A. Form of the Material
– Reversible hydrocolloid ( Agar ) is premixed by the manufacturer and supplied as a semisolid
material in tubes and sticks.
– The sticks look and feel like a long, thin pencil eraser, except they feel wet because of their high water content.
– Reversible hydrocolloid is predominantly water with added agar (a carbohydrate polymer).
–
Agar
is also the material used in microbiology as a growth medium. Other components of
reversible hydrocolloid include colorants, flavors, mold inhibitors, and a sulfate compound.
– The sulfate compound improves the hardness of the gypsum material that is poured into the impression.
B. Use
1. Equipment
– Reversible hydrocolloid (Agar) requires special equipment to heat, store, and temper the materials.
– In addition, special impression trays that circulate cooling water are needed.
– The special equipment that is required to use reversible hydrocolloid limits the popularity of this excellent material.
– Reversible hydrocolloid itself is very inexpensive and results in a very accurate impression.
2. Preparation of Material
– Reversible hydrocolloid ( Agar ) must be prepared for use before taking the impression.
– This can be done at the beginning of the day or the week.
– Special equipment, called a hydrocolloid conditioner is required.
– This equipment has three compartments:
(A) In the first compartment, the reversible hydrocolloid is boiled to change the rubbery
material (the gel) into a viscous liquid (the sol).
(B) In the second compartment, the material is stored in a 150°F (65°C) water bath until needed. Material can be stored for as long as several days.
(C) Several minutes before it is to be used for an impression, the reversible hydrocolloid is placed in a 110°F (45°C) water bath.
This step is called tempering.
Tempering lowers the temperature to a point at which the oral tissues are able to tolerate the impression material.
At mouth temperature, the material gels and returns to its elastic state.
Circulating cold water through tubing and a special tray facilitates cooling.
A hydrocolloid impression.
3. Hysteresis
– Hysteresis is unlike the common phase changes of water, which melts and freezes (or boils and condenses) at the same temperature.
It is important to note that reversible hydrocolloid does not melt at the same temperature at which it gels.
It melts at a much higher temperature that of boiling water.
On the other hand, it gels at a much cooler temperature, the mouth temperature.
The characteristic of melting and gelling at different temperatures is called hysteresis.
4. Advantages and Disadvantages of Reversible Hydrocolloid ( Agar )
– Reversible hydrocolloid works well in a wet environment.
– Therefore, reversible hydrocolloids are very useful for taking impressions when the margins of a crown preparation are subgingival or not easily kept dry.
– In fact, some dentists wet the teeth with water just before taking the impression.
– On the other hand, these materials require special equipment for heating, cooling, and use.
– Another shortcoming of these materials is their poor tear strength compared with
non aqueous elastomeric impression materials.
5. Popularity of Reversible Hydrocolloid ( Agar )
– With the various advantages and disadvantages of reversible hydrocolloid material, one would expect dentistry to have a love–hate relationship with this material.
– Few dentists use reversible hydrocolloid, but those who do, love it.
– They work around the weaknesses of the material and are able to produce excellent results.
