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.


  • 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


  • 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

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

Latest Comments
  1. nuJA^-^

    Nice Page!!

    This article helps me so much.

    Thanks a lot!

    but i still wanna know about w/p ratio altering & tear strength.

    ?^ _______ ^

  2. Prof. Brian W. Darvell

    I am afraid that the “setting reaction” is not – what is given is an example of a retarder reaction. The setting reaction is missing.

  3. dr pratik chopra

    nice article.but i want 2 know the sorce of alginate.

  4. celia vinny v

    nice article
    helps me a lot
    i want to know that what is the important of Coloring & flavoring agents or what s role of that in alginate.

  5. Dr. Tricie

    ^ Coloring and flavoring agents are added to make the alginate palatable and pleasing to look at.

  6. sk

    alginic acid obtained from brown seaweed is the source of alginate

  7. sukanya

    great article !

  8. ash

    i have a viva this coming monday

    portion is manipulation of impression comp alginate n zinc oxide eugenol
    can i know wat is likely to be asked?

    the previous batch was asked why is kneading for a long time avoided???

  9. subu thomas

    nice and simple piece of information

  10. Pankaj Rajoriya

    nice article
    helps me a lot…..twmroow is my prac. n viva…..!! it is commanly used for which type of patients…??

  11. Rodrigo Perez Villarreal

    very good post! I need someone to recommend me some brands of this product to test. thank you very much.

  12. payaal rani

    nice article thanx for publishing this artticle, this helps me alot,,, what is the ratio of powder & water?

  13. Adeline

    This is a very nice article. Thank you for publishing it, really helpful.

  14. Zahid

    Thanks for there articles they help me so much,
    plz add all the remaining articles

  15. nidhi

    I have problem in manipulating alginate…impressiontray me dalne se pehle hi set hone lagte hai…wat to do??

  16. Varun

    Use Cold water to mix Alginate and if using metal tray cool them before use which will retard the setting time of Alginate considerably

  17. Aditi Upadhyay

    I need to know the W/P ratio of alginate.

  18. krimal patel

    Your notes is very helpfull
    Thanks a lot sir

  19. dr marudhaa

    i know the composition of alginate
    but cant tell what is the differant btween chromatic and soft dustfree and hight stability fast setting

  20. Kdcz

    I would like to know if alginate, used for non-oral impressions, can be safely composted.

  21. Varun

    Yeah alginate is also used in many other impressions for making prosthesis like Eyes, Ears, nose etc

  22. Courtney Birke

    I do life castings of children/babies hands and feet. I’d like to know why in most videos I’ve watched the complete set alginate mould is stiffer & dryer than when I do mine? Am I not using the correct mixture? Please help

  23. Varun

    What type of Alginate are you using??
    There are many factors which play a role in the end result like, the water to powder ratio, Mixing style and type, water temperature.

  24. Mangesh Satam

    Why chromatic alginate is not stable at higher temperature 40°C. What raw material is needed to add to keep the powder stable through out its shelf life besides addition of Diatomeous earth powder, Calcium Sulphate, potassium hexaflouro titante, Sodium Phosphate, zinc oxide, Potassium Alginate, Indicator, color and flavour.
    kindly give your suggession.

  25. priyalal

    Very helpful information in short

  26. Nick Mast

    i was burned with the initial impression for a tooth cap. My mouth developed kanker sores from the chemical in the plastic like impression material. it was terriable.

  27. jennifer

    Hi. Do you know the perfect ratio of W/P?
    I tried 2/1ratio. But it starts to come out with water after setting. I tried two brands, Kromopan and Heraplast.
    If I used 1/1 ratio, the results is good, elastic and precise impression. But it is really difficult for me to stir the mixture, when I need to cast a hand. It is too heavy to stir it vigorously.
    Except for the temperature of water, what else should I pay attention to?
    Can I pour the mixture 3-4 times in the some container? Decrease the amount of mixture so that i am able to stir it quickly and then increase the number of pouring times?

  28. Varun

    To cast a hand it takes a lot of alginate and in the post we have talked about dental impressions which is a lot less in quantity. So apart from the water temperature you can make sure that the quantity of the water to the is not premeasured but decide as you start mixing.

    What I mean to say is, take the required amount of Alginate powder in the mixing bowl and slowly start pouring cold water as you mix so that you can decide while mixing what is the correct consistency you require. This is one of the best ways to mix any alginate in large quantities.

  29. Thomas

    Great post! Have nice day ! :) ysneb

  30. Mario Marini

    I inhaled the powder from back filling the trough. I keep getting bronchitis now is there a connection

  31. Varun

    I have certainly heard it for the first time, will get back to you after doing some research on whether Alginate powder can cause Bronchitis.

  32. Faiez N. Hattab

    Nice concise review but lacks the essential element of not giving any references!

  33. Varun

    There is some problem with the website design which I not showing the references box, so added the references to the post itself. Thank you for pointing it out.

  34. Marie

    If you have allergies to color and dyes can this material be used to take an impression of your teeth?

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