What is Plaster Anyway?

Plasterer, by John Cranch, 1807

Originally posted February 2016 on Traditional Building Magazine Online

Plaster is as old as civilization. I'll go out on a limb and proclaim that without plaster civilization was impossible! Mankind’s ability to leave the metaphorical cave, raise a shelter of stones or reeds and coat that shelter with an earthen plaster enabled him to create the cave wherever he desired. Building permanent dwellings close to fresh water, upon a fortifiable position or adjoining arable land allowed extended families to gather and the first cities to be born.

The English word plaster has a rather direct lineage from the Classical Greek ‘emplassein’ (εμπλασσειν) ‎meaning to ‘mould or form’ as well as the related term ‘emplastron’ (εμπλαστρον) conveying the sense of ‘daubing, to salve.’ So it is that our contemporary speech has effectively retained these ancient meanings more or less unaltered, the word plaster still being used to describe a range of materials for casting and for coating. Physically plaster begins as a wet, mineral slurry characterized by either a chemical set or a mechanical one, meaning that it simply dries out. Having now a general idea, let's take a closer look at what makes up a plaster.

Plaster Ingredients  

The most important component of a plaster is its binder. As the name implies, it’s the component that binds or holds the plaster together. Think of it as a kind of mineral glue. Lime and gypsum are very common heritage binders and plasters that exclusively use one or the other are commonly referred to as lime plasters or gypsum plasters respectively.  Aggregates are typically the ingredient that physically constitute the bulk of a plaster. Materials such as silica sand act mostly as filler, something relatively inexpensive for the binder to cement together. Aside from sand many other aggregates have been used that impart very distinct properties to a given plaster. We’ll address several of these in a subsequent essay.  Plaster has to be wet to be ductile, spreadable enough to use. The proper level of moisture also makes a plaster sticky so that it can form a good bond or adhesion. Potable water is almost exclusively the material of choice to provide a fluidizing agent, being both inexpensive, readily available and safe. Sometimes the water will have additions that either ‘accelerate,’ speed up the set of the plaster or ‘retard’ it, slowing it down. On occasion loose fibers such as hair, cow dung or even woven fabrics such as burlap might be added to a plaster to increase its tensile strength if applied in a system that subjects it to unusual shear forces.

Types of Plaster  

There is a surprisingly varied range of vocabulary used to describe and subdivide types of plaster. Many of these are regionally specific and vigorously defended. You might hear terms such as render, coating, grout, mud, dash, harling, parging, daub, to name a few. I’ll describe the three most common divisions for describing plaster that have come into widespread use in the United States and Canada.

Plaster – plaster used as a coating in interiors, for moldings or for ornamental casting

Stucco – plaster used as a coating in exteriors

Mortar – plaster used to bond masonry units or rubble

I have definitely heard exceptions to these generalizations here in North America. Some masons would vehemently deny that mortars are plasters whereas others are more accommodating. Furthermore, I would say that all of the aforementioned descriptions are practically meaningless throughout the United Kingdom and Europe where local tradition and terminology predominates. Perhaps that is only to be expected in places where folks have had their own way of doing things for centuries. For thousands of years until as recent as the 19th century only a small handful of binders, that is to say unique minerals, were used to make plasters, stuccoes and mortars: clay, gypsum and an entire family of limes. In my next essay in the series, I'll introduce these amazing minerals including an overview of the physical properties that make them so special and useful for plaster.


Contributed by Patrick Webb