Congrats on finding your first civil war artillery shell, Mark.
In a PM, you asked me about the best way to clean it. I'm replying publically because in the PM you said its iron body is "Graphitized" from being buried for 150 years in "sandy grey creek soil." Your photos here show you're right, that iron is definitely Graphitized. For anybody who is not already familiar with that term, Graphitization is a form of iron corrosion caused by Tannic Acid in slow-moving groundwater (swampy ground, slow-moving creeks, marshes, and "low ground"). The Tannic Acid gradually leaches iron molecules out of the iron relic's skin, and the result looks like Graphite (the same mineral as "pencil lead"). Therefore, the iron is the dull silvery-grey color of Graphite... although sometimes rusting will stain the grey to a brownish-grey, like we see on your Read shell.
VERY IMPORTANT: Graphitized iron relics must NOT be cleaned by either Electrolysis or the Zinc-&-Lye-Bath method. If those cleaning-methods are used, the Graphitized skin of the iron will dissolve -- and the result will be ugly.
Therefore, Graphitized iron relics must be cleaned "manually." I've had good success by using a coarse file and a fine-grain file. I use the coarse file to very gently file down any concretion, until I first start seeing the soft Graphitized iron under the concretion. Caution-note: the Graphitized iron is extremely soft... so you must be super-careful not to file into it. When I first start seeing the soft iron under the concretion, I switch to using the fine-grain file, and use a very light circular motion to smooth the relic's surface. Then I apply four coats of Satin-finish Polyurethane.
Your CS 3" Read shell shows some areas of brownish-grey Graphitized iron, and some areas of orange-ish concretion on top of the Graphitized iron. As described above, I'd use the coarse file to gently file down the concretion, and the fine-grain file to smooth the Graphitized iron which has no concretion on it.
Since you've (probably) never done by-hand cleaning with files on Graphitized iron, I urge you to practice doing it on some Graphitized shell fragments (or other low-value dug iron) before you try it on a precious relic like your first civil war artillery shell.
If you are coming to the huge Richmond relic-show on July 19 & 20, bring your Read shell to my table and I'll demonstrate the filing technique for you.
Here are some photos of Graphitized iron, to help y'all recognize it. Note that the Schenkl shell is dull silvery-grey, and the brass-saboted Parrott shell is brownish-grey in places. The other two photos are a Graphitized cannonball, before and after cleaning by Electrolysis... which stripped off the Graphitized surface of the iron, resulting in a look which resembles the moon's surface.