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.
Developments and changes in the construction industry have led to the gradual shift to gypsum plaster mix especially during the early parts of the 20th century. Plasterers soon came up a composite material made out of the combination of lime and gypsum plasters although, eventually, gypsum became the more popular plaster material.
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.
A coarse plaster is the result of the mixture of lime, water, sand and fiber. Plasterers apply one or two coats of this coarse plaster material in order to attain the desired thickness of walls. For best results, the normal practice is to use 3 layers of the plaster material. The initial two layers of the plaster material are the brown coat and scratch coat while the final layer is applied as the setting stuff of the plasterworks.
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 initial two coats of plaster material are 3/8 inch in thickness while the finishing plaster coat is about 1/8 inch in thickness. There are instances where plasterers use additives such as whitish sand and marble dust to attain a specific finish and texture. In order to reduce the setting time of the final coat, plasterers add plaster of Paris to the plastering material.
Gypsum started to be used as a plastering material at the early parts of 1900s. One of the main advantages of gypsum is that it immediately reacts as soon as it makes contact with water. Gypsum as a plastering material also sets faster than lime plaster and its drying period is much shorter. Another upside of gypsum is that it is proven to be more rigid than lime. Unlike lime, gypsum plaster will not require fiber component in order to attain the binding effect needed in the plastering material.
Notwithstanding all of these distinct advantages of the plastering material, gypsum is prone to damage when exposed to moisture and damp conditions. Thus, lime plaster is more appropriate as plaster coating for masonry and when doing external plastering jobs.
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