Agar

Clinical Use:

Supplied in sealed tubes to prevent evaporation of water. The tubes are heated in boiling water (in a water bath) for 10-45 minutes. Once the impression is taken the tray can be cooled with water to aid gel formation. A higher temperature is needed to convert the gel into a sol. The first material to set is that which is in contact with the tray since it is cooler than the tissues. Thus it is the material in contact with the tissue which stays in the sol state for the longest time. Agars have been largely superseded by alginates and elastomers, although are still used for complex impressions for advanced restorative work. They are often used in labs to duplicate model because they can be reused many times.

Chemistry:

• Agar (colloid)
• Borax (strengthen gel)
• Potassium Sulphate
• Water (dispersion medium)

In its natural state it a gel, but on heating becomes a sol.

Properties:

• Good surface detail
• Can be used on undercuts, but liable to tear on deep undercuts
• Evaporation or imbibition
• Non toxic and non irritant
• Slow setting time
• Poor tear resistance
• Adequate shelf life
• Can be sterilised by an aqueous solution of hypochlorite

Advantages:

1. Good surface detail
2. Reusable and easily sterilised

Disadvantages:

1. Need special equipment (water bath) and special technique
2. Dimensional instability