Classification of dental gypsums: how to choose the right one

Dental gypsum, in other words calcium sulphate, is one of the most commonly used materials in dentistry and orthodontics; its use is not limited to the development of dental arch impressions, it is in fact implemented in many other phases, such as blocking the models in the articulator and creating flasks.

Owing specifically to the multiple applications of this material in the dental sector, several gypsum formulations are available, each with its own characteristics, capable of adapting to every phase in the laboratory.

Gypsum is found in nature and undergoes pulverisation and calcification before being marketed in the form of a powder which, once mixed with water, creates a mixture malleable for a short period of time, capable of solidifying shortly after having assumed the desired shape.

To obtain quality models, the utmost precision is required during the various processing phases; specifically, the liquid-powder ratios recommended by the manufacturers should be observed, as well as the times frames and methods of spatula spreading, which should avoid the inclusion of air pockets which would cause bubbles to form.

The setting reaction occurs during the mixing process, the duration of which determines the setting time.

Dental gypsum types

Depending on the processes the gypsum undergoes before being marketed, distinctive characteristics are obtained, requiring different quantities of water during the mixing phase, allowing it to be classified into 5 types.

  • Type I/II – Soft gypsum for impressions
  • Type III – Soft gypsum
  • Type IV – Dental stone for models
  • Type V – High strength dental stone for models

As you move down the table, the setting time of the gypsum decreases, as does the quantity of water necessary for the same quantity of powder and the setting expansion percentage. Consequently, the hardness of gypsums and their ability to retain the details reproduced faithfully over time increase.

Differences between different types of gypsums

The choice of the right type of gypsum to use depends on the situation in which it needs to be applied. Type I and II gypsums are very soft and can be considered clinical impression materials rather than laboratory materials.

When developing a model, a unitary model or a divisible model may be desired. In the first case, type III and IV gypsums are more commonly used, especially type 3 which is normally used for duplicates, whereas type IV is used to create waxing, or to mould provisional plaques.

Type IV gypsum, like Zhermack Elite Rock, owes its popularity to its easy use, its relatively quick setting time and to good accuracy in reproducing details; in addition to details and to the presence of a smooth, even surface, it is important to try to find a suitable hardness.

Indeed gypsum undergoes incessant manipulation and stresses which could impair its integrity. It is advisable to keep the models in good condition until the end of the care plan because they constitute a useful tool in the evaluation of the patient’s prior status.

Type V gypsum is used in those circumstances which require specific dimensional stability and resistance. The choice of the type of gypsum is crucial and it constitutes a vital step in allowing the proper development of certain clinical and laboratory phases.

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