Diagnostic Impressions

By Dr.Swathi Pai on Tuesday, January 13, 2009 with 0 comments

Irreversible hydrocolloid (alginate)-ingredients:

1.Sodium alginate (18%): dissolves in water and reacts with calcium ions.

2. CaSO4 2H2O (14%): reacts with sodium alginate to form an insoluble calcium alginate gel.

3. Diatomaceous earth (56%): controls consistency of mix and flexibility of set impression.

4. Potassium sulfate (10%): counteracts inhibiting effect of a colloid on gypsum.

5. Trisodium phosphate (2%): retarder (controls w.t.)—amount of this differentiates slow from fast set!!

6. Glycols: limits dust.

7. Wintergreen: taste.

8. Disinfectants

Setting reaction of alginate:

H2O + potassium alginate + CaSO4 2H2O Calcium alginate gel + KSO4

There must be 3-6mm of material uniformly surrounding the structures being impressed.

The water temp is the true variable—rate of set is doubled with an increase in temp 10C.

-Mixing time:

-Type I (fast-set): 45 sec.

-Type II (reg-set): 1 min.

-Working time:

-Type I (fast-set): 1.25-2min.

-Type II (reg-set): 2-4.5min.

Leave impression in place for 2-3min. after it loses its tackiness—this maximizes tear strength and limits permanent deformation.

Impression should be poured within 15min.—if not:


distortion accompanied by the exudation of salts and H2O.

This is not evaporation.


distortion accompanied by the imbibing of fluids.

Trim cast 1-3 hours after separation.

Category: Prosthodontics Notes



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