Potter's Friend



What is Pottery Plaster?

The history of plaster

The story goes that a shepherd built a fire for his night's vigil, on an outcropping of soft white rock. It burned there throughout the night, and in the morning the shepherd was quick to note that the rock on which he had built the fire, had crumbled under its heat. When the shepherd emptied his water jug into the embers of the fire to extinguish it; a portion spilled into the white powder which turned from a liquid into a mysterious stone-like slab.

The first known use of plaster dates back over 9000 years and was used in the building of the first pyramids of Egypt. Then known as alabaster, it was used to decorate and seal the tombs of the Great Pyramids.  

So plaster as we know it today was found to be made from the mineral gypsum.

The process of making a plaster mould was discovered about the 18th century when plaster moulds of different types were known and used for the forming of plastic clay.

Gypsum- the mineral

 Physical Characteristics

  • Color is usually white, colorless or gray, but can also be shades of red, brown and yellow. Gypsum crystal
  • Luster is vitreous to pearly especially on cleavage surfaces.
  • Transparency crystals are transparent to translucent.
  • Crystal System is monoclinic; 2/m
  • Crystal Habits include the tabular, bladed or blocky crystals with a slanted parallelogram outline. The pinacoid faces dominate with jutting prism faces on the edges of the tabular crystals. Long thin crystals show bends and some specimens bend into spirals called "Ram's Horn Selenite" Two types of twinning are common and one produces a "spear head twin" or "swallowtail twin" while the other type produces a "fishtail twin". Also massive, crusty, granular, earthy and fiberous.
  • Cleavage is good in one direction and distinct in two others..
  • Fracture is uneven but rarely seen.
  • Hardness is 2 and can be scratched by a fingernail.
  • Specific Gravity is approximately 2.3+ (light)
  • Streak is white.
  • Associated Minerals are halite, calcite, sulfur, pyrite, borax and many others.
  • Other Characteristics: thin crystals are flexible but not elastic, meaning they can be bent but will not bend back on their own. Also some samples are fluorescent. Gypsum has a very low thermal conductivity (hence it's use in drywall as an insulating filler). A crystal of Gypsum will feel noticeably warmer than a like crystal of quartz.
  • Notable Occurances include Naica, Mexico; Sicily; Utah and Colorado, USA; and many other locallities throughout the world.
  • Best Field Indicators are crystal habit, flexible crystals, cleavage and hardness.

    Making plaster -the technical process

    The mineral gypsum is found in large deposits throughout the world. It is a mineral which chemically, is calcium sulphate plus about 20% water. Gypsum rock is mined in the midlands of the UK but deposits are also found in Nova Scotia, in the western part of the USA and many other areas of the world.

    Although usually white, it may be found in shades of pink, yellow, brown or even black. To change gypsum rock into plaster, the rock must undergo a heat treatment, called calcination. This is done by placing the crushed rock in a heated vessel (calciner) at C for a period of time. Approximately two thirds of the water content of the rock is removed during this process to produce pottery plaster (beta plaster). But if the gypsum is made into a slurry first and then put into large autoclave (pressure cooker) heated to around 220C then the harder alpha plaster is formed.

    Pottery plaster is off white in colour and chemically is the hemihydrate of calcium sulphate. The powder when mixed with water is used to produce working moulds for the production of pottery items. During this remixing with water a chemical reaction takes place called ‘recrystallization’ which creates a strong rock like material. This rocklike material is highly porous allowing the water to be sucked from the slip during the casting process.

     Recommended reading

    Find out more about using plaster in my article how to make pottery plaster moulds.


    A range of pottery related books are available depending on your interest. Click here to find my recommended list of pottery books



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