Alginate impressions Notes

Elastic Impression Materials Agar And Alginate

The rigid impression materials described previously are best suited for recording edentulous areas. Teeth or severe undercuts if present, would make the removal of such impressions difficult. The impression could distort or fracture on removal.

The ideal impression material for accurately reproducing tooth form and relationship would be an elastic substance that can be withdrawn from the undercut area and return to its original form without any distortion.

By definition, an elastic impression material is one that can transform from a semisolid, nonelastic state to a highly elastic solid state.

Read And Learn More: Basic Dental Materials Notes

Two systems are used

• Hydrocolloids
• Elastomeric materials

Hydrocolloids

Solution and Suspension

• In a solution (e.g. sugar in water) one substance, usually a solid is dispersed in another, usually a liquid and the two phases are microscopically indistinguishable. Thus, a solution exists as a single phase because there is no separation between the solute and the solvent.
• A suspension on the other hand, consists of larger particles that can be seen under a microscope or even by the naked eye, dispersed in a medium. Similarly, liquid distributed in liquids are emulsions. Suspensions and emulsions are two-phase systems.

Alginate Impression Material

Colloids They are often classed as the fourth state of matter known as the colloidal state. A colloid is a two-phase system. The ‘colloidal solution’ or ‘colloidal sol’ is somewhere between the smaller molecules of a solution and the larger particles of a suspension.

The two phases of the colloidal sol are

• Dispersed phase or dispersed particle (the suspended particle).
• Dispersion phase or medium (the substance in which it is suspended).

Types of colloids Colloidal sols may be

• Liquid or solid in air (Aerosol)
• Gas, liquid, or solid in liquid (Lyosol)
• Gas, liquid, or solid in solid.

Hydrocolloids consist of gelatin particles suspended in water (Lyosol). Since water is the dispersion medium it is known as hydrocolloid. The particles are larger than those in solutions and size ranges from 1 to 200 nanometers (1 nm = 10–9 m). There is no clear demarcation between solutions, colloids, and suspensions (emulsions).

Gels, Sols, and Gelation Colloids with a liquid as the dispersion medium can exist in two different forms known as ‘sol’ and ‘gel’. A sol has the appearance and many characteristics of a viscous liquid. A gel is a jelly-like elastic semisolid and is produced from a sol by a process called gelation by the formation of fibrils or chains or micelles of the dispersed phase which become interlocked. Gelation is thus the conversion of a sol to gel. The dispersion medium is held in the interstices between the fibrils by capillary attraction or adhesion.

Gelation may be brought about in one of two ways

• Lowering the temperature, e.g. agar.
• By a chemical reaction, e.g. alginate.

Gel strength The gel strength depends on

• Density of the fibrillar structure Greater the concentration, the greater the number of micelles and hence greater the brush heap density.
• Filler particles are trapped in the fibrillar network. Their size, shape, and density determine their effectiveness. Fillers also increase the viscosity of the sol.
• In reversible hydrocolloids, the lower the temperature, the greater the strength, as gelation is more complete.
• Types of hydrocolloids.

Based on the mode of gelation, they are classified as

• Reversible hydrocolloids are called reversible because their physical state can be reversed. This makes them reusable.
• Irreversible hydrocolloids Once these are set, it is usually permanent, and so are known as irreversible.

Alginate Impression Material

Reversible Hydrocolloids – Agar

In 1925, Alphous Poller of Vienna was granted a British patent for a totally different type of impression material. It is said that Poller’s objective was to develop a material that could be sterilized and applied without pressure to the exposed surface of the dura mater for perfectly recording its convulsion and the bony margins of the skull.

• Later Poller’s Negacol was modified and introduced to the dental profession as Dentacol in 1928.
• Agar hydrocolloid was the first successful elastic impression material to be used in dentistry. It is an organic hydrophilic colloid (polysaccharide) extracted from a type of seaweed. China and South America are major sources of farmed seaweed.
• Agar is a sulfuric ester of a linear polymer of galactose. Although it is an excellent impression material and yields accurate impressions, presently it has been largely replaced by alginate hydrocolloid and rubber impression materials.

