Map cataloger Carolyn Hansen was going through the warehoused archives of the Brooklyn Historical Society this May when she found a rolled-up linen canvas. When she gingerly attempted to unfurl the crispy brown shellacked roll, she heard it rip. She stopped immediately but not before she saw a name: Ratzer.

Bernard Ratzer was a British Army officer who surveyed the east coast of America during the French and Indian War and the early days of the American Revolution. His 1770 map, “Plan of the City of New York,” is famous for its sharp detail and in-depth rendering of the topography, geography, streets and notable buildings of Manhattan.

“It’s one of the ways we know about how this place looked before the grid really took hold,” said Matthew A. Knutzen, geospatial librarian in the New York Public Library’s map division.

It’s also incredibly rare. There were only three known to survive. One copy the publishers sent to King George III, now in the British Library in London, and two are in delicate condition in storage at the New York Historical Society on the Upper West Side where they’ve been since they were donated by NYHS founder John Pintard on Jan. 4, 1810.

The Brooklyn Historical Society, on the other hand, had no record of owning any Razter map, never mind the ultra-rare 1770 “Plan” one. The map Hansen found had been shellacked and mounted on linen at some point, probably to decorate some wall. The name Pierrepont, a prominent Brooklyn family, was on the back of the linen. The hard-coated map had been cut into strips so it could be rolled and stored. When Hansen and other BHS personnel oh-so-carefully managed to unroll just enough to see what they were dealing with, they found the 1770 Ratzer “Plan of the City of New York.”

Realizing they had found a previously-unknown fourth Ratzer 1770 but that it was so fragile that it might not even exist for long — three strips of shellacked paper crumbled at the touch — the Brooklyn Historical Society called in paper conservationist Jonathan P. Derow who promptly swooped in to save the day and restore the map to a condition that will shock and amaze you.

Derow made a little plastic tent over the map in the middle of the BHS office and put a humidifier inside. That softened the paper enough so that he could transport it back to his office for conservation.

He washed the map for four days in an alkaline bath that removed acid and grime, and he cut away the linen backing. He aligned the pieces, using a strong magnifying glass and tweezers, and let the map dry, only to see tiny gaps appear between strips, the result of the paper’s shrinking. He rewet it and started over, but let the pieces overlap slightly. That worked: the map shrank perfectly in place.

White lines were visible where the map had ripped, the brighter inner fabrics of the paper standing out from the stained surface. Mr. Derow visited Argosy Book Store on the Upper East Side and bought a handful of obscure old books — among them, for example, “The Select Dialogues of Lucian, to Which Is Added, a New Literal Translation in Latin, With Notes in English,” from 1804 — that were printed on cloth paper, like the map, and not wood pulp.

He performed on them a technique that should chill the blood of any author, wondering where his books will be in 200 years: he baked them in his kitchen stove and boiled them in water. He painted the resulting brackish stew onto the white lines, matching them to the rest of the map.

The astonishing result:

Now that the map is back in top shape and safe behind plexiglass, the Brooklyn Historical Society plans to put it on public display. You can zoom in for closeup views of the before and after pictures on this page.