Turning update
After turning down the surface of the casting several tenths of an inch, we’re finding even more porosity, including some large, and relatively speaking, deep holes. While this is disappointing, discovering such problems was the reason for this initial pour.
We currently are trying to determine whether the gas causing the porosity was in the bronze or the mold. If the latter, then the solution will be fairly simple: burning out/drying the mold more thoroughly. If the problem lies with gas in the molten metal, then dealing with it is more complex.
It is my understanding that the main culprit is hydrogen actually dissolved in the bronze. As the bronze cools, hydrogen bubbles can form, leading to porosity. Vibrating the molten metal will not solve this problem, but there were eighteenth-century means of controlling it.
“Poling,” mixing of the molten bronze with fresh wood sticks, is critical, both in terms of its timing and duration. Preventing the absorption of water vapor and fluxing are also important. We need to look carefully at all three (and quite possibly something else we have yet to identify).
The first process under scrutiny probably will be the poling. We did this to some extent, but because we were concerned that the bronze was taking longer to melt than anticipated, we were hesitant to open the furnace door and allow the temperature to drop. This limited the amount and timing of the stirring we did. We now know that we can be more flexible, and we’re hoping this will help.
Another possibility is that the mold was not as porous as required. This would prevent gases from escaping. When we make the mold for our next pour (another coehorn), we may experiment with including more fiber in the loam.
For more information, see The Art of Gunfounding, edited by Carel de Beer (Jean Boudriot Publications, Rotherfield, England: 1991). It is one of the principal sources of information we are using for the casting processes, and the source of the explanation given above.
See images of the coehorn's surface in project multimedia.
Except for its mouth, the mold is completely buried in an earthen mound on the pour-side of the furnace. It is pretty effectively insulated. We have discussed, however, the possibility of placing charcoal on top of the bronze once the pour is complete. It would ignite, slow the cooling a bit, and also provide a reducing atmosphere.
We are talking with some modern founders who are assisting us about cooling rates and grain size. It appears that, during the period, they were trying to cool the castings as quickly as possible, and therefore trying to achieve a smaller grain. Like most aspects of this project, it is turning out to be a fine balance!
I can't really tell from the pictures, but how much insulation did you surround the mold with? The slower the Bronze cools the larger the grain size - the stronger the final product. Since the Bronze travels down the trough in the open air, it may be cooling to quickly.
If I remember my differential equations correctly; Newtons law of cooling showed that a hot substance cooled quickly to ambient temperature exponentially. This all depending greatly on the constant of proportionality (k) is tied to the volumes ability to give off heat. [T(0) = (T(0) - T(a))e^{kt}]
I would think surrounding the volume with dirt covering coals might give you a slower cool down and allow the gases to rise while the metal is solidifying.
Wow this is really impressive craftsmanship. Trial and error is just part of the process I guess, but it must be amazing to have such talent.
I know how disappointing it is to find major flaws in one's casting but it might be worth the trouble to go ahead and bore the piece even though it will eventually be melted. At least then you will see what the inside looks like and know whether there are even more issues to be resolved.
The one mortar we’ve cast was done with a top-fill mold and a rather substantial deadhead. That’s the way they were done in the period, and we want to stick to that technique. As we’ve said in other postings, we’re looking at a number of areas where we can make adjustments in our techniques—and still be using the period technology.
I have poured several half scale bronze mountain howitzers in both sand and ceramic moulds using a bottom fill, tiered gating system that gets hot metal to the top several times during the pour but still fills from below. I also cast one half scale Coehorn using a top fill technique and ran into porosity problems with that one. Do you know that the Coehorns cast at Williamsburg were done with a top fill mould?
Just my two cents worth. I worked in and ran Navy foundries for 20 years, so I have a little experience.
From what i see you have two possible problems, one is slag or dross, second and the more likely of the two is the metal wasn't degassed enough. I believe you noted that in the blog, I don't know how they would have degassed in the 18th century but we would have used phosphorous copper to de-gas.
Some bronzes are more gassy than others, and all bronzes are very drossy.
You probably already did this but, if you didn't divert some of the first flow your porosity could just be slag/dross.
I have a number of old foundry books and I will have a look and see what I can come up with.
I apologize if I am stepping on toes with this comment, but I am very interested in what you are doing.
Dennis Baecht
I know nothing of pouring bronze. I do make musket balls from lead and have tried to do so in the cold once before. The molten lead is fine in the pot, but once it hits the cold air it starts to solidify quite quickly and in a non uniform manner. I noticed the date of the pour was in November. Also, I noted the the long distance the bronze had to flow in the open air into the mold. Pouring lead into a cavity mold is tough and it takes several tries once the lead and the cavity mold is heated up. Although not a perfect analogy, it's just an observation from my lead pouring experience.
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