ALGINATE IMPRESSION MATERIAL

General Objective:
To demonstrate the proper handling and manipulation of alginate as an impression material.
Equipment and materials:
Alginate impression material 3 Cardboard squares 2 x 2
Plastic spatula Boley gauge
Rubber mixing bowl Watch / timer
Inlay ring / PVC tubes Plastic container
Vaseline / petroleum jelly Ice cubes
Wax & wax spatula Alcohol lamp
Indelible pencil 1 small Ziplock bag
Alginate measure cup and spoon

Exercise 1: Setting Time

Specific Objectives:
1. To determine the effect of temperature on the setting time of alginate.
2. To determine the effect of water-powder ratio on the setting time of alginate.
General Information:
The setting time or gelation point for the alginate hydrocolloids can be considered as the point where the
adhesive property of the mix vanishes. In clinical applications, the impression should remain in place for approximately
two minutes after gelation since additional fibrils continue to form and thus the strength of the gel increases. However,
if allowed to remain as long as five minutes after gelation, the elasticity of the gel is reduced and distortion may occur.
Two types of alginate are generally available in the market today, the fast set (1 to 2 minutes) and the normal set (2.5 to
4 minutes). There are however ways within the clinicians control were it is possible to manipulate the setting of
alginate. The most common methods are altering the water/powder ratio, the mixing time or the temperature of the
water. The temperature of the mixing ingredients as well as the environment has an appreciable effect on the gelation
time. The use of cold water to prolong the setting of alginate is perhaps the safest method that can be employed without
having any subsequent effect on its properties.

Procedure:
1. Three tests will be made using the 3 conditions given in the table below.
2. To mix, place the required amount and condition of the water (as stated in the table below) in the rubber bowl. Then
measure 1 scoop of alginate. When dispensing the alginate powder the measuring spoon should be slightly
overfilled, then leveled off using the straight edge of the alginate spatula (avoid over compacting). Sift the powder
into the water and stir slowly to wet the powder particles. Once the powder has been moistened, continue mixing
using a vigorous figure-8 folding motion. Swiping the material against the sides of the bowl and the spatula ensures
that all the powder particles are wetted by water and presses out any entrapped air. Rotating the rubber bowl slowly
during mixing helps spread out the material against the sides of the bowl. Mixing for 45 60 seconds is generally
sufficient to obtain a homogeneous and creamy consistency. An inadequately mixed alginate will appear grainy.
3. Record the time of the start of mix and the time when the alginate gels. Alginate is said to have gelled if it the mix
does not adhere when touched (loss of tackiness)
Mix Condition Gelation time
1 Cold water; W/P ratio 1:1
2 Tap water; W/P ratio 1:1
3 Tap water; W/P ratio 1.25:1

Guide Questions:
1. What are the factors that can affect the setting time of alginate?
2. Of the factors that can alter alginates setting time, what are within your manipulative control?
3. How would you rank the 3 mixes in terms of ease of manipulation?
Exercise 2: Storage Conditions
Specific Objectives:
To relate how the environment can affect the accuracy of alginate as an impression material.

General Information:
Since hydrocolloids are approximately 70-85% water, once the material is removed from the mouth and
exposed to the environment, the material gaining (imbibition) or losing (evaporation and syneresis) water is bound to
occur. This gain or loss in water is accompanied by a significant dimensional change in the material. In cases where
pouring of the stone must be delayed, storing the impression in an environment with 100% humidity is recommended
to minimize any loss or gain of water. There is however no adequate storage for any hydrocolloid impression material
without risking any dimensional changes. For best result, impressions made from hydrocolloids should be exposed to
air for the shortest time as possible and that the construction of the cast be done immediately.

Procedure:
1. For this exercise 3 mixes of alginate will be placed in three different environments. Make 3 cardboard boxes 50mm
x 50mm x 10mm in dimensions. Place a thin film of petroleum jelly on the floor of the boxes.
2. Measure the inside diameter of the inlay ring using the Boley gauge and set markers on opposite sides to serve as
the reference points. The same reference points will have to be used for all the 3 mixes of alginate. Place a very thin
film of petroleum jelly on the outer and inner surface of the inlay ring.
3. For each of the 3 mixes use 1 measure of water to 1 scoop of alginate powder. Spatulate until a smooth and creamy
consistency is obtained (mix the alginate in the same manner as described in the previous exercise). Gather the
alginate with your spatula, place it on the box and spread the material to an even thickness.
4. Seat the end of the inlay ring (with the markers) into the center of the unset alginate and allow to gel undisturbed.
When the alginate has gelled, carefully break the seal of the impression material with the outer surface of the inlay
ring. With the cardboard box held firmly on the bench, snap the ring from the impression with a purely vertical pull.
Do not rotate or twist the inlay ring as this may change the reference points in relation to the set alginate.
5. While the alginate core is still enclosed within the inlay ring, mark the core to correspond with the markers placed
on the inlay ring. Remove the alginate from the inlay ring and immediately measure the diameter. The diameter of
the alginate core should be the same as the inner diameter of the inlay ring. Avoid any compressions or pressure on
the alginate core when measuring.
6. After measuring store 1 each of the alginate cores at the given storage conditions below. Thus 1 alginate will be
placed in a container with water, 1 exposed to air, and the other placed in a humidor. Measure the alginate cores
again after 1 hour and 24 hours. Record the dimensional accuracy of the three mixes on your data sheet.
Dimensional accuracy may be qualitatively determined by placing the core back into the ring at the times indicated.
Mix Water temp. Storage Time to check accuracy
1 70F Water Immediately, 1 hr., 24 hrs.
2 70F Air Immediately, 1 hr., 24 hrs.
3 70F 100% relative humidity Immediately, 1 hr., 24 hrs.

Note: 100% relative humidity can be achieved by placing the core impression in a water-saturated paper towel
under an overturned rubber bowl or in a closed container lined with moistened urethane foam rubber. Use the
same caliper throughout this exercise.
Mix/Storage Immediately 1 hour 24 hours Fit in inlay ring
1 Water
2 Air
3 Humidor
* For each time check, specify whether the fit of the alginate core into the inlay ring is good, poor, very poor and
indicate the nature of dimensional change: expansion (+), contraction (-), or no change (0).

Guide Questions:
1. Define evaporation, syneresis, and imbibition.
2. Is there any adequate storage condition for a hydrocolloid impression?
3. How may one ensure a dimensionally accurate cast made from a hydrocolloid impression?