Wednesday, April 18, 2007

More Thoughts On Plaster

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.

When was sheetrock invented?
Before 1860
1860 to 1870
1871 to 1880
1881 to 1890
1891 to 1900
1901 to 1910
1911 to 1920
1921 to 1930
1931 to 1940
After 1940
Free polls from


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.


StuccoHouse said...

Have you seen the National Park Service Preservation Briefs? They are short on practical "how to", but pretty good on providing historical background. #21 covers the everything-you-ever-wanted-to-know history of plaster.

Greg said...

That was very helpful, but some things that the brief said don't jive with my observations.


"However it is difficult to tell the difference between lime and gypsum plaster once the plaster has cured"

The Worley Way plaster method looks nothing like the old lime plaster in my house. Mainly because there is no sand added, but also the color is all wrong.


"Although lime plaster was used in this country until the early 1900s, it had certain disadvantages. A plastered wall could take more than a year to dry"

I've read this before, but from what I’ve read – bits and pieces, here and there – this does not seem to be a problem. The information I got from some UK sites refer to lime plaster as being "fat" or "lean", and the difference between “Slaked lime” and “hydrate lime”. I don’t fully understand it, but it seems that it is the slaked lime putty that can take a year to set and not the hydrate lime. I’m still a little confused, but here are a few quotes from the UK site.

“The word 'lime' refers to quicklime or slaked lime, widely used to form the binder for mortars, plasters, renders and washes prior to the mid-19th century. Quicklime is produced when limestone is heated in a kiln. Slaked lime is obtained when this is then combined with water to create a putty (stored in tubs), hydrate (bagged powder) or, where the reaction takes place in sand, ‘dry-slaked’ mix. Lime-based products harden by absorbing carbon dioxide to revert back to calcium carbonate (‘carbonation’). Chemically, this is the same as the original limestone, hence the term ‘lime cycle’.”

“There are two main types for conservation work:

Non-hydraulic lime from relatively pure limestone. This hardens only by carbonation. It is classified as CL under European standards. The purest is described as ‘fat’, the less pure as ‘lean’. Impurities in the latter impart a slight hydraulicity, but insufficient for it to be called ‘hydraulic’;

Natural hydraulic lime from limestone with reactive silica and aluminum impurities. These have a harder set, as calcium silicates and aluminates form in the presence of water in addition to any calcium carbonate from carbonation. Natural hydraulic lime is classified as NHL 2, 3.5 and 5.”

These quotes can be found at

This is sort of the frustrating thing about getting information about working with lime based plaster. If I read and are told that it will take a year to set up and you will need to wait to paint, it scares me away from trying it. Yet, I read where others have worked with lime plaster without this problem. Very frustrating.

And it still doesn't answer the question as why gypsum wasn't used in the 19th century.

Inquiring minds want to know.

Gary said...

Horse hair plaster isn't what it seems. I think goat hair is more common or cow hair even. I read that somewhere. When I re-plastered my basement walls I used lime plaster which I mixed up by slaking a bag of masons lime to make lime putty and then adding 3 to 4 parts sand with one part putty and I used polypropelene fibers instead of kitten hair which we have a lot of! If you skip the hair or fibers your plaster will crack quite a bit. The stuff works great, takes days to dry which makes smoothing easier and a finish coat would consist of 1 part fine sand with one part lime putty. You could just use lime putty. If you play with it a bit you get a Venetian plaster texture. I know how much you hate JOINT COMPOUND but go read the ingredients. It is basically lime, gypsum, chalk, powdered perlite or sand and an acrylic medium instead of water. I think the long drying time of one year would depend on humidity. England is a damp place and if you don't have air flow this could be the case. I suspect gypsum wasn't used commonly because the rock beds weren't discovered or mining of it wasn't profitable since limestone is much more abundent. Gypsum is a product of drying lake beds in a hot climate. Quicklime is calcium oxide. Slaked lime is calcium hydroxide. Masons lime is powdered calcium hydroxide. When drying the hydrated lime reacts with carbon dioxide in the air to form calcium carbonate (limestone).
Lime walls is why people used milk paint and whitewash to cover them allowing them to breath. Don't know if this info helps you. A 50 lb. bag of masons lime should cost you no more than $7.00.

Greg said...

Yes, that does help. I agree that horse hair may or may not have been the most common. I think it did become more common in later years, but regardless, it’s kind of hard to say. I put “Horse Hair” in quotes for the reason that it may not have always been horse hair, but it is a common name.

