There were widely believed to be only three copies of this exact map in existence. One of them belonged to King George III and remains in the British Library in London, where it is displayed occasionally. The other two  one legible, the other tanned and dark with shellac  are at the New-York Historical Society on the Upper West Side and remain in storage but for two or three times a year, when they are pulled out for students.

Restoring this surprise fourth map, aged beyond its 240 years by its destructive shellac coating, became an immediate priority in Brooklyn. Its transformation from literally untouchable to clearly legible and mounted behind glass, to be unveiled at a private party at the society on Wednesday night, involved science, patience and more than a little bit of kitchen-sink cunning, calling to service, at one delicate point, boiling pots of old books used to distill the color of aged paper.

Not that anyone at the Brooklyn Historical Society knew what it had. The map had been delivered from the society’s warehouse in Connecticut. The society said it had no catalog listing the map or when it had been acquired. It had been shellacked and mounted on linen, with a wooden pole attached at the bottom, presumably to bestow a more artistic air. It had probably hung on a wall somewhere for who knows how long, but in May it was in disastrous shape.

The map had been cut in long strips to allow it to be rolled up for storage. The strips were so brittle they broke when touched. It took a lot of squinting and bending, breath held in, to discover that it was a Ratzer 1770  its name perhaps an error, as it was most likely completed in 1769.

A British Army officer in America, Lieutenant Ratzer was a surveyor and draftsman, and his map was immediately praised as a step forward from those of his predecessors. For his trouble, his name was misspelled on initial versions of his maps, called the “Ratzen plan.”