Classification Based On Viscosity (ISO 21563:2013)

• Type 1 — Heavy bodied (for use as tray material)
• Type 2 — Medium-bodied (for use as tray or syringe material)
• Type 3 — Light-bodied (for syringe use only)
• Type 3A — Light bodied for agar-alginate combination technique

Uses

• Widely used at present for cast duplication (e.g. during the fabrication of cast metal removable partial dentures, etc.).

• For full mouth impressions without deep undercuts.
• It was used extensively for FPD impressions prior to elastomers.
• As a tissue conditioner.

Supplied As

• The gel in collapsible tubes (for impressions).
• As cartridges or gel sticks
• In bulk containers

Commercial names Syringe materials include herculloid, cartriloids (Van R), etc. Duplicating materials include Wirogel (Bego), and Dubliform (Dentaurum).

Composition

Functions of the Ingredients

• Agar Basic constituent 13–17% for tray material and 6–8% for syringe material.
• Borates Improve the strength of the gel (it also retards the setting of plaster or stone cast when poured into the finished impression—a disadvantage).
• Potassium sulfate counters retarding effect of borates, thereby ensuring proper setting of the cast or die.
• Hard wax acts as a filler. Fillers affect the strength, viscosity, and rigidity of the gel. Other fillers are zinc oxide, diatomaceous earth, silica, rubber, etc.
• Thixotropic materials act as plasticizers. Examples are glycerine and thymol. Thymol acts as a bactericide also.
• Alkylbenzoates act as a preservative.
• Coloring and flavoring For patient comfort and acceptance.
• Water acts as the dispersion medium.

Alginate Impression Material

Gelation or setting of agar Agar changes from the sol to the gel state (and vice versa) by a physical process. As the agar sol cools the dispersed phase groups form fibrils called micelles. The fibrils branch and intermesh together to form a brush-heap structure.

• The fibrils form weak covalent bonds with each other which break easily at higher temperatures resulting in gel turning to sol. The process of converting gel to sol is known as liquefaction which occurs at a temperature between 70 and 100 °C.
• On cooling, agar reverses to the gel state, and the process is called gelation. Gelation occurs at or near mouth temperature which is necessary to avoid injury to oral tissues.
• The gelling property of agar-agar is due to the three equatorial hydrogen atoms on the 3,6-anhydro-L-galactose residues, which constrain the molecule to form a helix. The interaction of the helixes causes the formation of the gel.

Manipulation The equipment and materials required for an agar impression are

• Hydrocolloid conditioner
• Water-cooled rim lock trays
• Impression syringes
• Connecting water hose
• Agar tray material in tubes
• Agar syringe material

Hydrocolloid conditioner 46 °C for about two minutes with the material loaded in the tray. This reduces the temperature so that it is tolerated by the sensitive oral tissues. It also makes the material viscous.

Impression Trays Rim lock trays with water-circulating devices are used. The rim lock is a beading on the inside edge of the tray border which helps to retain the material (as agar does not adhere to the tray). It also has an inlet and outlet for connecting the water tubes. The tray should allow a space of 3 mm occlusal and laterally and extend distally to cover all teeth.

Making the Impression

• The tray containing the tempered material is removed from the bath. The outer surface of the agar sol is scraped off, then the water hoses are connected, and the tray is positioned in the mouth by the dentist. Water is circulated at 18–21 °C through the tray until gelation occurs.
• Rapid cooling (e.g. ice cold water) is not recommended as it can induce distortion. To guide the tray into position, three stops of the compound are prepared on non-involved teeth. A post dam is constructed with a compound to prevent the distal flow of the impression material.
• In a deep palate case, a compound is placed on the palatal aspect of the tray in order to provide a uniform thickness of the hydrocolloid. The mandibular tray is prepared by placing a compound on the distal aspect to limit the impression material. Black tray compound is used as it is not affected in the tempering bath.

Working and Setting Time The working time ranges between 7 minutes and 15 minutes and the setting time is about 5 minutes. Both can be controlled by regulating the flow of water through the cooling tubes. Since the cooling tubes are on the periphery, the material sets from the periphery towards the teeth surfaces.

