Re: Cast Iron Cupola


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Posted by Stuart Marshall on October 12, 1998 at 09:11:11:

In Reply to: Re: Cast Iron Cupola posted by jon presson on October 12, 1998 at 08:44:04:

I must first take issue with you as to cupolas normally producing brittle iron. This is only common when you are not used to a cupola or to ironfounding. You will in fact get brittle chilled iron with ANY furnace if the casting is too thin for the metal composition. If you use a cupola regularly and know what you are doing, you can easily produce perfectly strong machinable iron every heat.

There is no difference in the quality of iron from a well-managed cupola than any other furnace. Most of the world's production of iron, including all sorts of precision, thin stuff, still comes from cupolas. With very small cupes, machinable iron is almost effortless, for the simple reason that you are breaking up iron that is already very thin, like old wood stoves, steam radiators, etc., so that the composition was already adjusted originally for thin strong work.

Contrary to normal belief, thin stuff like old wood stoves and such are usually very high quality iron or you would not be able to pour such thin pieces.

A small bore cupe requires very small iron pieces to not block the blast, so you cannot break up heavier section iron this small if you wanted to; thus you are assured of the correct metallurgy without even worrying about it. If by chance you do get some chill in very thin castings, you add a bit of ferro-silicon, just a pinch to the laddle, to adjust the silicon content. This takes the chill right out.

Too much however, lowers the strength of the iron appreciably, so it is a matter of experience. Not something you are going to learn in a week!

Right now, I may get one or two chilled castings by accident in a years time. And it is invariably very thin unusual work where you would get this chill regardless of what type furnace you were using. Nothing to do with the cupola at all.

Now, as to the other part of your question, to get a stronger iron, this is not too hard to do. But you are going to

get frustrated if you get too high-tech or scientific about it, because there are so many variables. Even the humidity affects melting. What I do when I want a strong, tough iron is to simply throw a lot of mild steel scrap into the melt, as much as half and half. The steel of course picks up some carbon and does not remain steel, but a cupola melts so fast that it may not pick up enough to completely convert back to iron and so you get something similar in strength to mehanite or semi-steel. Don't use thin steel plate or very small nails. Use stuff like short chunks of re-bar, large bright framing nails, old steel nuts and bolts, etc. Cupolas are so fast, you have to move quickly. There is not time to worry too much about exact conditions once you start the blast. You are literally tapping and pouring the metal as fast as you can get rid of it; and you will have poured off the whole floor of moulds while your friends with ordinary crucible rigs are waiting for their first load to melt. No exaggeration! You have hot iron in 6 minutes or so, and maybe every 5 minutes thereafter as long as you keep loading at the top. Compared to 30 to 45 minutes to melt one crucible full (not to mention the wear and tear on expensive crucibles at 3000 degrees). Sorry to run on so, but I get passionate about cupolas! And I run into so many misconceptions. Many people tend to make it much more complicated that it really is, and most modern books are no help.

So much of the printed high-tech stuff is completely useless to the small operator.

Cheers, Stewart


Marshall Machine & Engineering Works Ltd.

P. O. Box 279 (Hilltop Lane, Upright Head)

Lopez Island,Washington, USA 98261


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