Correct me if I’m wrong, and this time I mean it.
I’ve been peiceing together information on plaster and drywall over the last few years. A bit of it is because I’m curious about different construction techniques, but mainly because I wanted to try and use historically correct building methods in the restoration of the house.
Getting information about plaster repair was not easy when I first started working on The Petch House. I didn’t even know there was more than one type of plaster. As I said yesterday, Dave from The Worley Place supplied me with very good information on the use of Gypsum Plaster. From what I’ve been able to gather, the two basic types of plaster are Gypsum and Lime. So the question is, why was one preferred over the other?
From an old house prospective, the Gypsum plaster seems to be the newer method for wall plastering, while the Lime plaster seems to be the older method. Plaster of Paris is gypsum plaster, and the ancient Egyptians, and many other cultures have used gypsum plaster for thousands of years. There is nothing new about gypsum plaster.
There is nothing new about lime based plaster either, from what I can tell. It seems to go back thousands of years as well. I assume the method for creating lime for plaster is the same as quicklime for lime mortar. Bare with me, I’m no chemists, so I’m sure this is over-simplified. Anyway, you take lime stone and cook it in a kiln to burn off the carbon dioxide. This creates what is known as Quicklime or Burnt Lime. Adding water to quicklime starts another process which I guess sort of reverses the process the lime stone went through in the kiln. Simply put, the water starts a chemical reaction in which the lime gets hard again.
The process for turning raw gypsum in to plaster is similar, from what I’ve been able to tell. I tried to figure out why one was used over the other. I thought maybe it was regional. If an area had a lot of lime stone and kilns near by maybe they used lime plaster, and if you had a source of gypsum you used gypsum plaster.
I know that there is both a source for gypsum and limestone in California, and I also know that there were many lime kilns in central California at the turn of the century. When I lived in Santa Cruz I would go hiking in the local mountains and there were a few old lime kilns still up in the mountains. They were crude structures built up against a small cliff.
In So. California there is Plaster City which is a source of gypsum plaster, which I assume has been an active source for a very long time. Given the amount of commerce going up and down the coast by ship it seems that either could be a source of plaster for my house. So why did they use lime instead of gypsum? Also, why were some houses skim coated with a finish coat and others were not.
My walls don’t have what you would think of as a finish skim coat of plaster. There may have been a scratch and a brown coat, but really, it looks the same all the way through for the most part. The only difference I can tell about the surface of the plaster, and what’s behind it, is that I never see any animal hair on the surface. Maybe it was skim coated, but there is definitely sand in the skim coat. It is not the smooth, white skim coat one normally associates with later plaster jobs.
If you have sandy plaster walls, then it’s lime based. Even if it’s smooth on top, that may just be a skim coat of plaster (lime or gypsum) without sand. Why gypsum plaster wasn’t applied to more walls in 19th century homes, I can’t say. Or maybe it was and I’m just not aware of it. From my limited perspective, it seems that plaster walls in 19th century homes were largely lime based horse hair plaster, while gypsum based plaster became more and more popular in the early 20th century. I could be wrong.
The lime based plaster is usually referred to as “Horse Hair” plaster, but it was not limited to just the use of horse hair. The hair in the plaster acts as a binding agent to hold everything together. At some point drywall, also known as sheetrock, became the predominant method for finishing walls in a house. I can’t say exactly when sheetrock got it’s start, but I do know when the first US patents for sheetrock were obtained by the US Gypsum Co.
And that will be today’s poll question.
Seconds after I posted this I had a thought. Maybe it was the rise of the automobile that lead to the demise of horse hair plaster. With fewer and fewer horses in the city there would be fewer places to obtain horse hair. Without a reliable supply of hair as a binding agent you would need to switch to gypsum plaster.
Horse hair plaster literally went out with the buggy whip.