Introduction
Hydrocolloids are colloidal suspensions of polysaccharides in water. They exist as a less viscous fluid, the sol, and as an elastic gel. The material goes into the mouth as a sol and turns into a gel by a physical or chemical change.
Shared properties
- High accuracy: low viscosity and good wetting of moist surfaces.
- Sensitive to water: they swell and shrink easily.
- Disinfection: they cannot be soaked in solutions. Makers add disinfectants, often thymol, and offer sprays.
- Effect on gypsum: colloids disturb its setting, so accelerators such as potassium sulfate or fluorides are added to the material.
Reversible hydrocolloids (agar)
These contain 10 to 15% agar-agar, a seaweed product, with 80 to 85% water, small amounts of borax for gel strength, potassium sulfate, fillers and additives. The change between sol and gel depends only on temperature and can be repeated.
- A three-compartment thermostat liquefies the material and holds it at the right temperature.
- It is placed at about 43 °C in a tray warmed to 45 °C.
- Cold water flows through the double-walled tray to cool it.
- After at least 5 minutes, the impression is removed with one sharp movement, without twisting, and poured at once.
Residual deformation is about 1%. The material has no volume change of its own when handled precisely, which once made it the standard. However, the gypsum expansion is then not compensated, and it needs extra equipment, so its use is now limited. It is still used in the laboratory for duplicating models, at 50 to 60 °C.