To restore to its original colour an old oil painting that is black with age and smoke, wash it with a sponge or soft leather and clean water, and dry with a silk cloth. If the painting is very dirty, take it out of the frame and lay over it a clean damp cloth. Allow the cloth to remain for a day or two, keeping it damp all the time. Then remove the cloth and place another clean damped one over the picture, and keep on renewing the cloths till the dirt is thoroughly soaked out of the painting, when it may be washed with a sponge and water. Then rub over the picture a little clear linseed oil, or give it a thin coat of mastic varnish applied with a clean flat brush till every part is covered, and set aside to dry where no dust will fall on it.