Plaster

Plaster of Paris

History

  • A house in Paris on a gypsium deposit burned down in the 1800s.
  • A gypsium mine still exists near Montmartre in Paris.
  • Rain falling on the remains revealed that footprints hardened in the material.
  • Initially used in industry and for coating walls.
  • First used by Mathyson of the Dutch military in 1852 to make splints from cotton bandages.

Current Form

  • A muslin roll stiffened by starch or dextrose and impregnated with a hemihydrate of calcium sulphate.
  • Chemical Formula: CaSO₄·2H₂O (Gypsium).
  • On addition of water, calcium sulphate crystallizes.
  • Setting Time:
    • Sets in 5-10 minutes.
    • Fully hardens in 72 hours when it becomes completely anhydrous.

Advantages

  • Cheap.
  • Mouldable.
  • No adverse allergic reactions.
  • More expansile in case of swelling.

Disadvantages

  • Heavy.
  • Partially radio-opaque.
  • Not waterproof.
  • Weaker than fibreglass.

Synthetic Casts

Composition

  • A polymer (fibreglass).
  • Chemical Formula: C₆H₅.NCO (Scotchcast).

Advantages

  • Stronger.
  • Lighter.
  • Waterproof.
  • Available in different colors (popular with children).
  • Hardens more quickly (10-20 minutes).

Disadvantages

  • Less expansile in case of swelling.
  • More expensive.
  • Not as easy to mould.
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