Definition: Agar is a reversible hydrocolloid impression material. Chemically, it is a galactose sulphate polysaccharide that forms a colloid with water. It exists as a hydrosol (liquid) when heated and a hydrogel (solid)when cooled, allowing it to record oral tissues and then be removed from undercuts as an elastic material.
Clinical Case
A 35-year-old patient needs an impression for an indirect restoration and you have access to a chairside agar system. You syringe a low-viscosity agar around the preparation for detail, load higher-viscosity tray agar into a water-cooled special tray, then cool it intra-orally to gel and capture an accurate impression before pouring promptly.
Uses
Accurate impressions where a reversible hydrocolloid system is available
Historically used for high-detail impressions in dentate patients
Still seen in some settings for duplicate models (in controlled environments), noting modern cross-infection constraints
How to use agar in clinical practice
Equipment required:
Temperature-controlled water bath
Special metal water-cooled tray
Syringe delivery option for low-viscosity agar
Step-by-step workflow:
Liquefy the agar in the high-temperature bath until it becomes a viscous liquid (around 8-12 minutes)
Store the liquefied agar at 63-66°C (keeps it workable without overheating)
Load tray material into the special tra and temper at 46°C so its comfortable and safe for tissues
Syringe agar around teeth/prep margins for surface detail
Seat the tray and connect the water supply only after placement
Circulate cool water through the tray (around 13°C) for patient comfort and to avoid thermal shock
After around 5 minutes, the agar gels and the impression can be removed.
Tips:
Do not overheat agar: overheating causes polymer breakdown and worsens performance.
Use ~13°C cooling water: too cold risks painful thermal shock.
Although agar is reversible and can be reheated, each reheating causes polymer degradation → it becomes stiffer; do not reheat more than ~4 times (and in modern practice, recycling is generally avoided due to cross-infection concerns).
Constituents
Component | Approx. amount (%) | Purpose |
|---|---|---|
Water | 85.5 | Continuous phase |
Agar | 12.5 | Dispersed phase / gel former |
Borax | 0.2 | Strengthens the gel (pH control) |
Potassium sulphate | 1.7 | Accelerates gypsum model setting (counteracts borax effect) |
Alkyl benzoate | 0.1 | Prevents mould growth |
Dyes/flavouring | Trace | Appearance and taste |
Note: Borax can slow gypsum setting and contribute to a soften model surface; potassium sulphate is included to offset this