As for the one year drying time, I’ve only read this in American literature. I never read that in the UK info I saw. That’s one of the reasons it’s so confusing. I think it has to do with the “fat” and “lean” designations of the lime putty. I’m not sure which it is, but I think the fat or the lean putty will not set quickly. In fact, some lime putties will keep for years under water without setting. Some versions of lime putty you can fill a pale with it, and pour on an inch of water and it will keep for 4 or 5 years. At least that’s what I’ve read. What is the recipe for this mystery lime putty. I don’t know. That piece of information is always left out.

Now for the gypsum thing. You suspected that sources of gypsum weren’t discovered so lime was used instead, but everything I’ve read says just the opposite. Gypsum plaster has been around as long as lime plaster and sources were known about in this country and in Europe for a long time. Gypsum is a very common all over the world, and as I said, Plaster of Paris is gypsum. Ornamental plaster is Plaster of Paris. So, I have a house with lime plaster walls and gypsum plaster ornamentation plaster. Obviously both were known about and available at the same time.

Finally, I wish you would give some more details. You write:

When I re-plastered my basement walls I used lime plaster which I mixed up by slaking a bag of masons lime to make lime putty and then adding 3 to 4 parts sand with one part putty and I used polypropylene fibers instead of kitten hair which we have a lot of! If you skip the hair or fibers your plaster will crack quite a bit. The stuff works great, takes days to dry which makes smoothing easier and a finish coat would consist of 1 part fine sand with one part lime putty. You could just use lime putty. If you play with it a bit you get a Venetian plaster texture.

It’s good information, but, with all due respect, it’s a bit vague. I mean I could get a bag of masons lime and play around with it, but why should I reinvent the wheel. What are the portions of water to lime. How long does it slake. How much fiber. What you’re writing is nothing new to me. I read it time and time again. It’s frustrating for me. I guess I’m being lazy, or maybe I’m just too busy. I don’t like wasting time experimenting.

And yes, regardless of what's in it, I stand by my aversion to the all powerful Joint Compound.

Angus said...

My comment on the long drying time is what Gary pointed out as well.
Sure it might take "a year" to dry, but from what I know it was often painted over with whitewash or milk paint, both of which can be painted on a 'non-dry wall' - in fact could even bond better because of it - and because they breath, will let the wall still dry slowly.

John said...

I'm not sure when sheetrock was invented, but World War Two killed off the art of plastering. With the US's entry into the war, the Army suddenly had a huge need for housing. Millions of men were entering the armed forces and they need someplace to house them. Plastering a whole building (barracks) could take months, but sheet rock could be thrown up in a matter of days or a couple of weeks. And, their was a shortage of skilled plasterers. Even if they weren't getting drafted, there just weren't enough of them to meet the demand. After the war, people had gotten use to the sheetrock, and contractors loved the speed with which it could be installed.

I'd love to tell you where I found this, but I can't remember. I think I may have even done a post about it a year or two ago.

Gary said...

It’s good information, but, with all due respect, it’s a bit vague. I mean I could get a bag of masons lime and play around with it, but why should I reinvent the wheel.

If I give you details then it wouldn't be so mysterious, would it? What the heck!
If you half fill an empty joint compound bucket (wait, you wouldn't have one of those) or kitty litter bucket with water and start pouring masons lime into it, you will find that half the bag will get sucked up. It is amazing that that much lime will absorb all that water. You can stir it a bit if you want to. If you let it sit for a day or two it will saturate and you will have lime putty. The viscosity will increase with more lime.
You don't need more than a pinch of fibers per batch where you use up to three parts lime putty. The polypropelene fibers should be available at a concrete supplier.
About six months ago I was exactly where you are now. Trying to figure all of this stuff out. All I can tell you to do is to stop trying to research and go make a batch and start using it. You will soon become THE expert that you are seeking for advice. After all, it is just lime and sand that you are working with. After you have worked with it a bit you will understand this last comment better. I can't wait till you research Stucco!

Angus said...

Wartime seemed to have given gyprock the start it needed:

"After more than two decades, the product they called Sheetrock still hadn’t really caught on. ... But the urgencies of wartime construction changed all that.
Of course, all this was only meant as a stopgap replacement for plaster, but as you’ve probably guessed, it didn’t turn out that way."

John said...

"Gypsum is a very common all over the world, and as I said, Plaster of Paris is gypsum. Ornamental plaster is Plaster of Paris. So, I have a house with lime plaster walls and gypsum plaster ornamentation plaster. Obviously both were known about and available at the same time."

Let me preface this by saying I may be completely wrong.

