Plaster goes back thousands of years and has changed very little over the years. From colonial times plaster was installed as a wall covering. It was often mixed with things like horse hair to give it added strength. The mixture changed locally depending on the availability of certain materials, but generally the plaster was applied in a 3 coat process that provided extremely strong and durable walls. The plaster could be mixed with sand for a rougher texture or the plasterer could use brushes or other techniques to create a unique wall texture. Typically the plaster was a mix of lime or gypsum, aggregate (sand), and water
The first step in plastering a wall is installing a horizontal wood lath. This lath is a thin (1/4″ thick) by 1″ wide board that is attached to the framing studs and layered up the entire wall. Once the walls and ceiling were covered in lath the plasterer would apply the base coat or “scratch” coat of plaster about 3/8″ thick. The coat would be scored with a brush or comb to give it the rough texture and allow the next coat the adhere well. The second coat was again 3/8″ thick and called the “brown” coat. The brown coat was not scored because it was rough enough to give the final 1/8″ finish coat that was placed on top a sturdy base. The work required a skilled craftsman and was a combination of science and art. The plaster required 28 days to fully cure before it could be painted or altered in any way.
During the massive housing boom that followed WWII, plaster fell out of favor because it was too expensive and too slow of a process. The finer homes still had plaster for a decade or two, but increasingly drywall took over and plaster became somewhat of a lost art. For a while in the 1950s you’ll find a combination of plaster and drywall call “gyp-rock.” This was a process where the new drywall would be installed and then the finish coat of plaster would be applied on top. Hiding the cheaper option of drywall and making the house look just bit classier than it really was.