Porosity
Further investigation of the “porosity” has revealed that many of the pockets have bits of material in them—some are likely loam from the mold, others could be dross. The furnace is tapped at the bottom of the melt, so I don’t think it likely we are being contaminated by the stuff floating on top of the metal.
But, we did not divert any of the run, and that could mean that we were washing out stuff from the furnace itself, picking up bits from the trough that runs from the tap hole to the mold, or that the mold itself may have spalled off bits as the metal heated and filled it.
Diverting the flow could possibly take care of the first two of these potential problems and is certainly something we’ll look into. We’re also looking into whether or not we fired the mold to a high enough temperature to dry/harden it properly and whether or not the mold-making mixtures we were using resulted in a porous enough shell to permit gases to escape.
As for degassing, we’d like to stick to the techniques used in the 18th century—but, again, we’ll need to experiment. Using a modern degasser in a test pour could help us determine whether or not that is the/a problem.
We’re experimenting with some of the mold making questions now and hope to get things ready for another test pour -- a coehorn again -- sometime over the next couple of months.
I am late to your project - but am a metallurgical PE with 35 years foundry experience - including casting my own cannons. If you need coaching, I'd be happy to help. A couple of first comments:
Poling is a lost art. The principal is to generate a stream of bubbles that then become the low free energy sites and the gas is drawn from the molten metal to the generated gas bubble and out. You need "green" pine (heavy resin content) of about 3/4" diameter (broomstick size) and you need to keep this up for about a minute (4 to five poles normally). It is very dangerous because there can be considerable splashing if your ladle does not have enough freeboard. People burn and whole foundries too!!
I can't tell from your pictures - but you do know to pour muzzle down - so crap (sand, dross, slag, refractory AND bubbles) all float to the breech end - which has an extension to catch this crud and make it go away when the barrel is turned. Muzzle down also minimizes the risk of the bore core floating, leading to a thin wall and a rupture the first time the cannon is fired (with black powder).
The old test for metal quality is to char an oak log (best if an old fall - year or more old) - dried and then charcoaled) [in the day, this would have been in a bee hive charcoal oven - developed ≈ tenth C (certainly pre Chaucer)]. Drill a cylindrical (≈2" X 2") cavity in one end. You are going to pour this without a cope - just fill the cavity (it's only a few ounces). Pour a "spoon" full of your liquid metal into it.
Let it freeze - if it is crowned at the top - you have gassey metal and should not attempt pouring it. It should sink down on the central axis - maybe as much as a 1/4" (convex surface).
Again I assume you know that liquid bronzes "shrink" - that is the volume of the liquid > (≈7%) than the solid and that you have falsified your patterns for this and allowed for extra feed metal to backfill the shrinkage porosity.
Again - if you still have problems - let me know and I can talk you through the solutions.
As a reference - get a copy of Jackson & de Beers "18th C Gunfounding" (it's a UK publication - my book review is dated 1974,
AND Georgius Agricola's De Re Metallica - written in 1556. Dover had a nice edition translated by President and Mrs. Herbert Hoover (in 1912) from the original vulgar Latin. It was the first and only metallurgical text for about 200 years! Your English gun foundries at the Arsenal would have known it well!!
My copy was published in 1950, and was a gift to me from a Swiss colleague - I see an ISBN number: 0-486-60006-8 It has a lot of other medieval and Roman methodology - but you can learn a lot from it.
William D. Scott, P.E.
Hello,
Look into the temperature of the mold and the rate of pouring to resolve the debris & porosity question.
Also try tracking down what info you can find about weapons production in northern India & Pakistan. The tribesmen are absolute masters at replicating anything that fires a bullet. While they have been copying very modern weapons for a long time there is also every probability that older skills like pouring complex large bronze pieces have not been lost.
Furthermore they produce these very lethal weapons with nearly none of the industrial tools & resources that modern manufacturers have. If anyone can pour a bronze twelve pounder it will be these guys, they take their guns seriously.
Good Luck, Bye, Scott T. Wilson, CPT,
Ordnance Corps, US Army, retired
What alloy are you using? Any idea of the actual temperature when filling the mould?
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