Alginate Impression Material

Removal of Impression When the agar has gelled, the peripheral seal is broken, and the impression is removed from the mouth rapidly. The impression is rinsed thoroughly with water and the excess water is removed by shaking the impression.

Storage of Agar Impression

• Storage of agar impression is to be avoided at all costs. The cast should be poured immediately. Storage in air results in dehydration, and storage in water results in swelling of the impression.
• Storage in 100% relative humidity results in shrinkage as a result of the continued formation of the agar network agglomeration. If storage is unavoidable, it should be limited to one hour in 100% relative humidity.

Separation from Cast When the gypsum product has been set, the agar impression must be removed promptly since the impression will dehydrate, become stiff, and be difficult to remove. Weaker portions of the model may fracture. In addition, prolonged contact will result in a rougher surface on the model.

Properties Of Agar Hydrocolloids The ISO 21563 (2013) sets the standard for properties required of agar–hydrocolloid impression materials.

Gelation, liquefaction, and hysteresis Most materials melt as well as resolidify at the same temperature. However, in agar, this does not coincide. Gelation (solidification) occurs at 37 °C approximately, whereas liquefaction (melting) occurs at a higher temperature, i.e. 60–70 °C higher than the gelation temperature. This temperature lag between liquefaction and gelation is known as hysteresis.

Syneresis and imbibition (dimensional stability)

• Since hydrocolloids use water as the dispersion medium, they are prone for dimensional change due to either loss or gain of water. If left in a dry atmosphere, water is lost by syneresis and evaporation, and if it is immersed in water, it absorbs water by a process known as imbibition.
• The exuding of fluid from the gel is known as syneresis. Some of the more soluble constituents are also lost. During syneresis, small droplets of exudate are formed on the surface of the hydrocolloid and the process occurs irrespective of the humidity of the surrounding atmosphere.
• Agar exhibits the properties of syneresis and imbibition. However, when immersed in water, they do not imbibe more than the original content which was lost by evaporation (unlike alginates).
• Importance Syneresis and imbibition can result in dimensional changes and therefore inaccurate casts. To avoid this hydrocolloid impression should be poured immediately.

Flexibility ISO 21532:2013 requires flexibility ranging between 4% and 15% when a stress of 12.2 N is applied. A few set materials, however, have a flexibility of 20%. On average a flexibility of 11% is desirable.

Elasticity and elastic recovery They are highly elastic, and elastic recovery occurs to the extent of 98.8% (ISO 21532:2013 – min. 96.5 %).

Alginate Impression Material

Gel strength including tear and compressive strengths

• The gel can withstand great stresses particularly shear stress, without flow, provided the stress is applied rapidly. Thus, the impression should be removed as rapidly as possible in order to avoid distortion.
• Agar has a tear strength of 0.8–0.9 kN/m and a compressive strength of 0.5–0.9 g/cm². (ISO 21532:2013, minimum tear strength for Type 1 and 2 is 0.75 N/mm; for Type 3 is 0.50 N/mm.) The above values are for tray materials. The syringe materials have poorer mechanical properties.

Factors affecting strength

• The composition—agar concentration, borate and filler content, etc.
• The temperature—the lower the temperature the greater the strength.

Flow The material is sufficiently fluid to record the fine details if correctly manipulated.

Reproduction of detail A reproduction of a groove of 25 µm (micrometers) is achievable with agar.

Accuracy and dimensional change

• Some contraction takes place during gelation. If the material is retained well in the tray, the material contracts towards the tray resulting in larger dies. Agar impressions are highly accurate at the time of removal from the mouth, but shrink when stored in air or 100% relative humidity and expand when stored in water. The least dimensional change occurs when the impressions are stored in 100% humidity (for not more than one hour). However, prompt pouring of plaster or stone models is recommended.

Laminate Technique (Agar – Alginate Combination Technique) After injecting the syringe agar into the area to be recorded, an impression tray containing a mix of chilled alginate that will bond with the agar is positioned over it. The alginate gels by a chemical reaction, whereas the agar gels through contact with the cool alginate, rather than the water circulating through the tray.

