DR AMIT AGRAWAL.
1ST PG PROSTHO DEPT.
JAIPUR DENTAL COLLEGE
A negative likeness or copy in reverse of the
surface or object.
An imprint of the teeth and adjacent structures
for use in dentistry
ZnO - Eugenol
O‟Brien, Dental Materials & their Selection
Alginate is classified as irreversible
Hydrocolloid because it consists of particles
of a gelatinous (colloidal) state in water
(hydro) and irreversible because once it has
jelled it cannot be returned to a liquid
The Word Alginate comes from the term
It is a peculiar mucous extract yielded by
certain brown seaweed ( algae ) .
The term „ALGIN‟ was coined by chemist
from scotland @ the end of 19th century.
TYPES Type I – Fast setting.
Type II – Normal setting.
MODES OF SUPPLY
It is supplied as a powder that is packed:
a) In bulk or in tins or in sackets
b) In preweighed individual containers.
A plastic scoop is supplied for dispensing the
bulk powder, & a plastic cylinder is supplied
for measuring the water .
1) Used for impression making :
a) when there are undercuts
b) In mouth with excessive flow of saliva
c ) For partial dentures with clasps.
2) For making preliminary impressions for
3) For orthodontic and study models.
4) For duplicating models.
from seaweed; is used as a thickening
with the potassium alginate to form the
Trisodium phosphate (2%)
to slow down the reaction time for mixing.
Diatomaceous earth( 60 %)
filler that adds bulk to the material.
Zinc oxide (4%)
bulk to the material.
Potassium titanium fluoride(3%)
so as not to interfere with the setting and
PHYSICAL PHASES OF ALGINATE
The first phase is a sol (as in solution). In
the sol phase, the material is in a liquid or
The second phase is a gel. In the gel
phase, the material is semisolid, similar to a
Reaction occurs by a chemical reaction.
Soluble alginate reacts with calcium sulphate
to produce insoluble calcium alginate as a
The production of calcium alginate is delayed
by the addition of a third soluble salt to the
solution,with which the calcium sulphate will
react in preference to the soluble alginate to
form an insoluble calcium salt.
In a nutshell , when powder is mixed with
water to obtain a paste ,two main reaction
occur during setting :
1) 2Na3P04 + 3CaSo4---> Ca3(Po4)2+3
2) sodium alginate +
CaSO4 + H2O -------- > Ca alginate +
First , sodium phosphate reacts with the
calcium sulphate to provide adequate
working time. Second ,after the sodium
phosphate has reacted, the remaining
calcium sulphate reacts with sodium alginate
to form an insoluble calcium alginate which
forms a gel with water.
An adult mandibular impression generally
requires two scoops of powder and two
measures of water.
An adult maxillary impression generally
requires three scoops of powder and three
measures of water.
Fluff or shake the powder to distribute the
powder particles evenly
Use the specific measuring devices (water &
powder) provided by the manufacturer for
Follow the manufacturer‟s direction regarding
the ratio of water to powder.
Use exact measurements
After the water and powder have been
measured, place the water in a clean dry
bowl. Shift the powder into the water. Adding
the powder to the water ensures the powder
particles are wet evenly. If mixed in reverse
(the water is added to the powder) the
chemical reaction will start early with some
particles setting faster than others.
Mix the alginate for the specific amount of
time and using a stiff spatula “swipe” the
alginate mass against the sides of the bowl
to avoid entrapment of air in the mix.
SETTING TIME OF ALGINATE
Since alginate reacts or gels chemically,
temperature is a major factor in the setting
time. The colder the temperature of the water
the longer it takes to set; conversely, the
higher the water temperature the faster it
sets. Ideal water temperature is 68 degrees
or room temperature.
LOADING OF TRAY
A perforated tray is used so that the material
is forced out slightly through the holes in the
tray during loading,thereby loacking itself
mechanically into the tray.
The surface of the alginate in the tray may be
smoothened out by moistening the finger
with water and running it over the surface of
SEATING THE TRAY
Before making the upper impression, saliva
should be wiped off from the palatal region.
In order to prevent the material from flowing
along the throat and causing nausea to the
patient,the posterior portion may be seated first
and then the anterior portion seated properly.
Since the material sets from tissues towards
periphery,any movement during gelation may
result in distortion.
So once the tray is seated ,it must be held in
place firmly without any movemnets.
REMOVAL OF THE IMPRESSION
An alginate impression when set develops a
very effective peripheral seal.
This seal should be freed by running the finger
around the periphery.
The impression must be removed suddenly
,with a jerk.
After removal from the mouth,
- washed with water to remove saliva.
-cast should be poured as soon as possible.
Normal set alginate
Working time of 2 minutes and a setting time of up to 41/2
minutes after mixing.
Fast set alginate
Working time of 11/4 minutes and a setting time of 1 to 2
The time allowed for mixing the alginate, loading the tray,
and positioning the tray in the patient's mouth.
The time required for the chemical action to be
TAKING AN ALGINATE IMPRESSION
Explain the procedure to the patient:
material will feel cold, there is no unpleasant
taste, and the material will set quickly.
Breathe deeply through your nose to help you
relax and be more comfortable.
Use hand signals to communicate any
AN ACCEPTABLE ALGINATE IMPRESSION
There is a complete "peripheral roll," which includes
all of the vestibular areas.
The tray is not "overseated," which would result in
exposure of areas of the impression tray.
The impression is free from tears or voids.
There is sharp anatomic detail of all teeth and soft
The retromolar area, lingual frenum, tongue
space, and mylohyoid ridge are reproduced in the
The hard palate and tuberosities are recorded in the
CAUSES FOR DISTORTION AND DIMENSIONAL
CHANGE OF ALGINATE
If an alginate impression is stored in water or
in a very wet paper towel, the alginate will
absorb additional water and expand. This
condition is called imbibition.
If an alginate impression remains in the open
air, moisture will evaporate from the material,
causing it to shrink and distort. This condition
is called syneresis.
RECENT ADVANCES IN ALGINATES
Dust free alginatesDe-dusting agent (glycerin or glycol)
Siliconized alginatessilicon polymers are added to strengthen the material
Alginates containing disinfectantseg quaternary ammonium salts or chlorohexamine are added
to alginate powder.
Hard and soft set alginatesby adjusting the amount of fillers
It is about 14% at a stress of 1000g/cm2.
Lower w/p ratio results in lower flexibility.
2)ELASTICITY AND ELASTIC RECOVERY
alginate are highly elastic but less when
compared to the agar.
97.3% elastic recovery occurs.
Permanent deformation is less if the impression is
removed from the mouth quickly.
The compressive strengths of alginate gels
range from 5000 to 8000 g/cm2.
The tear strengths vary from 350 to 700 g/cm2.
Both of these properties are time dependent
with higher values being obtained if the time
of removal is delayed.
4) DIMENSIONAL STABILITY :
Alginate impression loses water by
evaporation and shrinks on standing in air.
If it is placed in water it absorbs water and
Therefore ,cast should be poured immediately
after making the impression.
Alginate impression material have a short shelf
They deteriorate rapidly when stored at
elevated temperatures in a humid
Therefore ,it is better not to stock more than 1
year supply .
ADVANTAGES OF ALGINATE
It makes an accurate impression
It allows for undercuts
The process is not time consuming
It is easy to work with, has good viscosity &
is low in adhesive qualities
It is comfortable to the patient
Inexpensive to the dentist
Requires little armamentarium
Can‟t be corrected.
Cannot be used alone for compressing the
It is not accurate enough for crown and