Now let it age naturally and gather a nice patina. BoomLover
I'm with you, BoomLover; that's what they should have done: Polished the barrel to a uniform finish, and then let oxygen and time form a natural patina (the two Dahlgren's had been kept indoors, and after this conservation work they were being moved to The Army Heritage & Education Center) on the surface of the gun, but the Army unfortunately had other wishes.
One thing that happens when something gets clear coated is that the clear coat eventually starts to come off in spots, allowing the patina to start only in those locations. It eventually becomes a total mess unless it is frequently stripped, re-polished and re-coated.
Rick is of course right in his observations about some clear coats applied to bronze and brass. The Army Brass (pun intended) probably requested that both barrels be brought to a high polish, and then clear coated with a finish that would make the barrels be maintenance free for a while. In this respect the conservation co. kind of pulled a fast one on the Army, because one of the golden rules professional conservators abide by is never doing something to an artifact that won't be relatively easy to reverse, so even though there are now tough epoxy clear finishes on the market you'll notice that the article states that an "appropriate lacquer" was used on both Dahlgrens. If the guns were put on display at The Army Heritage & Education Center in a manner where the public (especially children) cant put their hands on them, then the clear lacquer finish might last for a decent length of time before dusting starts to wear off the lacquer, and the gun's once glossy finishes start to look like heck.
The Prussian 1745 cannon looks to be a well made piece of bronze artillery, but the one thing that I couldn't figure out is why there were no heads on the dolphins. They have such a wealth of details, like the coiled tails, the indentations along the spines, the wings, and the leg and paw on the side towards the center of the gun, that it didn't make sense that they were headless. After enlarging the photos as much as possible I could see (I think) that there were heads after all. I think that the ouside of the handle (2nd photo) shows the neck of the creature coming down along the side then curving forward showing a serpent's head with an eye and mouth (maybe).