Laminate Technique (Agar – Alginate Combination Technique) Advantages

• The syringe agar gives better details than alginate.
• Fewer air bubbles.
• Water-cooled trays are not required and therefore more convenient.
• It sets faster than the regular agar technique.

Wet Field Technique In this technique, the areas to be recorded are actually flooded with warm water. Then the syringe material is introduced quickly, liberally, and in bulk to cover the occlusal and/or incisal areas only. While the syringe material is still liquid, the tray material is seated.

The hydraulic pressure of the viscous tray materials forces the fluid syringe hydrocolloid down into the areas to be recorded. This motion displaces the syringe materials as well as blood and debris throughout the sulcus.

Cast Duplication With the introduction of alginate, agar slowly lost its appeal as an impression material. However, it is still popular as a duplicating material primarily because

• When liquefied it flows readily (like a fluid) over the cast to be duplicated. This makes it an ideal mold material.
• Large quantities can be prepared relatively easily.
• It is economical because it can be reused.

In the construction of cast removable partial dentures (RPD) the relieved and blocked master cast is duplicated in investment material. This is known as a refractory cast. The master cast to be duplicated is placed in a duplicating flask or mold former.

The agar is broken into small chunks and loaded into the liquefying machine where it is liquefied and stored. The liquid agar is poured into a mold former to create a mold. Later, investment is poured into this to create a refractory cast which is used in the fabrication of the cast partial denture framework.

Impression Disinfection Since the impression has to be sent to the laboratory, the need to disinfect it is very important. Most manufacturers recommend a specific disinfectant. The agent may be iodophor, bleach, or glutaraldehyde. Apparently, little distortion occurs if the recommended immersion time is followed and if an impression is poured promptly.

Advantages and Disadvantages of Agar Hydrocolloid

Advantages

• Accurate dies can be prepared if the material is properly handled.
• Good elastic properties help reproduce most undercut areas.
• It has a good recovery from distortion.
• Hydrophilic, moist mouth is not a problem. It also gives a good model surface.
• It is palatable and well tolerated by the patient.
• It is economical when compared to synthetic elastic materials.
• It can be reused when used as a duplicating material (reuse is not recommended when used as impression material).
• Low cost because it can be reused.

Disadvantages

• Does not flow well when compared to newly available materials.
• It cannot be electroplated.
• During insertion or gelation, the patient may experience thermal discomfort.
• Tears relatively easily. Greater gingival retraction is required to provide adequate thickness of the material.
• Only one model can be poured.
• Has to be poured immediately. Cannot be stored for too long.
• Requires special and expensive equipment.
• A soft surface of the gypsum cast results unless a plaster hardener is used.
• Although it can be reused, it is impossible to sterilize this material. Also with repeated use, there may be contamination of the materials and a deterioration in its properties.

Irreversible Hydrocolloid – Alginate

The word alginate comes from ‘alginic acid’ (anhydro-β-d-mannuronic acid) which is a mucous extract yielded by species of brown seaweed (Phaeophyceae). Alginic acid is a naturally occurring hydrophilic colloidal polysaccharide.

Alginate was developed as a substitute for agar when it became scarce due to World War 2 (Japan was a prime source of agar). Currently, alginate is more popular than agar for dental impressions, because it is simpler to use. Alginate is perhaps the most widely used impression material in the world.

Irreversible Hydrocolloid Types

• Type 1 — Fast setting.
• Type 2 — Normal setting

Irreversible Hydrocolloid Supplied As A powder that is packed

• Commonly in bulk packing (tins, bins, or sachets).
• In preweighed packets for individual impression.
• A plastic scoop is supplied for dispensing the bulk powder and a plastic cylinder is supplied for measuring the water.

Modified alginates In the form of a sol, containing water. A reactor of plaster of Paris is supplied separately.

• As a two-paste system, One contains the alginate sol, while the second contains the calcium reactor. These materials are said to contain silicone and have superior resistance to tearing when compared to unmodified alginates. They may be supplied in both tray and syringe viscosity.
• One product is supplied in low density for use with a syringe
• Dust-free alginates Concern over the inhalation of alginate dust has prompted manufacturers to introduce ‘dust-free alginates’.
• Chromatic alginates Alginates change color on set.
• Commercial Names Zelgan (DPI), Jeltrate (Dentsply), Hydrogum (Zhermack), etc.

