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Traditional Plastering
From the earliest times until the advent of comprehensive transportation systems, plastering was always done with local materials. by Rory Brennan
There was lime plaster in early New England homes and in ancient Rome, plaster of Paris in medieval France, gypsum stone mortar in early Egypt, and earthen plaster in Native American pueblos. The better the transportation, the wider the distribution of plastering materials.
A case could be made that plastering is the oldest trade, although masons really came first, giving the plasterers something to plaster.
In the United States, traditional plastering means lime plaster. The oldest plaster for which there is evidence is in southern Turkey and Syria, and that plaster is also lime. The age of these plasters suggests that lime plaster is a great material to use, a great material to maintain and repair, and has a track record of longevity second to none.
A little history
The earliest plaster found to date was found in the Levant section of the Middle East. The area includes parts of southern Turkey and northern Syria. The plaster was applied to the houses around 12,000 B.C., which means that it predates pottery technology. While these people were still hunter-gatherers, they were plastering their houses.
Plastering knowledge was not universal, but when people built structures to live in, they used whatever material they had locally. In ancient Egypt, gypsum was used for stone mortar as well as plaster. For the Egyptians it worked. This is what they had. The rains in Egypt were few and the mortar was stable enough. The Greeks picked up plastering from the Egyptians and used their own materials. The Romans picked it up from the Greeks, and traveling tradesmen spread the knowledge throughout the Roman Empire. Our plastering traditions in the U.S. came from the Romans through the English.
Plastering has not changed much in 14,000 years. Plasterers still spread wet “mud” on lath and work it until it’s as smooth as required. Tools evolved from wooden tools to modern heat-treated golden stainless steel and plastic trowels. The cementitious binders used in plaster changed as the Industrial Revolution advanced through technology and the availability of controllable energy. Oil provides the heat necessary to process these binders, and more importantly, to provide transportation for processed cement to be disbursed over a wide geographic area, negating the reliance on local materials.
Due to the English influence in our culture, the first plaster used in the U.S. was a clay-lime-hair-sand mixture applied upon wattle and daub. This was soon followed by lime-sand-hair plaster, which was used in this country until the early 20th century. Gypsum and portland mortars and plaster were being phased in during the early 20th century, as they allowed building production schedules to move forward faster. I have seen gypsum plaster from the 1870s, lime plaster from as late as the late 1920s, and gypsum sheetrock from 1926.
Today we have a complete mix of options available for plaster, ranging from modern gypsum formal work, veneer plaster, Venetian plaster, and traditional lime-sand-fiber plaster to ultra-modern synthetic plaster. Every one of these materials has its inherent strengths and weaknesses. Knowing the properties of each material allows us to determine the correct plaster to use in any given circumstance. It’s all about science.
It’s always time for lime
Traditional plaster in this country generally means lime-based material. In understanding and repairing historic plaster, it is important to know the properties of this plaster. Lime sets in a specific manner, interacting with atmospheric carbon dioxide. For this mechanism to occur efficiently and completely the lime needs to have an extremely large surface area. The lime is burned at a low temperature, and the high surface area allows the lime to hold onto water longer during the curing process and increases the lime’s plasticity. It is also important for complete carbonation for the lime to be at least 98 percent calcium. To preserve these qualities, it is made into putty immediately, a condition it can maintain indefinitely. Lime plasters can be used inside and out. They are flexible, water-resistant, and transpire moisture readily.
Modern hydrated bagged lime is burned to fuse the exterior of the lime particle, allowing the lime to act as a plasticizer in gypsum and portland plaster and letting the chemical set act as the setting agent. Immediately upon being bagged at the factory it starts to carbonate immediately, giving it a limited shelf life.
Often I’ve heard the thought, “I got rid of that old soft plaster and replaced it with something that is much stronger.” Experience in England tells us that softer plasters last the longest. The difference between softer plaster and harder plaster comes down to a mixture of science and philosophy.
From the philosophic perspective, which tree will last longer in a windstorm — an oak tree or a willow tree?
Science tells us the oldest plaster on the planet is “soft” lime plaster. No 18th or 19th century lime plaster can be replaced with any material that is as good or better. Any replacement for traditional historic plaster will be inferior to the original. The replacement may be harder, but it will not outlast the original. The softer the plaster, the greater the ability of the plaster to move with the structure and the longer it will last.
