Lath and Plaster was created by filling strips of wooden lathe with old style lime plaster that included horsehair and acted as a bind for the material.
Over time parts of the old plaster can dry out completely and crumble. This normally occurs when several layers of wallpaper or woodchip have been removed as they have a tendency to hold up these loose parts of the wall.
Depending on the size of the damaged area, this can be filled with a bonding material back onto the original lathes, or if a larger area, plaster-boarded and then skimmed over to the required level ready for decorating.
Removing old crumbling plaster can cause a lot of dust which requires suitable protective clothing and a respirator to be worn and to ensure the room is sealed to prevent dust spreading throughout the property.