Plaster is hard.
It cracks as houses settle and time passes. To really understand why plaster cracks you need to understand what makes up a plaster wall. You often hear of these walls being referred to as "Plaster and Lath" or "Plaster on Lath" with the latter being a more correct description. The lath is a series of wood strips nailed to the wall joists. The plaster is made up of two different materials, the first (which can actually be a couple of layers) being called the scratch coat which is a darker and more coarse material and the second being the finish coat which is white, very smooth and usually fairly thin in comparison to the scratch coat. The scratch coat's purpose is to lock into the lath. This occurs when the dead men who built these homes would put a healthy layer of the scratch coat onto the lath, pressing it in deeply to create "Keys." These Keys were the portion of the plaster which curled around the back side of the lath and held the wall onto the lath. The finish coat was then laid on and tooled to create the wonderfully smooth surfaces we know and love.
