Mending Breaks in Plaster.
Mix together half a pint of powdered lime, one gill of plaster of Paris, and cold water enough to make a thick paste. Fill the holes with this and smooth the surface with a knife. Work quickly. If there be many breaks mix only as much plaster as can be used in ten minutes, as it hardens quickly.
Another method is to fill the breaks with putty. When the plaster or putty is dry, the places can be touched with water colors to correspond with the rest of the wall.