28 June 2010

Wooden Boat Show - Bronze Casting

This Sunday, Helena and I drove up to Mystic, CT for the annual Wooden Boat Show. One day really isn't enough for this great show. Next year, we plan to sail the Blue Moon down the Sound to Mystic so we can anchor off the show (as several other boats did this year) and spend 2 or 3 days there.

But working guys and gals have to be happy for what they can get, so we made the most of the time there.

One thing I wanted to see was Sam Johnson's demonstration of bronze casting. This is something I have been interested in for a while, so I was super keen on seeing this, and Sam did not disappoint.

Sam Johnson and his various tools of fire
photo jalmberg

Sam uses a fairly simple furnace for melting bronze. The burner itself is a propane jet, with an electric fan standing in for the traditional bellows.

The furnace is a container filled with a special kind of high-temperature concrete. There's a chamber inside the furnace that holds a special crucible in which the actual bronze is melted. You can see the open furnace in the foreground above left, with the gray crucible to the right.

Here's what the furnace looks like in action. Note the glow coming out of the small hole in the top.

The simple furnace in action
photo jalmberg

There are chunks of bronze drying out on top of the furnace. They are so hot, the edges of the bronze chunks were glowing red.

Most of the demonstration was devoted to making a mold for the casting. From what I can tell, melting the bronze is fairly simple, as long as you don't make a mistake that causes an explosion that showers everyone within 100 yards with molten metal. The difficult part is making the molds.

Sam used an oiled sand mold for the demonstration. The mold is made in a two part box, that looks like this:

A two-part, keyed mold box
photo jalmberg

Note that the two halves of the box are keyed, so they can only fit together one way. The two small bits of wood are used to clamp the two halves together.

The mold making starts by laying the bottom half of the mold face down on the bench, with the object to be copied laid in the box. For the demonstration, Sam was copying a bronze oar lock, which is already in the box, below. Note that the oar lock is laying directly on the table. The box does not have any top or bottom. It's just a frame.

'Dusting' the object to be copied, to prevent the sand from sticking to it
photo jalmberg

The object that's being copied is then dusted with a powder that prevents the oiled sand (next step) from sticking to the object. Eventually, the object will be removed from the sand, leaving a void that will be filled by molten bronze. The easier the object is to remove, the cleaner the mold.

Various powders are used in this process, depending on the location, temperature, humidity, etc. I may have heard Sam wrong, but I'm pretty sure he said he was using corn starch.

When the object is well dusted, a special kind of oiled sand is sifted onto the object until it is evenly covered. This sand has enough oil in it so that when you squeeze a handful, it sticks together. Perfect for making sand castles!

Covering the object with sifted, oiled sand.
photo jalmberg

After that, it's okay to fill the rest of the box with sand.

Filling the mold box with sand
photo jalmberg

Then the sand is firmly tamped down.

Tamping down the sand
photo jalmberg

After tamping, the sand is 'sticky' enough that the box can be lifted off the table and flipped over. Now the object to be copied must be removed from the mold. Notice the powder on the surface of the mold. This powder also keeps the mold from sticking to the table.

The bottom mold box, flipped over
photo jalmberg

The object is carefully extracted from the sand.

Digging out the object that's being copied (an oar lock)
photo jalmberg

Sam makes a kind of beveled edge around the oar lock
photo jalmberg

If the sand is dug away properly, when the box is inverted, the object will fall out cleanly, leaving a half-mold of the object.

The completed half-mold
photo jalmberg

Notice the rectangular 'channel' below the oarlock mold. This channel will be used to feed molten bronze to the mold.

The object is placed back into the half-mold, along with some other bits of wood that are strategically placed in order to create a complete channel for the bronze to flow through. Then the whole assembly is dusted again, so the two halves of the mold will not stick together.

It's actually a bit more complicated than that. The channel is constructed so that any gunk in the molten metal will get stuck in the channel, while clean metal will flow into the mold.

As always, this is the "How NOT To" blog... only an insane person would try to do bronze casting from my brief description! Creating the right type of channel to the mold is a perfect example of what I mean... There is more to it than meets the eye, I am sure!

The bottom half of the mold nearly done.
photo jalmberg

Then the top half of the mold box is added, more sand is sifted on top of the objects to be molded, and finally the box is filled with sand and tamped down.

The top half of the mold being prepared, notice the big bit of wood sticking out of the 
sand. This will be removed, creating a channel from the outside of the box, into the mold.
photo jalmberg

Then the top half of the mold box is lifted off the bottom, and the two halves of the mold are revealed.

Sam is gently tapping the oarlock, which breaks it free from the bottom mold again. The box is inverted and the oarlock falls out.

The two halves of the mold
photo jalmberg

Here's a good shot of the bottom mold, with the oar lock removed, and part of the channel clearly visible. The rest of the channel is in the top mold.

The completed bottom half of the mold, ready to go.
photo jalmberg

The mold box is then reassembled, clamped, and moved near the furnace.

The cup of coffee is smaller than it looks.
photo jalmberg

The crucible full of molten bronze is then lifted out of the furnace and metal is quickly poured into the mold. So quickly, that I didn't have time to snap a picture of it! We are talking a few seconds, here.

After the pour, flames shoot out of the channel, where the metal was poured. These flames last a few minutes. Not sure what is burning, but it is HOT in there, brother!

Flames erupting out of the channel, after the metal has been poured.
photo jalmberg

After cooling a bit, the boxes are separated and the newly forged oar lock removed from the mold.

The newly born oarlock still wreathed in smoke.
photo jalmberg

And, hey, it looks pretty darn good, even to Sam.

The solid, but still hot, oar lock
photo jalmberg

Notice that the channel has also been filled with bronze, and is still attached to the oarlock. This will be cut off and then the oarlock will need a bit of polishing to get the mold lines off. But the casting part of he process is complete.

My hat off to Sam Johnson and Wooden Boat Magazine for a fantastic demonstration. The demo answered many question that I had about the process, and I was pretty psyched about taking one of Sam's two-day courses on the subject at a later date.

Helena, who had stood back 20 yards or so after hearing about possible explosions and molten metal rain, was NOT so keen on me playing around with 2000 degree furnaces, but I'll work on her!

Disclaimer: To state the obvious, once again: This is really dangerous stuff, folks. I will jump into lots of things and just see how I do, but after seeing this demo, I would NEVER attempt this without taking a course from a real expert, like Sam.

In particular, no one should think they 'get' how to do this from reading this blog post. I take no responsibility if you do!

Here's a link to the info at the WoodenBoat School: Bronze Casting for Boat Builders

Great stuff!

Tomorrow, more from the Wooden Boat Show, including my next build???

>>Next Episode: Wooden Boat Show -- Boats!

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  1. Hey John, that's my coffee cup. It's a shame that we didn't meet, must have been elbow to elbow. A very nice record of the demonstration. My photo's are less sequential or documentary. Could you email me ? I would like to borrow one, and maybe two of your photos, things I missed, one especially of the oarlock before casting. I loved the sort of pentimento of the ...cope? drag? My email is available in the intro to 70.8%. Good job and I'm sorry we didn't meet.


  2. The flames are from the oil in the sand burning. A thin layer around the object usually chars and then is disguarded when the rest of the sand is recycled for the next casting.


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