Alginate information

Alginate can deteriorate in its powder form within 3 days of opening the container. Exposure to humidity or heat causes thin mixes, altered setting times, reduced strength, high deformation. Contamination with gypsum products in the bowl should be avoided as it can accelerate its set. Don't mix stone and alginate in the same bowls.

The water powder ration affects the consistency and setting time of the materials and strength and quality of the impression. a thin mix will flow out of the tray and away from the tissues with increased tear risk. a thick mix will capture less fine detail. Ideally work with a predetermined amount of powder and alter the water.

Primary mechanisms of alginate distortion is water absorption (imbibition-Do not wrap in wet paper towels or immerse in liquids) and evaporation of liquids (syneresis occures even at 100% humidity). Distortion exists if impressions are not poured up within 12 minutes of removal.

Alginate can stick to teeth (hydroxyapatite) usually on the facial surfaces of teeth and the cusp tips. The impression will appear 
roughened or scaly. Excessive polishing (Cleaning of tooth surface), drying of teeth (air drying is contraindicated as it minimises the moisture content of teeth surfaces), repeated impressions (as a separating film on the teeth is removed) increases the risk of sticking. If sticking occurs, delay appointment or use a prophy paste/milk on the teeth

Position of patient and dentist during impression-making procedures

The patient should be seated upright and the dentist should be standing during these procedures.The patient should be positioned so the occlusal plane is parallel to the floor when the patient's mouth is open. The height of the chair should be adjusted so the patient's mouth is at the same level as the dentist's elbow. 

Checking maxillary tray for correct size

The width of the dental arch is the most important factor in determining tray size. Ideally, there should be a clearance of 5 to 7 mm between the inner flanges of the tray and the facial surfaces of the remaining teeth and soft tissues. This space is necessary to ensure the impression material will be thick enough to spring over the undercuts. Too large a tray may be difficult to insert because of interference with the coronoid processes of the mandible. 

Checking mandibular tray for correct size

A mandibular tray should provide 5 to 7 mm of space both facial and lingual to the remaining teeth and residual ridge. If a tray extends too far lingually, there is a tendency to trap the tongue or the floor of the mouth. The problem should be corrected by selecting a tray of a different size or by bending the lingual flanges of the tray to provide the required clearance. As the tray is rotated into position, the patient should be instructed to gently raise the tongue. This will ensure that the tongue is not trapped beneath the tray. Upon passing the corner of the mouth, the tray should be straightened and positioned over the teeth. The patient should then be instructed to rest the tip of the tongue on the anterior section of the tray. By depressing the lower lip, the dentist can easily see the buccal and lingual clearance between the teeth and the tray.

Customizing a stock impression tray

Softened compoound in a hot water bath can be used to modify deficient areas of the tray. this should be placed in the patient's mouth to mould to the tissues. It should be taken in and out to avoid being locked into undercuts. 5-7mm clearance should then be trimmed.

Control of gagging

Under no circumstances should the dentist bring up the subject of gagging, since this may cause the patient to become nervous or hesitant; it should only be addressed if the patient reports past difficulties with gagging during impression making.

The dentist should employ the following procedures to minimize or prevent gagging.

1. The patient should be seated in an upright position with the occlusal plane parallel with the floor.

2. When indicated, the maxillary tray should be modified using modeling plastic. A narrow band of unrelieved modeling plastic should be maintained at the posterior border of the impression area. This band of modeling plastic should prevent alginate from running posteriorly as the tray is inserted.

3. The patient should be directed to use an astringent mouthwash and cold water rinse just before the impression is made. The use of an anesthetic spray is usually contraindicated. Anesthetic spray will cause numbness of the tongue and soft palate and may contribute to gagging.

4. The impression tray should not be overfilled with impression material.

5. The posterior portion of the impression tray should be seated first. Then, the anterior portion of the tray should be rotated into position. This permits excess impression material to be displaced in an anterior direction—away from the soft palate and airway.

6. The patient should be instructed to keep the eyes open during the impression procedure. This encourages the patient to focus upon the surroundings rather than the impression procedure. It may be helpful to have the patient focus on a small object.

