Re: Silicone bronze casting and chasing safety.


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Posted by bruce paul fink on November 23, 1999 at 12:55:39:

In Reply to: Re: Silicone bronze casting and chasing safety. posted by Mike Langianese on November 23, 1999 at 10:07:11:


That is a very good point and illustration of what we really are dealing with at these higher-than-being-used-to temperatures.

I very seldom have anyone human even around when working or especially pouring metal and that is both a pro and con situation. It would be nice to have an additional hand sometimes but then it also requires they be intelligently up on what is going on and is possible in every cause and effect situation. Makes it rather challengingly fun (funnily enough) and for me a much better way to get the feeling others may have to go skydiving for.

The room temperature spoon (or stir rod, or slag skimmer, or even another added ingot of the same metal) will have some latent surface and/or crevasse moisture on it and when plunged or dropped under the surface of the metal at 1100 degrees F or more will become the equivalent of all hydrogen and oxygen gases. This is the same as having just plunged a cherry bomb under the surface of the molten metal. Even my ingots are preheated on the lid or over the exhaust flame port before being added to the crucible if there is any molten metal in it.

As for the comment about plaster investment venting...the permeability is not the problem as much as the moisture escape route and the elimination of it before hand. All plaster investments need to be kiln dried or 'burned out' before pouring.

Plaster when mixed combines with the water BOTH chemically and mechanically. The mechanical amount is the greatest and will evaporate out eventually even in the hot sun after a good period but the chemical is the smaller amount that will be released when the metal comes in contact and THAT is what will blow the metal back out.

You do need to add some other elements to the plaster like sand or sand flour or luto (old investment previously used which is the combination of the sand and plaster burned out... a great way to recycle while taking advantage of the properties at the same time). Straight plaster will crack and craze in the kiln and has none of the strengths of a concrete. The sand is part of the mix in that it adds granular strength and also helps conduct the heats more evenly through it during each phase.b The plaster is being used only as a conglomerate BINDER and not as a full mix investment. This might be misleading if you buy a commercial dental investment or jewelers casting mix as the sands are very fine and in the form of a fine flour, but they are in there.

I use basically a 50 / 50 proportion of sands to plaster but there are many other formulas others prefer.

And yes, Green sand (or damp sand with a little clay or oil with moisture as a binder) without any plaster will also work for the mold but this is a case where the sand investment is so permeable that the gases can go straight away anywhere from the new hot metal surfaces being poured in.... But not if it is a damp sand sealed interior CORE surrounded by the metal as that brings up a new set of conditions and again needs interior venting escape routes.

The safety key for me has been to design a pour system that allows me to evacuate the room at a seconds notice and for everything to be able to take care of itself. I actually never am holding anything heavy that could not be dropped, and everything would automatically swing back into it's safest positions.... There is still risk, keeping alert and ahead of any imagined possible effects helps and I would say a Guardian Angel or two have probably been watching but during the last 39 years of casting have never had an accident... yet.

I told you it was fun. I wish you the best with discovering and conquering the challenges. bpfink

If interested in a top view drawing of my furnace layout with the extending / telescoping / lifting and radius swing pour capabilities see:

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