This heating process drives out water from the mineral. When calcium sulfate is processed during the manufacture of plaster, it is heated up. This refers to how their ingredients are heated during production. Broadly speaking, different types of plaster can be divided into two categories. On the one hand, the term Plaster of Paris is sometimes used to refer to something called ‘beta gypsum’. Different uses of the term ‘Plaster of Paris’. Since then, Plaster of Paris has become a term used to refer to various kinds of gypsum. The response in Paris was to protect their flammable timber buildings with a layer of fire retardant plaster. Fire tore through the city of London consuming flammable timber buildings. This was because in 1666 the city of London, England was destroyed by the Great Fire of London. Back in the 17 th century, King Louis IV of France demanded that the buildings of Paris were clad with gypsum. The city of Paris rests on a large supply of high-quality gypsum ( source). Plaster of Paris gets its name from its historical origins. This can be a bit confusing, so I will try to tease out these differences. Plaster of Paris is an umbrella term that is often used to refer to quite a few different types of plaster. Plaster of Paris is cheaper and more easily available than Pottery Plasters, so let’s take a look at it first… What is Plaster of Paris? This in turn hardens the slip, turning the liquid clay into greenware pottery. The water from the clay is drawn out of the slip by the pores in the plaster mold. For example, when it’s used as a casting mold, liquid clay (called slip) is poured into the plaster mold. One of the reasons plaster is so handy for potters is that is porous and absorbent. These capillaries are tiny microscopic open channels in the plaster that make the hardened plaster porous. The crystals also form capillaries in the plaster. The crystals form a matrix-like structure in the plaster that gives it strength. These crystals are what make the plaster harden. When the calcium sulfate bonds with the water, it changes from hemihydrate to dihydrate.ĭuring the setting reaction, crystals are formed in the mixture of water and calcium sulfate. This is a chemical reaction where the water recombines with the calcium sulfate particles. This is because, when it is mixed with water a ‘setting reaction’ takes place. In its partially dehydrated state, it becomes usable as an ingredient in plaster. Once it has become partially dehydrated it is called calcium sulfate ‘hemihydrate’. During the heating process, the calcium sulfate loses some of its water content. The heating process is called ‘calcining’. Part of the processing involves grinding it up to a powder and heating it. The main ingredient in each of these types of plaster is calcium sulfate, which is similar to chalk.Ĭalcium sulfate is a mineral that is mined from the ground and processed. There are a number of different types of plaster.
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