7. The patient should be directed to breathe through the nose. The dentist should encourage slow, deep breaths.

8. All instructions should be given in a calm, firm manner.

It is important to recognize that most gagging problems are psychological rather than physical. Confidence in the dentist will help eliminate many of these problems. However, a very small percentage of patients have a true, uncontrollable gag reflex. In these patients, the simple procedure of introducing an empty tray into the mouth may initiate severe gagging. Therefore, additional measures may be necessary to complete the impression process. 

The following procedures will allow a dentist to make impressions for almost any patient who is physically and psychologically able to follow instructions:

1. The patient should be instructed to take a deep breath and hold it while the dentist checks the size and adaptation of the tray. Most patients will not gag while holding their breath. As a result, the dentist can complete short procedures without the danger of the patient gagging.

2. The patient should be directed to rinse the mouth with astringent mouthwash and then with cold water. This combination will minimize the flow of saliva during impression procedures and will decrease the likelihood of gagging.

3. A fast-setting alginate should be used to hasten the set of the impression material. Slightly warmer water also may be used to shorten the gelation period.

4. The "leg-lift" technique may be used during the impression procedure. The patient should be directed to lift one leg off the dental chair and to keep it raised at all times. As fatigue sets in, it will usually be necessary to firmly command the patient to keep the leg lifted. The mixing of alginate should not be initiated until the patient appears to be tiring. When fatigue is noticeable, the alginate should be mixed and the impression made. The dentist should ensure that the patient's leg remains raised throughout the procedure. The leg-lift procedure is intended to distract the patient by focusing attention on another process. Its success is probably based on a combination of distraction, muscular fatigue, and anger directed at the dentist. However, once the patient understands that an impression can be made with little or no discomfort, additional procedures may be accomplished with a reduced tendency toward gagging.

Control of saliva

Excessive amounts of saliva can displace alginate impression material and contribute to an inaccurate impression. The patient's mouth should be packed with 4 × 4-inch gauze that has been unfolded to form an absorptive strip . In the maxillary arch, one gauze strip should extend from the posterior portion of the right buccal vestibule to the posterior portion of the left buccal vestibule. The patient should be instructed to hold a second strip against the tissues of the palate. In the mandibular arch, one gauze strip should extend from the right buccal vestibule to the left buccal vestibule. A second gauze strip should be positioned in the lingual sulcus by having the patient raise the tongue, placing the gauze, and then having the patient relax the tongue. The gauze should be gently removed immediately before the impression is made.

Mixing alginate impression material

Mixing should begin slowly using a stiff, broad-bladed spatula (Fig 5-35). When all of the powder has been thoroughly wetted, the speed of spatulation should be increased. During this process, the spatula should be used to press the alginate impression material against the sides of the bowl. Spatulation should continue for a minimum of 45 seconds. Insufficient spatulation can result in failure of the ingredients to dissolve sufficiently resulting in a significant reduction in the strength of the material. An incompletely spatulated mix will appear lumpy and granular and will exhibit numerous areas of trapped air.

Loading the impression tray

Small increments of the impression material should be placed in the tray and forced under the rim. Placing too large a portion of alginate at one time increases the probability of trapping air. Impression material should be added until it is level with the flanges of the tray. Overfilling should be avoided. 

Making impressions

The mandibular impression should be made first because it usually entails less patient discomfort, and patient confidence is increased when an impression has been successfully completed. While holding the tray with the left hand, the dentist should use the right hand to remove gauze pads from the patient's mouth. The lips and cheeks should be pulled apically and then outward at a 45-degree angle to properly form the peripheries of the impression. It is essential that the dentist maintain the position of the tray during the entire gelation period. This can be accomplished most effectively by placing the forefinger of each hand on top of the tray in the premolar area, and by placing the thumbs under the patient's chin. The dentist should maintain an even amount of pressure on the tray even if the patient opens or closes the mouth. Any movement of the tray during the gelation period will result in an inaccurate impression. Therefore, allowing the patient or the assistant to hold the tray should be avoided. 

