March 9, 2010
Looking ahead to the light three
No, the cannon project hasn’t become a flash in the pan. We’ve just been on hold to get some other things done while we wait out this winter.
We have cleaned up the exterior surface of the second coehorn casting, and it is much improved over the first one. The porosity and shrinkage problems are much less, diminishing toward the lower part of the casting. So far, it does not appear they are serious enough to prevent the casting from being machined into a workable barrel.
The larger problem this time, somewhat to our disgust, is inclusion of what appears to be refractory mortar near the outside of the casting, especially around the moldings. This probably happened with the first casting as well, but there were so many inclusions that we didn’t focus on it as one of our primary concerns. We believe the mortar material was knocked loose when we rammed the crook into the tap hole.
We’re now looking at the construction of the tap hole, and more promisingly, a gentler and more controlled way to knock the plug loose. Despite these problems, we have decided to go for broke and proceed with casting the light three when weather permits this coming spring.
The toolmaker is about to return to boring the coehorn so that we have full information on its porosity. The founders are making the strickle for the light-three so that they can begin to make the pattern and mold when the weather allows them to set up outside again. The masons are checking the furnace after our recent snows to make sure it is ready to go. When we have set a pour date, we will post it.
In the meantime, the wheelwrights and blacksmiths are moving ahead on the limber construction, with the hope that it will be done within four or five months. We are tackling the limber before the cannon carriage because we need the exact dimensions of the finished barrel trunnions in order to forge the carriage ironwork which receives and secures the barrel. The carriage is best tackled only after we have a barrel in hand.
- Bubbles and inclusions plagued the first pour.
- Porosity was widespread in the first coehorn.
- The second pour has fewer inclusions and less porosity.
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Todd says:
Thank you for the updated pics! This is very interesting. Question: How did they make hollow cannon balls in those days??
Colonial Williamsburg says:
The mold has a “core” made of sand in the shape of what becomes the hollow inside of the shell. The core is supported by an extension that passes through what becomes the fuse hole when the shell is poured.
The metal runs in the void between the core and the inside of the mold. After it cools, the mold is broken apart to reveal the outside surface of the shell, and the sand is removed from the inside, leaving a hollow iron ball with a round hole for the fuse.
Todd says:
Many thanks! Very interesting.
Benjamin Sunderlin says:
Was looking at the pictures of the coehorn, and was thinking if the porosity still remains a problem, you might try to whisk in the end of a willow branch right outside the tap hole before the metal enters the mold, or maybe just dip it into the flow. The natural Salicylic acid in the willow should act as a degassing agent and produce a cleaner cast.