- Plaster is a combination of lime (or gypsum —calcium sulfate) and water used as a coating on walls or other surfaces.
- Cement is ground calcinated limestone mixed with clay, making it harder than plaster.
- Concrete is made by mixing cement with sand and pebbles to increase its strength.
Because all three of these materials are widely used today, we tend to forget that they were also used in antiquity and in some regions even in prehistoric times. Since the three all tend to grade into each other as the proportions of different ingredients vary, it is sometimes difficult to differentiate one from another, and archaeologists tend to use the term "plaster" for all three, and may therefore speak of a "plastered floor," for example.
The critical ingredient in all of these is lime, a cover term for various forms of calcium oxide, procured by smashing and heating ground limestone, gypsum, or seashells. The combination of smashing and heating eventually results in a fine powder. Mixed with water to make a kind of mud, this powder produces a hard, smooth surface, and one capable of being painted, qualities valued in the past just as they are today.
The pictures here, from a small, traditional cement plant in Honduras, show (1) quarried rocks, smashed with sledge hammers, (2) the underground lime oven, and (3) the resultant cement being packed in bags. The fuel used is wood chips, a byproduct of logging.
When various kinds of plaster and cement were widely used in antiquity, the immense fuel requirements necessary to produce this product sometimes resulted in deforestation and other environmental damage, particularly if the same people cultivated animals such as sheep or goats that tended to eat the small shoots of trees or other important fuel plants.
Photos by DKJ
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