The surface of those guns look like they have been sand blasted with very coarse sand.
This pair and perhaps another cannon (now missing: described only as 24 pounder howitzer dated 1871) were monuments in a cemetery in Brooklyn, NY, nearby, until given to SUNY sometime in the late 20th C. before 1993. The exposure to the elements for over a century would certainly have been enough to corrode the bronze to the extent seen in the photos. It would of course have left a greenish patina and corrosion products which have certainly been removed, however I doubt if Bernie K. would have sandblasted the bronze. Walnut hulls I could envision, but not sand.
In the closeups you can see what I think are mold and/or file marks from finishing process, and I've seen those on other Dahlgren boat howitzers. This shows up as longish cut lines, not the close-packed, continuous stippling you'd see if sandblasting had been used. Does that make sense?
I'd suspect that either walnut hulls or a chemical bath may have been used to remove the old patina and corrosion products. Museums of course can do whatever they want, but collectors largely prefer to leave as much of the old patina on a bronze gun as possible. Removing it all can decreases the value of the item.