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So, what is plaster actually made of?

Have you ever tapped on a plastered wall with your knuckle? It feels like solid rock, right? Well that’s because plaster is, essentially, rock… processed rock, reconstituted and spread on to walls and ceilings.

Of course, you can’t just grab a wheelbarrow full of gravel from your driveway, grind it to dust, add water and spread it on your wall. There’s more to the recipe.

What’s in plaster anyway?

Clay, lime, gypsum, sand, animal hair, cement and mud have all made their way into various mixes. Plaster has been around for thousands of years, and in the days before motorized transportation, builders simply used the materials available close by. Builders in Louisiana, for example, used a technique mixing swamp mud, Spanish moss and deer hair to make a plaster foundational material they call “bousillage.”

There are dozens and dozens of different ways to mix plaster, but for now let’s talk about interior walls and ceilings in 19th century American homes.

In American architecture, lime was once the most common material used in interior mixes. When mixed with water, sand and cattle hair, the putty like substance was applied and allowed to set. Once dry, you were left with a durable surface that resembled limestone.

Around the early 1930s, however, gypsum became the “rock” of choice in American plastering. Made up of calcium sulfate dihydrate, gypsum is still the standard we see in most interior mixes today (it’s also the main ingredient in drywall).

What holds plaster together?

Still, something is needed to bind all those ingredients together once they dry. In the same way that thin rods of re-bar are encased in concrete to give it enhanced structural integrity, lime-based mixes use some sort of fiber binder. In antique America homes, cattle or ox hair was a common binder.

Modern mixes, however, doesn’t need fibrous binders. Instead, it utilizes the crystalline structure inherent in the gypsum. When those crystals are allowed to become wet, they lock into each other during the curing process.

Have more questions about the makeup of plaster?