Modern methods of making plaster have come out of years of learning, and modern plaster survives better than its predecessor. Plaster in the 20th century was made in many different ways; older plasters used many different additives and combined horse hair in the mix for added strength. For the plaster in older homes that is now cracking, it is hard to tell what the actual fault is because there are many different possible causes.
A poorly proportioned mixture that uses cheaper materials, such as sand, weakens the plaster, which will eventually crack. Plaster that is too thick or has the wrong retarding agent added will become powdery when dry and crack. If a plasterer skimps on materials, the job may look good for years but it will eventually show its flaws. Applying the finishing coat of plaster must be done at the right time in the drying period for proper adhesion to the undercoat.
Over the years, homes develop areas that are weaker and bend from the weight and stress of the upper floors and roof structures. Old plaster on walls and ceilings can crack from that stress. After 10 or 20 years, plaster naturally will start to show a few wrinkles and cracks that are due simply to ageing -- just as our skin wrinkles and cracks at its stress points surrounding the joints, so will your plaster wrinkle and crack at stress points.
Environmental and Structural Stress
Moisture changes in the home and wind storms shaking the walls over the years can cause a change in the shape of the walls, putting stress on the plaster. A shifting foundation causes the house to move, which can crack plaster. Rusted nails loosening the structure of the lath -- the thin wooden lattice the plaster is spread over -- will bend the plaster. Add to all those factors vibrations from planes and trucks, even loud music, and plaster walls will crack.
When plaster repeatedly gets damp and then dries, its bonds will weaken. That tends to happen most in bathrooms, kitchens, laundry areas, and basements. Excess humidity in the home over the years is more damaging to plaster than a one-time leak. Another concern is using the wrong types of paint on the walls. Some paints dry to a hard finish so the wall can't breathe; the plaster can sweat behind the paint and retain moisture.