Modified alginates Applications

• It is used for impression-making
  • When there are undercuts.
  • In mouths with excessive flow of saliva.
  • For partial dentures with clasps.
• For making preliminary impressions for complete dentures.
• For impressions to make study models and working casts.
• For duplicating models.

Modified alginates Composition

Modified alginates Setting Reaction

• When alginate powder is mixed with water a sol is formed which later sets to a gel by a chemical reaction.
• The final gel, i.e. insoluble calcium alginate is produced when soluble sodium alginate reacts with calcium sulfate (reactor). However, this reaction proceeds too fast. There is not enough working time. So the reaction is delayed by the addition of a retarder (trisodium phosphate) by the manufacturer.
• Calcium sulfate prefers to react with the retarder first. Only after the supply of the retarder is over does calcium sulfate react with sodium alginate. This delays the reaction and ensures adequate working time for the dentist.

In other words, two main reactions occur during the setting

Reaction 1 \(2 \mathrm{Na}_3 \mathrm{PO}_4+3 \mathrm{CaSO}_4 \longrightarrow \mathrm{Ca}_3\left(\mathrm{PO}_4\right)_2+3 \mathrm{Na}_2 \mathrm{SO}_4\)

Reaction 2 \(\text { Sodium alginate }+\mathrm{CaSO}_4+\mathrm{H}_2 \mathrm{O} \longrightarrow \mathrm{Ca} \text { alginate }+\mathrm{Na}_2 \mathrm{SO}_4\)

• Initially, the sodium phosphate reacts with the calcium sulfate to provide adequate working time. Next, after the sodium phosphate is used up, the remaining calcium sulfate reacts with sodium alginate to form insoluble calcium alginate which forms a gel with water.
• Gel structure
  The final gel consists of a brush heap of calcium alginate fibril network enclosing unreacted sodium alginate sol, excess water, filler particles, and reaction byproducts. It is a cross-linked structure (i.e. each fiber is tied to each other at certain points). Calcium is responsible for cross-linking.

Properties Of Alginate Hydrocolloid

Taste and Odor

• Alginate has a pleasant taste and smell. Over the years, manufacturers have added a variety of colors, odors, and tastes to make it as pleasant as possible for the patient. Flavors include strawberry, orange, mint, vanilla, etc.

Flexibility It is about 14% at a stress of 12.2 N. However, some of the hard-set materials have lower values (5–8%). A lower W/P ratio (thick mixes) results in lower flexibility (ISO 21563:2013—minimum requirement ranges from 5 to 20%).

Elasticity and Elastic Recovery Alginate hydrocolloids are highly elastic (but less when compared to agar) and about 98.2% elastic recovery occurs. Thus, permanent deformation is more for alginate (about 1.8%). Permanent deformation is less if the set impression is removed from the mouth quickly.

Reproduction of Tissue Detail Detail reproduction is also lower when compared to agar hydrocolloids. ISO 21563:2013 requires the material to reproduce a line that is 20 µm in width. A number of products exceed this minimum value.

Strength

Compressive strengths Range from 0.5 to 0.9 MPa.

Tear strength This is an important property of alginates. Values range from 0.4 to 0.7 kN/m

Factors affecting strength are

• Water/powder ratio Too much or too little water reduces gel strength.
• Mixing time Over and under mixing both reduce strength.
• Time of removal of impression Strength increases if the time of removal is delayed for a few minutes after setting.

Syneresis and Imbibition Like agar–agar alginate also exhibits the properties of syneresis and imbibition. When placed in contact with water alginates absorb water and swell. Continued immersion in water results in the total disintegration of the alginate.

Dimensional Stability Set alginates have poor dimensional stability due to evaporation, syneresis, and imbibition. Therefore, the cast should be poured immediately. If storage is unavoidable, keeping in a humid atmosphere of 100% relative humidity (humidor) results in the least dimensional change.

