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We are renovating an apartment in Windsor Terrace, and are exposing a few brick walls throughout the place. We have opened the walls, and to our surprise saw that brick on external walls is black, as if painted with something (see links to pictures below). An internal wall is of regular red colored brick. We like both colors, and would like to keep the black wall exposed.
Does anyone know what was the brick coved with, and whether it is toxic or not? How can we clean the black walls to retain the color? Will the acid solvents ruin the color, or just strip it and expose the red underneath?
Any help/advice will be greatly appreciated!
Picture1

Picture2