Military History/Probably not a brass cannonball
I've read a response you posted some years back about what someone thought was brass cannon-ball, but as I can offer a little more information about the ball in my picture I'm hoping it might give you a clue to its true identity.
The ball was dug up by a farmer around 50 years ago, and as he was on the site of the Battle of Sedgemoor (in the UK, 1685) he assumed it was a cannonball. It is 57 mm / 2¼ inches diameter and weighs about 800 grams / about 28 ounces, which seems about right for a brass or bronze.
I've recently had it analysed and found that it's a form of leaded bronze (Cu 9½% Sn, 5% Pb, 3½% Zn). On the photograph you can just about see a circular area about 16 mm / 5/8 inch across which appears slightly paler than the rest, but the composition was essentially the same.
The plug is the big mystery for me - as the density is right for solid bronze, it can't be hollow or filled with lead, yet there's corrosion at the joint. We've tried ultrasonic testing in the hope of finding out whether it's solid or hollow, but the tester couldn't get a response at all, suggesting that whatever the "inside" is it has a rough surface.
I also read the discussion at http://www.thehighroad.org/archive/index.php/t-31721.html
, but as I'm not American I'm not sure whether the sizes "softball" and "quarter" are like mine (and anyway they don't tell us the outcome).
Any suggestions you might have would be much appreciated.
While later cannon balls were made of iron almost exclusively, I don't think anyone can say that other alloys were not used. In several cases people have in their possession hollow brass balls that were really just ornamental balls from fences or some other public structure or sculpture.
Then there are the cases of the steel balls from industrial ball crushing mills.
You have done what I usually recommend, taking a measure of the ball using Archimedes principle by submerging the ball in a graduated vessel of water to get its true volume displacement, then comparing the weight against similar volumes of iron or brass.
You seem to have a good grasp of the balls composition. One can do the same measurement to assess whether the weight would be comparable to a solid shot, vs a shell (one filled with powder) which would be considerably lighter.
The diameter does not correspond directly to any bore diameter used in later years, but does come close to a 3lb shot which is listed as 2.9 inches and a weight of 3.05 lbs for iron.
That means for a 2lb cannon that there would be about .5 inch or 1 centimeter of windage around the ball.
Considering the possible age, when cannon foundries followed no set unified patterns, and cannon were used till they exploded, the windage may not have been too excessive.
Napoleon was the first to force standardization of cannon on an army. The Grimbeauval system standardized calibers or poundage of shot, and bore diameter. So the 1600's probably saw a lot of artillery of odd bore and shot sizes.
Since the shot was found on a battlefield, I would say it tips the scale in favor of it being a shot despite the odd alloying of its composition.
Since shot was a disposable commodity, I would expect them to skimp on it a bit, so the surface pitting might be due to variable weathering or corrosion of elements in the original metal. The larger divots might be impact scars, since the lead component of its composition would make it less hard than iron.
The bronzing of the outside might have ben an attempt to lessen the deformation of the lead which would be prone to deformation and going out of round due to rough handling in transport.
Now having said all that, and reading a bit about the battle of Sedgemoor, it seems that the battle was fought at night, between 10pm and 1am and the rebels were routed by being outflanked by cavalry and infantry in the dark. So I do not know how useful artillery was during the battle. There was a royal artillery park and artillery was deployed, but it seems dicey to me to use it in the confusion of your own men and cavalry. However that does not eliminate the possibility that it was a dropped ball, lost in the heather by carelessness.
As to the "plug". Do not forget that even solid shot has a sprue generated by the casting process. This is cut off and then hammered to blend with the curve of the shot, this can leave what looks like a seam.
For shells, the plug was threaded later, but in earlier days a fuse set in a wooden plug was used. It was nothing more than a black powder fuse that would be lit and then loaded rapidly and fired. Later they discovered that the act of firing would ignite the fuse which made firing shells safer for the cannon crew. If it is a shell, there would be a marked difference between the weight since powder is considerably lighter than the alloy of which it is composed.
So at this early date if it was a shell, I would expect an open hole non threaded. Later shells with threaded holes the plug would have a slot to facilitate its removal with a tool and replacement with a threaded fuse. These innovations did not come along until considerably later.
Just for the record, there are three sizes of softballs, by diameter there are 11 inch, 12 and 16 inch. The sizes are used for different forms of play. The smaller for fast pitch, the medium for slow pitch (the ball is thrown in a high arc of no less than four feet from the horizontal from pitcher to plate, I used to pitch this form when I got too old to play baseball). The larger is truly a soft ball and is played without gloves. The medium ball is around 4 inches in diameter.
With regard to the ultrasound, I am a geoscientist and ultrasound is just another form of seismic but uses shorter wavelengths of sound to image the internal structure of things. We used sound to image the earth. What you need to have a return at all is an interface between two materials. You have seen a clear glass of water, and stuck a spoon or straw in it. and it looks as if the object is bent. That is refraction. Same principle applies to sound. If there were different materials inside the ball, you would see them, as the density changes would show up as faster or slower reflection/refraction times. So if there was no response, I would take that to mean the object is solid and homogeneous, with no voids. Seismic shows us a multitude of layers of rock that make up the ground beneath our feet, and the layers be they horizontal, or the near vertical layers you see in Wales and Scotland, show up clearly.
Hope this answers your questions.