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. Crafty builders in Louisiana, for example, have been known to mix swamp mud, Spanish moss and deer hair to make a plaster 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 American homes. In American architecture, lime was once the most common material used in interior mixes. When mixed with water, sand and horsehair, the peanut-buttery substance was applied and allowed to set. Once dry, you were left with a hard (yet still somewhat pliable), durable surface that closely resembled a slab of limestone.

Around the early 1900s, 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 is also the main ingredient in drywall). It is the main ingredient in Plaster of Paris, too.

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 particles inherent in the gypsum. When those crystals are allowed to become wet, they lock into each other during the drying process.

Have more questions about the makeup of plaster? Leave us a message in the comments below.

To date, the earliest evidence of plaster was found in portions of southern Turkey and northern Syria. Plaster was applied to the houses there around 12,000 B.C. That’s before pottery was even invented.

In its most pure form, plastering has not changed much in 14,000 years. Wet “mud” is still spread over some type porous surface until it is smooth. It is functional, durable and looks great – that hasn’t changed. The tools and ingredients have undergone some evolution, though.

In the beginning, simple wooden tools and clay/dirt mud were used. During ancient Egypt’s height of power, gypsum was incorporated. The Greeks had lime and the Egyptians had gypsum… they both used the local materials that were were available.  The Romans, in turn, took cues from the Greeks, spreading the power of plaster through the Roman Empire.

Nowadays we have access to modern, heat-treated stainless steel and plastic trowels. The Industrial Revolution of the late 1800s brought big changes in binders used in plaster – oil provided controllable energy and the heat necessary to process these binders, and more importantly to provide transportation for plaster to be distributed over a wide geographic area, negating reliance on local materials.

Due to English influence on our culture, the first plaster used in the U.S was a clay-lime-hair-sand mixture called daub, which was applied to wattle, thus the name “wattle and daub”. Gypsum and Portland mortars and plaster were phased in during the early 20th century, as they allowed building production schedules to move forward faster.

Today we enjoy a wide selection of  plaster types, each tailored for a specific purpose or climate, be it plaster repair in Milwaukee or new veneer in Miami. Whether it is gypsum formal work, veneer plaster, Venetian plaster, traditional lime-sand-fiber plaster, or ultra-modern synthetic plaster, builders have a myriad of options. Every one of these materials has its inherent strengths and weaknesses. Knowing the properties of each material allows a builder to determine the correct plaster to use in any given circumstance.