Alginates can also be stored in sealed plastic bags. Modern alginates, both regular and extended pour varieties have shown to have good clinically acceptable dimensional stability for periods ranging from 1 to 5 days according to some studies.

Biological Properties No known chemical or allergic reactions have been identified for alginate. Silica particles present in the dust which rises from the can after fluffing alginate powder, are a possible health hazard. Avoid breathing the dust. Some manufacturers supply ‘dust-free’ alginates. Dustless alginates contain glycol. It acts by coating the powder.

Adhesion Alginate does not adhere well to the tray. Good adhesion is important for the accuracy of the impression. Retention to the tray is achieved by mechanical locking features in the tray or by applying an adhesive.

Shelf Life and Storage Alginate material deteriorate rapidly at elevated temperatures and humid environments.

• The material should be stored in a cool, dry environment (not above 37 °C).
• The lid of the bulk package can, must be replaced after every use, so as to minimize moisture contamination.
• Stock only for one year.

Manipulation

• Fluff or aerate the powder by inverting the can several times. This ensures uniform distribution of the filler before mixing. The top of the can should be taken off carefully to prevent the very fine silica particles from being inhaled.
• Mixing equipment includes
  • A clean flexible plastic bowl and
  • A clean-bladed, reasonably stiff metal spatula.

Note It is better to use separate bowls for plaster and alginate as plaster contamination can accelerate the setting.

The proper W/P ratio as specified by the manufacturer should be used (usually one measure of water with two level scoops of powder. The water measure and scoop are supplied by the manufacturer).

The water is taken first. The powder is sprinkled into the water in the rubber mixing bowl and the lid of the metal can is replaced immediately. The mixing is started with a stirring motion to wet the powder with water. Once the powder has been moistened, rapid spatulation by swiping or stropping against the side of the bowl is done. A vigorous figure-eight motion can also be used.

This helps

• Remove most of the air bubbles.
• Wipe dissolved algin from the surface of the yet undissolved algin thereby promoting complete dissolution.

Mechanical devices are available for spatulating alginate.

Their main advantages are

• Speed
• Convenience

• Elimination of the human variable.

A proper mix is smooth and creamy with minimum voids and does not drip off the spatula when it is raised from the bowl.

Mixing Time

1. For fast set alginate—45 seconds.
2. For normal set alginate—60 seconds.

Over-mixing results in

• Reduction in final strength as the gel fibrils are destroyed.
• Reduction in working time.

Undermixing results in

• Inadequate wetting, lack of homogeneity, and reduced strength.
• The mix is grainy and has poor recording of detail.

Working Time

1. Fast set alginate—1¼ minutes.
2. Normal set alginate—2 minutes.

Gelation Time (Setting Time)

1. Type 1 (fast set)—1.5–2.0 minutes
2. Type 2 (normal)—3–4.5 minutes

Control of gelation time The ideal gelation time is 3–4 minutes (at 20 °C room temperature).

• Gelation time is best controlled by adding retarders (which is in the manufacturer’s hands).
• The dentist can best control the setting time by altering the temperature of the water for mixing alginate material.
  • The colder the water, the longer the gelation time.
  • The warmer the water, the shorter the gelation time.
  • Even the mixing bowl and spatula can be cooled.

Note Control of setting by changing W/P ratio is not recommended.

Tray Selection Since alginate has poor adhesion, tray selection is very important. Alginate can be retained by

• Mechanical locking features in the tray
  • A rim lock (a beading around the edges of the tray)
  • Perforations (holes or slits) in the tray
• Applying adhesive (available as liquid or sprays)
• A combination of the above.

The tray should cover the entire impression area and provide a space of at least 3 mm on all sides.

Loading The Tray

• The mixed alginate is pressed and swiped into the perforated rim lock tray so that the material is forced out through the holes in the tray, thereby locking itself mechanically into the tray.
• The surface of the alginate in the tray may be smoothened out using a moist finger. However, this is not mandatory.

A small amount of material may be taken on the index finger and applied on the occlusal surfaces of the teeth and on the rugae area. This helps to reduce voids and improve accuracy.

