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.