Modern drywall didn't come into wide usage until the 1940s. Prior to that, plaster was the construction material of choice for walls and ceilings. Since most plaster is in older homes, time and moisture eventually lead to damaged plaster. Repairing it can be a bit more of a challenge that repairing drywall, but with the right techniques, it isn't too hard.
History
Plaster walls have been around for centuries, from the adobe constructions across South and Central America to lime plaster used in medieval times. The oldest use of true plaster has been dated back almost 10,000 years ago in the Middle East, when a soft rock known as gypsum was powdered and mixed with water to form bricks. These were used on some parts of the interior construction of the Great Pyramids in Egypt. The more modern system known as "lathe and plaster" has been around for several hundred years. Since the 1940s, the overwhelming majority of walls and ceilings are done with drywall, joint compound and tape. If your home is of particular historical significance, it might be worthwhile to find someone who can do a true plaster repair.
Function
Thin strips of wood called lathe were nailed in rows to the wall studs and ceiling joists. Over this was spread a scratch coat of mortar, often with horse hair or chicken feathers mixed in to give it strength and flexibility. Over this goes a "brown coat" of sand and plaster, which dries very hard. Once this was dry, the plasterer spread a thin layer of lime plaster. This wall system was much more durable, better insulating and more soundproof than modern gypsum drywall walls, but it was a lengthy and expensive process done by true craftsmen. Once finished, the walls had to dry for at least a month before painting. Obviously, this would not be practical in today's faster paced building process.
Considerations
The number-one enemy of plaster walls is moisture. This can be from obvious sources like leaks or more insidious sources like years of high humidity in the home or a sprinkler regularly hitting an exterior wall and raising the moisture level in that particular area. Poorly insulated doors and windows often cause slight dampness within the walls, causing more plaster damage around the frames or below windows. Signs of moisture-damaged plaster include peeling and crumbling walls, brown stains or bowing out of the area.
Features
Barely perceptible hairline cracks on the wall surface are a normal feature of old plaster walls and may not indicate underlying damage. If they are neither spreading nor very unsightly, it is OK to accept them as character in an old house. Cracks that regularly appear at the corners of door frames, around windows or across the ceiling despite repeated efforts to spackle and repair are stress cracks and, unless repaired properly by an expert by addressing underlying problems in the wall like loose or broken lathe, it will probably keep reappearing.
Prevention/Solution
Applying drywall mud or spackle to plaster cracks is never a permanent solution, as the cracks will inevitably reappear, although it can be used effectively on holes and dings. Plaster of Paris and setting compounds will give you a more permanent plaster repair and should be available at any good hardware store. Remember that these compounds dry extremely hard and cannot be sanded. Be prepared with wet sponges to smooth the repair before it dries. Long stress cracks in plaster may be repairable with drywall mud if you first lay flexible fiberglass tape over the crack and put at least two wide applications of mud over the tape, then sanding and feathering it in with the surrounding surface. Another solution for walls with extensive cracking is to use a wallcovering like smooth NuWall or textured Anaglypta, which hides all minor imperfections and once painted over, will look like new.
Warning
Avoid breathing old plaster dust by wearing at least a dust mask while working and use plastic sheeting to mask off other areas of the home, including heat registers, if making a lot of mess. Keep children away from the work area and clean up thoroughly when done. Working with old plaster inevitably includes working with the old lead paints that were used prior to 1980, and lead residue is particularly harmful to children.
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