Once the tray is in the mouth, the thumb and forefinger of the left hand should be used to raise the upper lip. This permits the dentist to see the relationship between the labial flange of the tray and the facial surfaces of the teeth or residual ridge. Care should be taken to ensure that the tray is properly aligned during the insertion process. This can be evaluated by looking at the patient's head from above and behind, and by observing the position of the tray handle. The handle should protrude from the center of the mouth and should be oriented parallel to the sagittal plane. After the proper orientation has been verified, the tray should be seated. During this process, the cheeks should be lifted upward and outward to prevent the buccal tissues from being trapped by the flanges of the tray. The upper lip also should be lifted upward and outward to allow good visibility and to avoid trapping the lip between the flange of the tray and the anterior teeth. The tray must not be overseated. Overseating produces contact between the internal surfaces of the tray and the occlusal or incisal edges of the teeth, thereby producing an inaccurate impression. The tray should be stabilized throughout the set of the impression material by maintaining light pressure over the premolar areas on both sides of the arch. Following completion of the gelation process, the impression should be removed and inspected. 

Removal of impression from the mouth

Clinically, the initial set of alginate is determined by a loss of surface tackiness. Early removal of an alginate impression may lead to unnecessary tearing of the impression material. It is important to note that the gel strength doubles during the first 4 minutes after initial gelation. Beyond 4 minutes, no further strengthening occurs. In fact, impressions left in the mouth for 5 or more minutes exhibit noticeable distortion. 

The physical strength of alginate gel is such that a sudden force is more successfully resisted than a slow, sustained force. The material also displays improved elastic recovery when an impression is rapidly removed. Therefore, alginate impressions should be removed from the mouth with a rapid, sustained tug. This results in a more accurate impression and cast.

The following technique makes it possible to remove an impression without significant distortion of the alginate. The lips and cheeks should be retracted to partially break the seal and facilitate impression removal. For a maxillary impression, the right thumb should be placed on top of the tray handle. The middle and index fingers should be placed on the underside of the handle and should extend onto the palatal segment of the tray. This prevents the tray from striking the mandibular teeth and tissues during tray removal. At this point, the impression should be removed with a rapid tug directed parallel to the long axes of the teeth. For a mandibular impression, the right thumb should be placed on the underside of the tray handle. The middle and index fingers should extend onto the upper surface of the tray to prevent damage to the opposing teeth. The mandibular impression also should be removed with a rapid tug directed parallel to the long axes of the teeth. Rocking or slowly teasing the impression from the mouth will result in irreversible distortion of the impression material and should be avoided.

Inspecting the impression

Following removal from the mouth, the impression should be inspected using a good light source and magnification. An impression should be repeated if there are any doubts regarding its accuracy. Common problems include layering of the impression material, improper positioning of the tray, and entrapment of the tongue or other tissues by the flanges of the impression tray.

Layering is caused by the premature gelation of the syringe material. Alginate applied with the syringe should be 3 to 4 mm thick. If the impression material is too thin, the heat of the oral cavity may cause the material to set before the tray can be seated. This produces a distinct border between the syringe material and the tray material, and results in a layered impression.

The fingers should be used to manipulate the lip and provide optimum visibility during the seating process. The tray should be carefully seated so its flanges are apical to the gingival margins of the teeth. Overseating may cause the cusps of the teeth to contact the tray and result in an inaccurate impression. In addition, great care must be exercised when seating the tray in a patient with tori or other exostoses since contact with the overlying soft tissues may cause significant discomfort.

Entrapment of the tongue and other soft tissues also should be avoided. Upon seating of the mandibular impression tray, the patient should be asked to raise, protrude, and then relax the tongue. By raising and protruding the tongue, the patient prevents its confinement by the lingual flanges of the tray. With the subsequent relaxation of the tongue, the form of the lingual vestibule may be recorded.

Following are other common reasons for rejecting an impression:

1. Inadequate extension of the impression

2. Voids in critical areas

3. Tearing in critical areas

4. Alginate sticking to the teeth

5. Alginate separated from the underlying impression tray (Although it appears the alginate can be pushed back into contact with the tray, this will result in an inaccurate impression and an inaccurate cast.)

6. Rough or granular impression with poor tissue detail (This indicates inadequate spatulation, delayed insertion, or premature removal.)

If the impression is acceptable, unsupported alginate material is removed.

