Alginate Impression material

Alginate Impression material is an Elastic, Irreversible Impression material which is used to take the impression of both Dentulous and Edentulous Impressions. It is made from seaweed and is made with components like Sodium alginate, Calcium sulfate and other ingredients which act as retarders. Alginate material is available in powder form which turns into Gel when mixed with water and further converts into elastic material on manipulation.

This process of conversion from powder to Gel and then solidifying into elastic material is used to take an impression of the oral cavity in dental clinics. In medical practice the Alginate impression material is used widely to take impression or other parts of the Face like Nose, Ear etc to help make prosthetic organs.

TYPES of Alginate based on Setting Time:

  • Type I fast setting : 1-2 min.
  • Type II normal setting : 2-4.5 min.

Available as :

  • Bulk powder
  • Pre weighed packages
  • Color changing alginates: violet during mixing, pink when ready to be seated and white when set

Composition of Alginate:

  • Ester salts of alginic acid(sodium or potassium or triethanolamine alginate) – 15%
  • Calcium sulphate (reactor) – 16%
  • Zinc oxide – 4%
  • Potassium titanium fluoride – 3%
  • Diatomaceous earth – 60%
  • Sodium phosphate – 2%
  • Coloring & flavoring agents traces

Setting reaction:

  • Primary reaction: 2Na3PO4 + 3CaSO4 -> Ca3 (PO4)2 + 3Na2 SO4
  • Reaction with Retarders to increase the setting time: Na Alginate + 3 CaSO4 -> Ca Alginate + 3Na2SO4

Setting takes place due to the reaction of Sodium Alginate reacting with Calcium Sulphate – this is the primary reaction which takes place. The other ingredients used like Calcium Phosphate acts as a retarder which is used to manipulate or increase the setting time to an adequate Type I or Type II setting time as required.

Setting Time:

  • Type I (fast set) : 1-2min
    Type II (normal) : 2-4.5 min.

Control Of gelation time:

Under control by manufacturers:

  • By the amount of retarder added during manufacturer

Under Control of clinicians

  • By altering the W:P ratio
  • By changing the mixing time.
  • By altering the temp.of water
  • Increase in temp. decreases gelation time & vice versa.
  • Premature gelation causes distortion in the impression & it is rendered useless.
  • Prolonged gelation time is tedious for both pt. & the dentist.

Manipulation :

The Powder is taken after inverting the can several time to produce uniform distribution of the filler before mixing with water. The top of the can should be taken off carefully to prevent the silica particles from being inhaled. The proper W/P ratio as pacified by the manufacturer should be used measured quantity of water and powder are added in the rubber bowl & mixing is started, with a stirring motion to wet the powder with water. Once its moistened rapid spatulation by swiping or stropping against the side of the bowl is done. A vigorous figure eight motion is used.

  • Mixing time:Working time
  • For fast set alginate45 sec.1 ¼ min.
  • For normal set alginate60 sec.2 min


  • Taste & odor: ·Pleasant
  • Flexibility:·lower w/p ratio results in lower flexibility.
  • Elasticity & elastic recovery: highly elastic, permanent deformation less if the impression is removed from the mouth quickly.
  • Reproduction of the surface detail: ·lower than agar.
  • Strength: ·compressive strength : 5000-8000 gm/cm2
  • Tear strength : 350-700 gm/cm2
  • Dimensional stability: Poor

Biologic properties:

  • Silica present in the dust which rises from the can after fluffing alginate powder is a health hazard.
  • Dustless alginate is also present in the market which contain glycol.


  • Used for making primary impression of edentulous patients with undercut ridges.
  • Used for preliminary impressions for complete dentures.
  • Used for making impressions for dentulous patients. For construction of study models & temporary removable prosthesis.
  • Used for making impression for orthodontic study models.
  • Used for making impressions for the preparation of mouth protectors for athletes.
  • Used as duplicating material even in medical field for making artificial prosthesis like nose, ear, eyes etc.


  • ease of mixing & manipulation.
  • no elaborate equipment required.
  • material is elastic & comes out easily from undercuts.
  • economical
  • impression can be removed easily & has pleasant taste & odor, so comfortable for the patient.


  • The impression cannot be electroplated.
  • Comparatively poor tear strength especially in thin sections and inter dental areas  in comparison to Elastomeric Impression material.
  • Dimensional stability is poor, due to loss of water the impression sets to shrink overtime.
  • Distortion may occur if tray is shifted during setting in the oral cavity. Cannot be manipulated once the impression is set.
  • Cannot be refined after taking the impression.

Types of failures:

1. Defects: Can be caused due to the following reasons, which can be human errors or material problem.

  • Improper mixing – over or under mixing with unset powder left in the improession
  • Prolonged mixing – mixing beyond the mixing time will lead to lack of bond formation leading to tearing of material
  • Undue gelation
  • W/P ratio is too low –


  • Inadequate bulk – thin segments tend to tear, in placed without adequate material tearing is common
  • Moisture contamination
  • Premature removal from mouth: Removal before completion of setting time will lead to tearing of the impression
  • Prolonged mixing

3.External bubbles:

  • Undue gelation preventing flow
  • Air incorporated during mixing

4.Irregularly shaped voids:

  • moisture

5.Rough or chalky stone model:

  • inadequate cleaning of the impression
  • excess water left in impression
  • premature removal of model.

6. Distortion:

  • impression not poured immediately


O’Brien, Dental Materials & their Selection,  Phillips Science of Dental Materials, Basics Dental Materials by John J Manappallil.

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