Seating The Tray Since the material sets from tissues towards the periphery any movement during gelation may result in distortion. So once the tray is seated, it must be held in place firmly without any movement.

Time Of Removal And Test For Set The alginate impression should be left in the mouth for at least 2–3 minutes after initial gelation. The strength and elasticity of the alginate gel continue to increase for several minutes after initial gelation.

Test for Set

• The material loses its tackiness when set. It should rebound fully when prodded with a blunt instrument.
• Color indicators Although chromatic alginates indicate a color change after setting, it is still best to test for set by prodding the material at the periphery with a blunt instrument.

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. In addition to holding the tray handles, additional displacing force may be applied with a finger on the buccal flange of the set material and tray.

The impression must be removed as quickly as possible. The brush-heap structure of a gel responds more favorably to a sudden force. A gentle, long, continued pull will frequently cause the alginate to tear or separate away from the tray. It also causes higher permanent deformation.

After removal from the mouth, the impression should be

• Washed with cold water to remove saliva.
• Disinfected by immersion in a suitable disinfectant.
• Covered with a damp napkin to prevent drying.
• The cast should be poured as soon as possible, preferably within 15 minutes after making the impression.

Impression Disinfection

• Disinfection of impression is a concern because of viral diseases such as hepatitis B, AIDS, and herpes simplex. The viruses can contaminate the gypsum models and present a risk to dental laboratory and operating personnel.
• Recommended disinfectants include phenol, iodophor, bleach, or glutaraldehyde. Irreversible hydrocolloids may be disinfected by immersion in, or spraying. The current protocol recommended by the Centers for Disease Control and Prevention is to spray the impression with disinfectant. The impression is then wrapped in a disinfectant-soaked paper towel. Immersion disinfection if used should not exceed 10 minutes to reduce dimensional change.

Storage Of Alginate Impression Alginate impressions must be poured as soon as possible. If it becomes necessary to store the impression, the following methods may be used

• Wrap the impression lightly with a moist paper towel and cover with a rubber bowl or
• Keep the impression in a sealed plastic bag.

Note Even under these conditions storage should not be done for more than one hour. Care should be taken not to use a soaking wet paper towel or gauze as it can cause imbibition of water.

Construction Of CAST

• The early alginates required immersion in a gypsum hardening solution, such as potassium sulfate, zinc sulfate, manganese sulfate, and potash alum (the most effective is 2% potassium sulfate solution). However, the formulas of presently available alginates have been adjusted so that no hardening solution is required.
• Alginate is a hydrophilic material and wets easily reducing the entrapment of air. After rinsing the excess water is shaken off. The impression is held against a vibrator to reduce the trapping off air. Freshly mixed stone is placed at one end of the impression.
• The impression is rotated to facilitate the flow of the stone around the arch. The stone displaces water and wets the surface of the impression as it flows. It is then allowed to flow out through the other side and discarded.

This helps to

• Reduce the trapping of air bubbles.
• Removes the water-rich surface layer which can result in a weaker cast surface.
• The impression is filled with the remaining stone and placed aside to set. The stone cast should not be separated for at least 30 minutes. For alginate, the best results are obtained if the cast is removed in one hour. The cast should not be left in the impression for too long a period either because it can result in a rough and chalky surface.

• Alginate dries and stiffens. Removal can break the teeth and other thin portions of the cast.

Advantages and disadvantages of alginate

Alginate Of Advantages 

• It is easy to mix and manipulate.
• Minimum requirement of equipment.
• Flexibility of the set impression.
• Accuracy if properly handled.
• Low cost.
• Comfortable with the patient.
• It is hygienic, as fresh material must be used for each impression.
• It gives a good surface detail even in the presence of saliva.

Alginate Of Disadvantages

• Cannot be electroplated so metal dies are not possible.
• It cannot be corrected.
• Distortion may occur without it being obvious if the material is not held steady while it is setting.
• Poor dimensional stability—it cannot be stored for a long time.
• Poor tear strength.
• Because of these drawbacks and the availability of better materials, it is not recommended where a higher degree of accuracy is required, e.g. cast RPD, crowns and FDPs, etc.