Importance of water-powder ratio in making the cast

All gypsum products, whether dental plaster or dental stone, require only 18.61 mL of water to react with 100 g of powder to form calcium sulfate dihydrate.¹⁵ All remaining water occupies space in the cast, thereby reducing the compressive strength. Gypsum products should not be stored in open containers, where air exposure will cause the hemihydrate to absorb moisture from the air. Moisture contamination will cause the formation of calcium sulfate dihydrate crystals within the powder. This may accelerate or retard the setting of dental stone depending upon the severity of moisture contamination. Moisture contamination will reduce both the compressive strength and the surface hardness of the resultant casts.

Two-stage pour technique

If an impression is filled with dental stone and inverted so the teeth and residual ridges are facing upward, a similar process occurs. Manipulation, movement, or vibration of the freshly mixed stone produces movement of water toward the surface of the impression. Therefore, the stone that makes up the teeth and residual ridges contains more water than do other portions of the cast. The surface of the resultant cast is weak and may be abraded easily. For these reasons, a single-stage or inversion technique should be avoided.

Pouring should be initiated within 12 minutes of impression removal. In the two-stage technique, an initial mix of stone is used to fill the impression. A small amount of stone should be added to one of the posterior extensions of the impression. The handle of the tray should be placed against a vibrator, and the impression tray should be tipped to permit a controlled flow of dental stone. The flow of the stone should be slow enough that it can be observed filling each individual tooth impression. Rapid flow or excessive vibration can cause air to be trapped at the impression-cast interface. Small increments of stone should be added to the posterior extension of the impression until all borders are covered by 6 to 8 mm of stone. Stone should not be permitted to flow onto the sides of the impression tray because this will lock the cast onto the tray. The exposed surface of the poured stone should be left rough. Irregular mounds of stone should be added to this surface to provide locking undercuts for the second pour (Fig 5-61).

The tray should be suspended by the handle until the stone has reached its initial set (Fig 5-62). This should occur in 12 to 15 minutes. If any movement or vibration occurs during this period, water will rise toward the free surface of the dental stone, causing the stone in the anatomic portions of the cast to become more dense.

After the initial set, the impression-cast assembly should be placed in a bowl of clear slurry water for 4 to 5 minutes to thoroughly wet the first pour of dental stone (Fig 5-63). Clear slurry is a supersaturated solution of calcium sulfate made by placing chips of dental stone in water for 48 hours. Because it is a saturated solution, clear slurry permits wetting of the first pour without dissolution of the stone. A cast should never be soaked in tap or distilled water because dental stone is soluble in these liquids. If a stone cast is immersed in running water, its linear dimensions may decrease approximately 0.1% for every 20 minutes of exposure (Fig 5-64).

After the first pour has soaked for 5 minutes, a second mix of dental stone is prepared as described earlier. Some of the freshly mixed stone should be vibrated onto the roughened surface of the first mix. The remaining stone should be used to form a patty, and the impression should be inverted and placed onto this patty of stone. A spatula or similar instrument should be used to shape the base of the cast. In the case of a mandibular impression, the tongue space should be smoothed. Special care should be taken to avoid locking the stone onto the impression tray. Between 45 and 60 minutes after the first pour, the cast and impression should be separated. An alginate impression should not be allowed to remain in contact with the associated cast for more than 60 minutes. Extended contact between alginate and dental stone will result in etching of the cast surface.

Trimming the cast

Each dental cast should be soaked in clear slurry water to facilitate grinding procedures and to prevent stone residue from sticking to the surface of the cast. The base of the cast should be trimmed so that the occlusal surfaces of the teeth are parallel to the base. The base should be trimmed until it is 10 to 13 mm thick at its thinnest point, usually the center of the hard palate for a maxillary cast and the depth of the lingual sulcus for a mandibular cast.

The posterior border of the cast should be trimmed to form an angle of 90 degrees with the base. When viewed from an occlusal perspective, the posterior surface should be perpendicular to the midline of the palate. Care should be taken to preserve essential landmarks such as the hamular notches and tuberosities of a maxillary cast and the retromolar pads of a mandibular cast.

The sides of the cast should be trimmed at 90 degrees to the base. Care should be taken to avoid overtrimming the lateral aspects of a cast, which could eliminate the vestibular and buccal shelf areas. A land area of 2 to 3 mm should be maintained around the entire cast. The sides of the cast should be joined to the posterior surface by trimming just posterior to the hamular notches or retromolar pads. 

