Historic PlasterMaintenance

Historic Plaster and Somebody’s Horse.

Plaster has served  as a durable coating for interior walls in both high style and common buildings for centuries.  19th century wall plaster was made from four ingredients: lime, sand aggregate, fiber and water.  The lime was the most important part of the mix as it cured and hardened much like concrete.  Sand was a filler that also added structure.  Water was the catalyst and the fiber held it all together while the plaster mix cured and hardened.   Gypsum replaced lime in the 20th century because it set more rapidly and provided a harder, more durable finish much faster.  Since gypsum sets in minutes and cures in weeks (lime plaster required months to cure before it could be painted or papered), it made the addition of fiber unnecessary.

The lime was often produced in kilns from local limestone deposits.  The sand aggregate usually came from a nearby river or lake which could also provide fresh water for mixing. What was the source of the fiber?  More often than not, it also came from local source: an unsuspecting cow or horse from the barn out back.

I recently attended a workshop on repairing historic plaster organized by the Preservation Alliance of Minnesota.  Our leader for the day was Anders Christensen of TigerOx, a Twin Cities painting and plastering company that frequently works on historic architecture. Rather than learning in a classroom we had the privilege of  working on an actual historic site, the 1867 Andrew Peterson farmhouse in Waconia, MN.

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This is one of the holes we repaired in the Andrew Peterson farmhouse.  Notice the single layer of plaster with a sand aggregate. 

In high style houses the plaster was applied in three coats over split or sawn lathe. The first two coats, called the scratch and brown coats, were thick, coarse and contained the animal fiber with lots of aggregate. This constituted the bulk of the wall plaster.  These two coats were finished with a thin layer of finish plaster. The finish coat had little aggregate and was troweled to a smooth finish.

However, when we looked at the walls in the Peterson house, we found something quite different. The Peterson house was built by Andrew himself, who was a farmer and not a professional carpenter or plasterer.  Rather than three coats, we found one, rather coarse coat of plaster.  This is not uncommon at all in vernacular buildings such as farmhouses or city buildings which weren’t built by professional tradesmen.

What was also interesting was what you could see in a few chunks of loose plaster. The animal hair used to bind the plaster was quite obvious.  I’m not sure what sort of critter donated its hair in 1867, but it was chestnut brown and rather soft. A horse, perhaps? Whatever it was, it is long gone. That is, with the exception of its hair, which survives 150 years later in the walls of the Peterson farm house.

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A piece of plaster with the animal hair fiber.

I also found traces of paint on another piece of plaster.  The color is a light pastel which was common in house interiors into the 20th century. I suspect it is a calcimine or distemper paint. Calcimine paints were water based and fairly durable because the pigments were bound to the wall with size, or diluted animal glue.

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Another piece of plaster with remnants of what appears to be a light, purplish-blue paint.

The next time you find yourself in a 19th century home with historic plaster, take a look at the walls.  Is the plaster coarse with lots of sand or is it troweled smooth and flat?  If it is smooth, it probably was applied by a skilled tradesman.  If not, it might have been plastered by the homeowner himself.  In either case, it is likely some sort of quadruped also made its own, valuable contribution.

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