Plaster is as old as mud, and used to help fill in the cracks between load-bearing building materials (framing timbers, stone, or bricks).
How plaster is used and what it is made of depends on two things: water and temperature.
In incredibly hot and dry places like North Africa, bricks and the mortar used to hold them together are often made of simple mud and straw, hard-baked by the sun. Shown here is the famous mosque of Djenne, in Mali- a World Heritage Site.
In North Africa, there is very little rain to wash away the mud and no freeze-thaw cycle to crack and break them.
Cue Exhibit B, the Pont du Gard near Nîmes, France.
This fabulous edifice (oh, those Romans, be still my fluttering heart) is an aqueduct, built to carry water from mountain springs to the town. Hurrah for indoor plumbing!
Clearly, in France you have 1) significant annual rainfall and 2) a freeze-thaw cycle, plus the weight and weathering qualities of thousands of gallons of water running over the aqueduct every day. Mud is not an option. So the Romans developed a wonderful thing called concrete, which is simply a harder, more resistant form of plaster used to bind the massive structural limestone blocks together. The concrete in the water channel at Pont du Gard is a mixture of slaked lime, pork grease, fig juice, and crushed pottery; this mess hardens to a slick, smooth surface that is extremely hard and impervious to water. Slaked lime, like mud, is the ‘sticky’ primary material that can be worked while wet but will harden near-permanently when it has dried.
This brings us to the temperature question. Why not build the entire aqueduct out of concrete? Because the harder things are, the less likely they are to flex under stress- instead they break.
Brick damage on a cathedral near Modena, Italy
One of the greatest and most frequent stressors on building materials is the freeze-thaw cycle, which not only causes temperature stress on materials by making them more brittle, but also drives water deep into existing cracks or cavities. Water expands when it freezes and as Scotty says, the laws of physics demand the water expand whether or not there’s a brick in the way. Think about it- there’s a very good reason why vernacular buildings in cold countries (Ireland, Norway, Nepal) are more likely to use a flexible, porous material such as wood or a relatively impervious material such as stone.
This is where the mixture of the plaster is important, because temper materials help mitigate the effects of temperature changes and make plasters more flexible. In the most basic terms, a temper is a material introduced into the plaster mixture such as straw, horsehair, sand, bone, or shell that will both hold its shape as the plaster hardens and has distinctly different expansion qualities. For example:
The straw and sand in these simple adobe bricks made by Mari at Gather and Grow expand and contract at specific temperatures, which are different from the expansion/contraction temperatures of the clay. As temperatures change throughout the day and throughout the year, the different materials are constantly in a state of flux (on a molecular level, hard to see) but create a more flexible material that resists cracking. No cracks, no room for water to get in and cause major failure. The long strands of grass also help to knit the clay together and provide a matrix or structural framework on the most basic level.
The other glorious thing about plaster mixtures is the idea of sacrificial material. This is similar to the idea of temper, but on a slightly larger scale. Within a brick or concrete block, temper creates flexibility and also helps to hold the primary material together. Multiple bricks and blocks (also stones) are joined together by mortar, a different type of plaster that again has different expansion/contraction properties. So next time you look at a brick wall, imagine it rippling ever so slightly- scary, right? But necessary to keep the whole structure from cracking and failing. Mortar and its cousin stucco are designed to help keep water away from the main structural material (i.e. brick and stone), but if water does get in (insidious thing) then the mortar will break first. This is exactly as it should be, because it’s much easier to slap a little extra mortar into the joint or patch a crack in stucco rather than start pulling bricks out of a wall. That’s why it’s called sacrificial, because it’s ok if it breaks- it can be fixed and the wall won’t fall down in the meantime.
Repointing demonstration from a UK hardware store- but in reality, please please don’t try this at home on anything but your garden shed. There’s a reason why brickmasons are master craftsmen.
(Preservation note a la Mama Boucher- Portland cement is the devil!! Why? Because it’s too hard, has very little flexibility, and a large expansion radius. Never ever patch anything with Portland, it breaks brick, stone, and other types of concrete, and the stuff in it is highly toxic. Learn more here at Metropolis.)
This is a tabby and stucco clad wall in Hilton Head, South Carolina- tabby is a concrete-like material made using shell temper, covered in a finer-grained stucco to give it a smooth exterior. The ugly blue stuff is some genius’ attempt to patch with Portland cement and you can see how it has broken the shell matrix over time.
This brings us neatly to the finer points (haha) of plaster. Plaster is also used as a sacrificial finishing material. In order to get a smooth outer surface, like the aqueduct water channel mentioned above, the grain of the plaster has to be very very fine. Fine grain means less temper and often a much finer temper material such as horsehair or sand. Less temper, smaller temper equals less flexibility and a more brittle surface that is prone to cracking. In the photo below, the horsehair at Shoreside Farm is clearly visible as both temper and matrix, which has failed due to stress from a settled sill plate. Note that the wall behind the plaster has no cracks.
Fine plaster makes a lovely smooth surface to paint over, but will crack as a house settles or as temperatures war between outside and inside. That is ok, because plaster is meant to be patched. Drywall, on the other hand, doesn’t patch as well and for damn sure doesn’t insulate as well.
Takeaway lesson for the day- previous generations designed living buildings, meant to be repaired, patched, and reworked because Nature never sits still. Man has not yet come up with a material that Nature cannot eat for breakfast- temperature changes, sunlight degradation, water, weathering, earthquakes, wind, bugs, squirrels, and kudzu are all in Her arsenal. Plasters and mortars are designed to work with Nature, not against it, and have been perfected to suit individual climates and environments over thousands of years. Embrace your plaster, love your plaster, and for the love of the preservation gods, always match your plaster (or mortar).