The anterior borders of maxillary and mandibular casts should be trimmed differently. The anterior border of a maxillary cast should be angular, originating from the canine area on each side and extending to a point anterior to the central incisors. The anterior border of a mandibular cast should be gently curved, originating from the canine area on one side of the arch extending to the opposite canine area. The curve should follow the form of the arch. In both instances, care should be taken to avoid damage to the teeth and vestibular areas.

The tongue space should be trimmed flat, while maintaining the integrity of the lingual frenum and the lingual sulcus. Nodules of stone caused by voids in the impression should be carefully removed from noncritical areas (Fig 5-76). After thoroughly soaking the cast in clear slurry water, voids in the base and other noncritical areas of the cast should be filled with a thin mix of stone. Accurate, properly trimmed casts are essential in a wide variety of dental procedures. As a result, care must be taken to ensure that impressions and casts accurately represent the hard and soft tissue contours of the oral cavity (Figs 5-77 and 5-78).

Causes of surface roughness on dental casts

There are several potential causes of surface roughness on dental casts. Perhaps the most common cause of surface roughness is adherence of alginate impression material to the enamel. This produces localized tearing of the impression material and results in noticeable surface irregularities on cast surfaces. If surface roughness is a consistent problem, one should suspect incompatibility between the alginate and the stone used for pouring the cast. Changing the brand of either the alginate or stone may correct the problem.

Surface roughness also may be caused by saliva or other fluids on the surface of an impression. Unwanted liquids should be eliminated from an impression by blotting with a dry tissue. As mentioned earlier, compressed air should not be used because it may cause dehydration and distortion of the impression material. Other possible causes of irregular surfaces on a cast include insufficient spatulation of the alginate, premature removal of an impression from the mouth, insufficient spatulation of dental stone, the use of contaminated stone, or the use of a single-pour technique.

It is also important to remember that an alginate impression should be removed from the cast 45 to 60 minutes after completion of the first pour. Leaving the impression in contact with the cast for an extended period may cause etching of the cast surface. This produces a soft, chalky surface. There is also danger that the cast will be abraded as the alginate shrinks and hardens.

A summary of the causes and solutions for common problems associated with diagnostic casts is presented in at the end of the chapter.

Table 5-1. Causes and solutions for common problems associated with making diagnostic alginate impressions

