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A Closer Look At Lime And Gypsum Plasters
Gypsum plaster and lime plaster are considered the two most common plastering materials. Lime plaster is the older material between the two, and most of the plasterworks during the 19th century are made of lime plaster. This plaster material is composed of lime, sand, fiber material and water. The lime component comes from oyster shells and lime stones while the fiber material is usually derived from hog or cattle hair.
Changes in the construction industry have led to the development of gypsum plaster as an alternative to lime plaster. Eventually, gypsum and lime plasters were also combined and used as a composite plastering material although gypsum slowly became the preferred plastering material for most tradesmen.
When constructing a house or building, the traditional practice of plasterers is to mix quick lime and water in order to hydrate the powder compound. As soon as the water is absorbed by the lime, heat is released. This is an indication that the chemical reaction has occurred and that the mixing process is complete. The resulting putty is then used a plastering material.
You get a coarse plaster mix when fiber material is added to lime, aggregate and sand. Plasterers normally apply at least two layers of the plaster coats on the masonry. However, in order to attain the desired thickness, up to three layers of the plastering material can be applied. The first two layers of the plastering mix are referred to as the scratch and brown coats while the finishing coat is referred to as the setting stuff of the plastering job.
This final layer of the plasterwork uses a plaster mix that has a higher amount of lime putty and lesser amount of aggregate. Further, the plaster mix does not contain any fiber in order to attain a fine and smooth finish.
The first two plaster coats must attain at least 3/8 inch of thickness each while the finishing coat must be at least 1/8 inch thick. Additives can be combined with the final plaster coat in order to create the desired kind of finish. The types of additives used include gypsum and whitish sand. Plasterers normally add minimal amount of gypsum in order to reduce the setting time of the plastering material.
It was during the 1900s when gypsum was first used as material for plastering. One of the distinct features of gypsum is that it instantly starts to cure as soon as water is mixed with the material. Gypsum immediately sets in a few minutes and the drying process is completed in a couple of weeks. However, for the untrained eye, it is difficult to distinguish gypsum plaster from lime plaster. Gypsum is acknowledged to be more rigid compared to lime and with this plaster material, there won't be a need for fibrous binder.
Despite all of these distinct advantages of this plaster material, gypsum has its downsides. Gypsum plaster is more susceptible to water damage compared to lime plaster. In most cases, lime plaster is the preferred material for masonry walls as it creates a strong suction bond the moment the material is applied. Today, gypsum is the most preferred plaster material of plasterers.
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