is being cast and the specifics of the part to be cast. Silica sand can also be used, again in a mix of grades, but it has health disadvantages and the proper protective equipment must be used. Olivine has very little free silica and does not cause silicosis, so it is the better choice.

The sand is prepared by “mulling” in a muller. This is a machine in which measured amounts of sand and clay are poured along with measured amounts of water. The mulling action bonds the clay to the grains of sand. This is what gives the sand its “green strength”, its ability to take and hold a shape. Molten aluminum is hygroscopic, meaning it will absorb moisture from the sand and from the atmosphere. In my experience, the less damp your sand is the less surface porosity your casting will have. This is where the “art” comes into play (and lots of experimentation).

In casting metals with higher melting temps, the bentonites will eventually burn out and the sand will have to be mulled again. We poured bronze at around 2,350 degrees F and the sand turns black. I pour aluminum anywhere from 1,150 to 1,350 degrees F and have yet to re-mull the sand.

As for melting aluminum, melt quickly and pour quickly. I’ve read that the crucible will lose one degree of temperature for every second of time once it is removed from the furnace. Once the furnace is shut down, and the crucible removed, don’t dawdle. Pour the molds as fast as the mold will take the metal. Remember the metal is cooling and cools even faster when it comes into contact with the cold sand. Once the pour is started don’t stop until the mold is full. Stopping the pour and then starting again will produce a defective casting every time. Get the pouring lip of the crucible as close to the mouth of the sprue as possible. This reduces the amount of aspirated air into the mold, and in aluminum, helps reduce porosity. For aircraft parts, I use only virgin metal of known analysis. Scrap aluminum can be used in general run work but make sure it is clean. Oil impregnated aluminum is really of no use to a foundryman.

As to safety, I can’t stress safety enough. Think through the steps for what you want to do and have the tools needed to do the job at hand. Do not put your foundry on a concrete floor. The molding floor (where the molds are made) may be concrete but the “pour floor” (where the molds are filled with molten metal) should be sand or pea gravel. The reason for this is when molten metal comes into contact with the moisture in concrete you get an instantaneous steam explosion. Concrete shrapnel is no fun. Gloves, face shields, and steel toed boots are a must. Lastly give molten metal the respect it is due. It will burn through leather and skin.

Read and study everything you can get your hands on relating to the foundry and pattern making. One of the best places to get books on foundry work and pattern making is Lindsay Books. The series by Steve Chastain and Lindsay’s reprint of the 1944 US Navy foundry manual are “must have” books.

Foundry work can be very rewarding. I get immense satisfaction knowing that I make my living in what is essentially a dying art and in my small way am helping to keep the trade alive. If anyone would like to contact me with respect to the foundry, they can do so via e-mail on my web site. – Barry A.


Mr. Editor:
Some feedback on the recent article series:

Don’t use concrete for the forge body. It can spall unexpectedly, fall in the melted aluminum, and decorate the personnel in various shades of pain.

Re-bar is made from an alloy that can red-short; in other words, if it gets up to dull red, it can suddenly fail catastrophically. There is a heavily spalled concrete floor up at a nearby university where somebody decided to cast 100 lbs. of copper using crucible handling gear made from re-bar. It failed catastrophically. Only one of the participants got a foot caught as the giant slosh of copper froze, and he lost the foot.

You can get enough heat out of a simple campfire to do this if you first bury several sections of pipe or conduit in a radial pattern , with the outer end sticking up to about knee high or so, and the inner ends converging in a simple divot under the middle of the fire. Then you have a bunch of friends sit down around the fire, and blow into their tube sequentially, around the circle. This will work for smelting Aluminum, or even for smelting Copper from ore, like Malachite. You don’t really need a foundry really until you start working with larger quantities, and [the higher temperatures required for] bronze or iron. This is how they did it in Africa right up until the Europeans came, and severed the oral histories and tribal craft knowledge. Constant air draft, no bulky equipment, no electricity or concrete (or re-bar!) needed. – D.J.