Problem	Probable cause	Solution
1. Alginate sticks to teeth	Teeth too clean from overly vigorous pumicing	Pumice lightly; delay impression making until after thorough prophylaxis; use silicone as protective coating for teeth
	Teeth too dry	Avoid air drying of teeth; isolate arch with gauze packs
	Loss of protective film from teeth due to repeated impressions	Use good technique so repeated impressions not necessary; delay impression until another day
	Any of the above	Use silicone protective film; have patient suck on sour (citrus) candy or swish with whole milk
2. Voids in impression	Poor mix of alginate	Spatulate for 45 to 60 seconds by hand or 15 seconds mechanically; wipe alginate along side of bowl during spatulation; use mechanical spatulation under vacuum
	Alginate did not flow to all areas	Wipe alginate on teeth, on palate, and into vestibular areas after mouth has been isolated with gauze packs; avoid mix that is too thick or too thin by using correct water-powder ratio;measure alginate by weight, not volume;avoid deterioration of alginate by heat or moisture contamination
3. Peripheral underextension	Alginate did not flow into peripheral areas or poor mix of alginate	See No. 2
	Tray too small, so material not carried into vestibule	Use tray with 5- to 7-mm clearance
	Tray incorrectly seated	Center tray with handle pointing straight out of mouth; retract lips with fingers so correct position of tray can be seen; seat tray so borders go below gingival marginal areas; avoid overly large trays, which will interfere with coronoid processes of mandible
	Cheeks, lips, or floor of mouth trapped under tray	Pull out cheeks; retract lips; have patient protrude tongue before final seating of tray
4. Alginate tears when impression removed	Mix of alginate is too thin or too thick	Use water-powder ratio recommended by manufacturer; measure alginate by weight instead of volume; avoid deterioration of alginate by heat or moisture
	Impression removed from mouth too soon	Keep impression in mouth 2 to 3 min after it loses its tackiness
	Inadequate bulk of alginate	Select tray with 5- to 7-mm clearance, center tray properly; relieve modeling plastic used to modify tray
	Use of deteriorated alginate	Store bulk alginate in airtight containers at room temperature
	Prolonged or insufficient spatulation	Spatulate for 45 to 60 seconds by hand or 15 seconds mechanically
	Improper removal from mouth	Avoid rocking or teasing out of impression; remove with snap, applying force along long axes of teeth
5. Lack of detail or grainy appearance	Prolonged or insufficient spatulation	Spatulate for 45 to 60 seconds by hand or 15 seconds mechanically
	Insufficient flow of material	Use tray that confines alginate; use correct water-powder ratio to avoid a mix that is too thin or too thick; measure by weight; avoid deterioration of alginate by heat or moisture
	Impression removed from mouth too soon	Hold steady in mouth for 2 to 3 min after tackiness is gone from alginate surface
6. Alginate sets before tray completely seated	Mixing water too warm	Use water temperature of 22°C (72°F), or lower if more working time required
	Particle of dental stone (calcium sulfate) in mixing bowl	Use different mixing bowls and spatulas for alginate and stone
	Prolonged spatulation of alginate	Spatulate for 45 to 60 seconds by hand or 15 seconds mechanically
	Use of deteriorated alginate	Store at room temperature; avoid moisture contamination by measuring and sealing all contents of bulk containers of alginate
	Layer of material painted in mouth too thin	Wipe larger amounts onto teeth and into vestibules; introduce tray immediately by having tray filled before painting in mouth
	Fast-set alginate used	Use regular-set alginate
7. Patient gags when tray is fit or impression is made	Patient is fearful and lacks confidence in dentist	Proceed with confident, well-organized manner; use simple explanations; avoid talk about gagging
	Alginate flowing out of tray and into patient's throat	Seat patient upright with occlusal plane parallel with floor; correct maxillary tray with modeling plastic; avoid overfilling of tray
	Patient is tense	Instruct patient to keep eyes open and focused on a small object; instruct patient to breathe through nose at normal rate
	Palate numb because of use of topical anesthetic	Avoid topical anesthetics; use astringent mouthwash and cold water rinses instead
	Patient has severe gag reflex	Ask patient to hold breath while tray is fit or corrected; use the "leg-lift" procedure; use fast-set alginate or accelerate the set of alginate by using warmer water
8. Alginate displaced by saliva in palate	Mucinous saliva not removed from palate	Have patient use astringent mouthwash and cold water rinse; wipe and isolate palate with 2 × 2-inch gauze
	Excessive secretion by palatal mucous glands	Use warm gauze pads to milk palatal glands, followed by cold pads to constrict gland openings
	Patient produces copious amounts of saliva	Premedicate with 15 mg of propantheline bromide (Pro-Banthine, Searle) 30 min before procedure if no contraindications
9. Alginate pulled away from tray	Alginate not forced under rim lock	Use small increments and force alginate into rim lock areas
	Alginate does not stick to modeling plastic	Use alginate to coat entire inner surfaces of tray and modeling plastic
	Alginate stuck to teeth	See No. 1

Table 5-2. Causes and solutions for common problems associated with casts made from alginate impressions