Your comment above, particularly the part about them using gypsum plaster for ornaments, made me wonder if gypsum holds details better than lime plaster.

In art, whenever I've used plaster, whether it is for making casts, scupltures, etc, we always use gypsum. It's great for getting hard lines and good detail, both characteristics would be very desirable for ornaments.

As someone with paster walls in their home, you may be able to answer this question for me: which plaster is harder? From what I remember, lime plaster is cool and very hard to the touch, much like stone.

Gypsum plaster, even when smooth, is chalky, a bit brittle, and doesn't feel quite as heavy or solid.

Maybe lime plaster is used for your walls because it is harder and more durable? And the ornaments are made from gypsum because of it's ablility to hold detail?

So, does that sound plausable?

Greg said...

All great comments. I love a good discussion.

After all of my belly-aching about not being able to find a lime plaster recipe I was going through some of my old book marked links. I had forgot that I had pestered Victorian Farm House Reborn for his plaster recipe.

Here it is.

Greg said...

Here's a better view of the link.

StuccoHouse said...

My knowledge of lime is limited. I do know the "curing" time comes into play with things like painting & wallpapering. Maybe why calcimite was used. Other than that, I don't think it's something you really need to worry about.

Gypsum plaster can be textured. Mine has sand in both the brown & finish coats in rooms that were intended for painting. In those rooms where the plaster is polished, it doesn't have it in the finish coat. I was also a bit confused by the Brief's comment about the two types looking similar. My gypsum plaster certainly does not look like lime paster. I will say though, that my old polished gypsum walls have developed a nice brownish patina to them that looks very different from new gypsum plaster....I guess this could be mistaken for lime (?) I do know that some people apply a thinned down varnish to the finish coat of gypsum plaster to replicate the patina of the old stuff.

I have found that many of the stucco companies in these parts still do plaster work, so the art isn't totally dead. I actually took a class from one a few years back.

It's just weird to me that there is so little clear info. that distinguishes between the two. And why is most of the repair info. on would think it would be on gypsum because it's easier.

Interesting topic.

Anonymous said...

hello. I have repaired large portions of my house using so called "horsehair" plaster. Making it is not terribly difficult. I do not measure, but go largely by feel. First, start with the lime. You can use mason's lime (a hydrated lime), as one person mentioned, hydrated finish lime (Type N or S), or slake your own from quicklime (CaO, not hydrated). Quicklime may be hard to find and is considered dangerous to slake, so I recommend using finish lime, at least for now. There is no need to measure. I use a 20 gallon Rubbermaid trash can mostly filled with water. Add hydrated finish lime (a bag is usually 50lb), mix and let it sit a little while. Make sure the hydrated finish lime is fuly covered with water. The dust is rather potent so be careful. One week is typically sufficient for soaking, but more is definitely ok. The excess water will rise to the top. The lime underneath will have the consistency of a soft cream cheese. Type S limes will set up faster than Type N. You should get better results from finish lime than from mason's lime. Once you you have this putty, you need merely add sand and mix. The proper sand is white textured mason's sand. On a microscopic level it is sharp, rather than rounded in other sands. White sands are low in iron, to prevent staining. Ratio of sand to lime is about 2:1 or thereabouts. It takes some effort to get everything mixed well. Basically, you need to end up with something that has enough lime so that you can work it, but not so much that it cracks up. I think hair is really only worth while when going over lath so that your keys are strong (ie in the scratchcoat). I have omitted hair since is difficult to find the right stuff. Polypropylene fibers are the best I have been able to find, but they are not ideal since they are synthetic. Natural hair will grab better since microscopically it is textured. You will use less aggregate on the finish coat. Some fancy finishes substitute marble dust for sand on the final coat. My house has a gypsum cement finish on top. I do not recommend using joint compound. The big reason is lack of workability compared to properly made lime plaster.
Lime is slow to set up and more work, but makes a superior plaster, in my opinion. It is not easily destroyed by water like gypsum. I have used my plaster mix to point masonry outside. It also is self healing, adjusting to structural changes better than anything else. It drys within 30 days. The year figure refers to carbonation of the surface (about .1 in deep). The active CaOH and possibly MgOH) msut react with CO2 in the presence of H20 to form carbonates. Prior to carbonation, the excess alkilinity will destroy typical paints that might be applied.

Spring City, Pa

Greg said...

Excellent, excellent, excellent. Roger, I can’t think you enough. I’m so glad I started these discussions on lime. The whole thing has really taken a lot of the mystery out of it for me.