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| ironman:
vtssteam It is by weight. The coal dust burns and slightly lifts the metal of the sand to give it a smooth surface. Coal dust may be hard to find so you could use other things that burn ie. sawdust or wheat flour. |
| vtsteam:
Thank you Swarfing, I will look into that for silica sand. Ironman, would powdered charcoal work? That would be easy to get/make. I'd prefer not to use other things like flour and sawdust that can decompose. I could carbonize sawdust -- I have quite a bit of that from my homemade sawmill. |
| awemawson:
Recalling something I read in one of my early foundry books, I think the coal is added to stop 'blistering' where the sand fuses to the cast. If I remember correctly the coal dust breaks down forming a gas layer that cushions the sand from the metal. Also improves surface finish. If the above is correct powdered charcoal wouldn't work as the gases have already been driven off. |
| vtsteam:
Thanks awemawson, I think the gas involved is CO2 or CO so any of the above might work. |
| awemawson:
Google "why is coal added to foundry sand" There are many references to the coal partially burning and off gassing organic compounds - here is one on wikipedia http://en.wikipedia.org/wiki/Molding_sand My feeling is charcoal won't work as the majority of the organic stuff has been driven off in the making process. |
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