Problem	Probable cause	Solution
1. Cast has rough surface	Incompatibility between alginate and dental stone	Change brand of alginate or stone to obtain compatible combination
	Insufficient spatulation of stone	Spatulate until smooth homogenous mix is attained (60 to 90 seconds by hand or 15 to 20 seconds by mechanical spatulation under vacuum)
	Sticking of alginate to teeth	See No. 1 in Table 5-1
	Saliva retained on impression	Rinse in running water until alginate has rough feel; use soap suds and camel-hair brush to remove saliva; use dry dental stone as a disclosing agent and remove saliva with camel-hair brush and running water
	Water left on impression	Blot water with dry tissue paper; avoid use of compressed air
	Poor mix of alginate; insufficient spatulation	Spatulate for 45 to 60 seconds by hand or 15 seconds mechanically
	Use of single-pour technique; water rose to tissue/tooth surface of impression	Use two-stage pour technique
2. Surface of cast has chalky appearance	Incompatible alginate-stone combination	Change brand of alginate or stone to obtain compatible combination
	Film of stone slurry on cast after dry cast trimmed on model trimmer	Thoroughly soak cast in clear slurry water before trimming; rinse periodically in clear slurry water while trimming
	Impression left in contact with cast for prolonged period	Separate impression from cast 45 to 60 min after first pour
3. Cast has a soft surface	Too much water in mix of stone	Use acceptable water-powder ratio; measure stone by weight instead of volume
	Use of inverted single-stage pour technique;water rose to tissue/tooth surface of impression	Use two-stage pour technique
	Use of moisture-contaminated stone	Premeasure stone and store in airtight container; avoid use of open bins for stone storage
	Water or stone powder added to improper water-powder ratio mix after mixing has been started	Measure correct amount of water and weigh correct amount of stone for acceptable water-powder ratio
	Stone spatulated too long	Spatulate for 60 to 90 seconds by hand or 15 to 20 seconds mechanically
4. Cast breaks when impression separated from cast	Premature removal of impression from cast	Separate cast from impression 45 to 60 min after first pour
	Too much water in mix of stone	Measure water and weigh powder for correct water-powder ratio
	Use of single-stage pour technique	Use two-stage pour technique
	Water left in tooth impression	Blot all water with dry tissue paper
	Low compressive strength of dental stone because of moisture-contaminated stone, adding powder or water while mixing stone, or prolonged spatulation	Store stone correctly; measure water and weigh powder before mixing; spatulate for 60 to 90 seconds by hand or 15 to 20 seconds mechanically
	Alginate impression left in contact with cast overnight	Separate impression from cast 45 to 60 min after first pour
5. Separation of cast between first and second pours of stone	Failure to leave surface of first pour with mechanical retention for second pour	Leave surface of first pour rough; add small irregular mounds of stone to soft surface of first pour
	Failure to thoroughly wet first pour before adding second pour	After initial set of first pour, soak cast and impression in clear slurry water for 5 min
6. Voids in surface of cast	Air trapped in mix of stone because of inadequate or improper mixing	Sift powder into water to avoid air entrapment; hand spatulate 60 to 90 seconds, avoiding any whipping action, or mechanically mix stone under vacuum for 15 to 20 seconds; lightly vibrate mix until no more air bubbles come to surface
	Cast poured too rapidly and air trapped on surface of impression	Add small increments of stone to the same posterior extension of impression with light vibration and allow stone to flow slowly to fill all areas of impression
	Overvibration during pouring	Use light vibration only; flowing stone should not bounce
7. Underextension of cast	Cast overtrimmed; hamular notch, retromolar pad, or vestibular areas obliterated	Take care in trimming of casts on model trimmer to avoid removal of critical areas
	First pour of alginate did not cover all peripheral areas of impression	Fill impression completely and cover all peripheral border areas with 5 to 6 mm of stone during first stage of pour
	Peripheral underextension of alginate impression	See No. 3 in Table 5-1
8. Erratic setting time of stone	Contamination of stone by heat or moisture	Pre-weigh and store stone in airtight containers
9. Cast is inaccurate; not a true reproduction of the anatomy of the mouth	Loss of moisture content of impression because of syneresis, resulting in release of strains	Pour cast within 12 min after removal of impression from mouth; avoid excessive drying of impression
	Release of strains and swelling due to water	Do not store impression in water or other solutions; do not wrap impression in wet paper towel
	Strains or distortion in impression caused by its movement during gelation	Maintain impression in position until it is ready for removal; do not have assistant or patient hold impression
	Impression removed before gelation complete	Maintain impression in position for 2 to 3 min after alginate has lost its tackiness
	Strains induced in impression during its removal from mouth	Remove impression with a snap, applying force directly along long axes of teeth
	Use of nonrigid impression tray	Avoid use of trays that lack rigidity
	Use of inaccurate impression	See Table 5-1
	Surface of cast lost by washing or soaking cast in tap water	Use clear slurry water whenever cast needs to be soaked or washed
	Teeth contacted tray during making of impression, allowing stone to flow between impression and tray	Retract lips for good visibility when seating tray; seat tray slightly beyond the landmark of the gingival margins
	Alginate displaced or strains induced by setting tray on bench top	Suspend tray by its handle in a tray holder or a slightly opened drawer
	Distortion in palate due to failure to correct tray	Correct palatal area of maxillary tray with modeling plastic; after modeling plastic chilled, trim to provide 5- to 7-mm clearance for alginate