Take the picture from the frame, and lay it on the table face upward. Prepare a bowl of tepid water and go carefully over the entire surface of the picture using a soft sponge.
Then take a large white potato, peel it, and cut in half, and after wiping it off, go over the picture with the smoothly cut part of the potato, while it is still moist from the cutting.
Do not rub hard, and the dirt will soon begin to soften and make a suds, quite black, continue to rub it until all the stains and spots disappear, and then carefully wash with the tepid water and the sponge.
Never use soap on an oil-painting. It may clean it, but the chemicals in soap do damage. Later, wipe the fly-specks and soot off, with tepid water and pearline, using a sponge. After it is cleaned and dried, varnish with picture varnish applied with a bristle-brush. Spread quickly, and don't go over the same place twice.