The modern age
Modern plaster, for its part, comes in several different varieties. Stucco (portland plaster) mixes absorb water, do not transpire moisture and are very rigid — so once set they do not move or change in size very much. These will set up in the presence of moisture and can stand wet environments. Gypsum mixes set quickly, are rigid, need water to set, and break down in the presence of water. Acrylic mixes are used with EIFS and are flexible but form a seal that prevents moisture from transpiring in either direction.
None of the qualities of modern plaster lend themselves to installation in historic structures. Many historic structures are built with inherent flexibility. For example, a typical 18th century house in New England is built in a number of layers:
Field stone foundation
Braced frame pegged
Wall studs notched and pegged
Wood lath nailed to studs
Plaster hung upon the lath
Painted on plaster
Between each one of these layers is an attachment that acts as an articulation. The structure of the building is inherently flexible.
Compare that to installing a modern building system — for example, poured concrete foundations bolted to nailed stick framing that is covered with wire lath and coated with a gypsum plaster (or sheetrock) that sets up the building for expansion and contraction conflicts. Each one of these layers represents a link in a chain. If any of the links break the whole structure can be threatened. The materials and techniques used historically have a proven track record. Why is it that well-meaning people will try to correct a problem that doesn’t exist or improve the architecture with a material that has a less-than-stellar track record in historic structures?
Tools and terminology
Tools have not changed in many years. The procedure began by slaking the lime in a sand lime pit located somewhere close to the home site. The lime was sifted into water and allowed to soak for a period of months. The lime settled out into different-sized particles with the heavier particles sinking to the bottom — these were used for the mortar and the scratch coat. The scratch was referred to as “coarse stuff.” Coarse stuff was created by beating the sand into the lime until it was workable. Just prior to the installation on the wall the hawk boys would put the hair into the mix. Then they would deliver the plaster to the mortar board. The plasterer would take his trowel and load his hawk from which he would take the plaster and apply it to the lath. The hair generally was only in the coarse stuff. The ratios of lime to sand ranged from 1-1 to 1-3. The sand was as important as the lime or hair.
The brown coat or the float coat is the layer where the walls and ceilings are filled out and flattened, made true and square with a browning rod and compacted with the darby. It is coarse stuff, the same consistency as the scratch coat. The finish or setting coat is the last layer. This layer fills in all the surface imperfections and can be opened up to accept paint or polished and left unpainted. The finish layer may or may not have hair or gypsum in it.
Plaster and stucco can be viewed as interchangeable terms. In general, plaster is allocated to the interiors of buildings and stucco is on the exterior of buildings.
Traditionally the plaster and the stucco were made from the same material as the brick, stone or block mortar — lime plaster. In the modern era stucco is made from portland cement or acrylics. Neither of these materials performs well on historic buildings.
Doing the repair
When time and money considerations are combined to analyze a plaster problem, a repair is the soundest investment one can make in a building. Lime sets over extended periods of time; it’s young at 100 years. This slow cure helps to impart to it the qualities that make it a viable candidate for maintenance and repair. Properly cared for, lime plaster will last forever — there is no other plaster that can make that claim.
The existing plaster must be reattached to the lath, making it stable and secure in its original position prior to patching. Without being stabilized, nothing else done to repair the plaster will last, while once stabilized, almost any repair will last. The preferred method of stabilization uses an injected adhesive, which is compatible with the characteristics of the lime plaster. The adhesive provides a dispersed area of attachment that is flexible, leaving nothing on the surface to cover. The flexibility of the adhesive allows the plaster to move as naturally as it did when first applied. After the adhesive sets all you have to do is fill the injection ports and cracks and the repair is completed.
The conventional approach using “plaster” washers and screws to compress the plaster back to the lath has a couple of issues that make it incompatible with repairing historic plaster. The washers and screws often crush the plaster to the lath, fracturing its fabric. What’s more, the screws and washers offer a very hard, narrow point of attachment. This method will not move with the building or the plaster. When the plaster moves around the washer, the plaster breaks, needing to be fixed again with a mesh to prevent the crack from showing. The crack does not go away or come together; it still is hidden under the tape and joint compound. The mesh tape and the screws and the washers need to be covered with multiple layers of joint compound and sanded or sponged. The crack will open up under the mesh tape and the washers will show at some point. This type of repair is short-term.
Repairing historic plaster saves plaster, time and money. There are samples of mortar and plaster (the same material and usually the same mix) thousands of years old that have been subjected to an amazing variety of environments from underground to sun, rain, fog and snow. Lime plaster properly prepared and maintained will last for a millennium. www.preservationplastering.com
Rory Brennan is president of Preservation Plastering Ltd.
