September 24, 2009
The Second Pour
The pour on August 26 went smoothly. We followed several suggestions offered both by Colonial Williamsburg tradespeople and experienced founders from outside the Foundation. Before the pour, the mold was fired in a modern pottery kiln at the College of William and Mary (Brad, thank you!) to approximately 1500-1550 degrees Fahrenheit. This resulted in a much more “ceramic-like” material, with all the fiber and any remaining wax burnt out of it. That eliminated several variables. We eliminated two others by using a modern flux and degasser. When the furnace was tapped, the flow of bronze appeared to be less viscous than that of our first attempt, and the material itself was a bright, gold color. A bit of the initial flow was diverted from the mold cavity.
The mold filled nicely, and the observed shrinkage looked good. The next day, Thursday afternoon, we excavated the mold and broke it away to reveal a smooth-surface casting with nice detail. On Friday afternoon, we sawed off the deadhead. While the surface revealed was a vast improvement over the first casting, it still had a ring of small (pin-head size) holes just below the surface and some small shrinkage gaps. Slicing the deadhead a couple of inches above the first cut revealed more porosity.
Although we won’t know until we clean the surface of the mortar-portion of the casting, our hope is that the defects diminish toward the bottom of the casting. If so, we may have a mortar. We’ll find out over the next couple of weeks and let you know.
Once we are satisfied with these experimental castings, we will begin “reverse-engineering” the modern technologies we are using in an effort to complete the entire process using only period methods.
August 17, 2009
Try, try again
We are in the process of completing the mold for a second coehorn pour, which is scheduled for August 26, weather permitting.
As a result of our own analyses, suggestions we’ve received via the blog, and discussions with several experienced and knowledgeable folks, we think we have discovered the source of at least some of our problems and are working to correct them.
The proof will be in the pour!
Listen to the podcast “The Joy of Discovery” to hear more about the upcoming pour.
April 23, 2009
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.
February 5, 2009
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.
This project is possible through a generous gift by the Ambrose and Ida Fredrickson Foundation. Research assistance was provided by Firepower: The Royal Artillery Museum, and the National Park Service. We are grateful to the Museum Restoration Service and The Royal Artillery Historical Trust for the use of their images.
Historic Triangle
iCitizenForum
