Does Six Hours of Contact With Alginate Impression Affect the Dental Cast Properties? (2023)

CompendiumJune 2017Volume 38, Issue 6

Does Six Hours of Contact With Alginate Impression Affect the Dental Cast Properties? (1)

Amna Adam Ibrahim, BDS, MSc; Mohammed Nasser Alhajj, BDS, MSc; Nadia Khalifa, BDS, MSc, PhD; and Magdi Wadie Gilada, BDS, MSc


Alginate impression (irreversible hydrocolloid) material is commonly used in dental practice because it is easy to mix, low in cost, and well tolerated by patients. The material is not dimensionally stable, however; thus, it is necessary to pour the impression immediately after the molding is accomplished, or within 60 minutes if the impression is kept in 100% humidity. Excessive contact of the alginate impression with the cast model over time may affect the model’s properties. In this study, the authors tested the effect of contact time between an alginate impression and type III dental stone on cast model properties. Sixty-seven cast models were obtained from a stainless steel cylinder by using irreversible hydrocolloid impression material and type III dental stone. Thirty-seven cast models were separated from the impression after 1 hour (control group) and 30 cast models were separated after 6 hours (study group). The samples were evaluated under light microscope for surface details and measured by digital caliper for dimensional stability. An indentation on the cast was made and the depth of the indentation was then measured with a digital caliper to measure hardness. The dimensional stability of the cast models was not affected when contact time was increased from 1 hour to 6 hours (P = .507). Surface details did not deteriorate when contact time was increased, as all of the samples could reproduce all details after the 1-hour and 6-hour interval periods. However, hardness was greater after 1 hour of contact time (P = .001) than after 6 hours of contact time. In conclusion, contact between alginate impression material and type III dental stone up to 6 hours did not affect the dimensional stability and richness of the surface; hardness, though, was significantly affected.

Cast models and dies are used daily in dental practice for many purposes, such as diagnosis, treatment planning, patient education, communication between clinicians and technicians, fabrication of custom trays, dental restorations, and orthodontic appliances.1-5 An impression is made to accurately record oral tissues and their spatial relationship. The impression is then poured using dental stone or other suitable materials to obtain a positive replica (ie, cast models and dies).1,3,4

Among the different types of impression materials, the most popular is alginate impression material (irreversible hydrocolloid). Despite its advantages over other materials, it has some problems with dimensional stability and water loss, which can affect the ability of alginate to reproduce details.1,4-7 Some clinicians still use alginate for definitive impressions.8 The American Dental Association (ADA) Specification No. 18 for alginate impression material regulates its use and accepts an error of 0.75 mm in the alginate impression as a maximum dimensional change.9

Alginate consists of a powder containing calcium or potassium sulfate and filler as primary components.3,9,10 Alginate impressions contain water, which inhibits the setting of the gypsum material at the surface. In addition, alginate produces insoluble alginate and sodium sulfate during the gelation process.11

Dental stone is extensively used in dental laboratories because of its low cost and easily modified properties.12 Chemically, dental stone is a mineral composed of calcium sulfate dihydrate. The gypsum used for dental applications is nearly pure calcium sulfate hemihydrates, produced by calcination of calcium sulfate dihydrate.11-13 According to ADA specification No. 25, there are five types of gypsum products for dental uses. One of them is type III dental stone, which is used as cast and/or die material to construct prostheses and has high strength and slight setting expansion.11,14 Type III dental stone sets when mixing the powder particles with water. That reaction process is an exothermic chemical reaction.11,12,15,16

The requirements of ideal gypsum casts and dies include accurate reproduction of details, dimensional stability over time, enough hardness strength to withstand the fabrication process, and being resistant to inadvertent abrasions that may occur during fabrication.12,13 According to ADA specification No. 25, detail reproduction is satisfactory when a copy line of 50 µm in diameter is reproduced continuously across the stone.14 The recommendation is to immediately pour the alginate impression with dental stone to avoid dimensional changes; however, pouring can be delayed up to 60 minutes if the impression is kept in 100% humidity.6,17

Few studies have discussed the effect of contact time. Some have recommended separating the cast from the mold before dehydration of the alginate18; others reported the effect of contact time between alginate impression and orthodontic plaster on cast properties after 12 hours of contact time.6 Another study reported the effect of contact time between alginate impression material and dental stone after 9 hours of contact time.19 These periods of time (9 hours and 12 hours) seem to be long with regard to a working day in a dental practice. Most practical working hours range between 25 to 40 hours per week, or approximately 6 hours per day.20

(Video) ALGINATE Impression Material | Setting reaction is a love story | Dental Materials

This study was designed to investigate the effect of contact time between alginate impression material and type III dental stone on cast model properties after 6 hours. To the authors’ knowledge, this is the first study to do so. The properties investigated were dimensional stability, detail reproduction, and hardness.

Materials and Methods

A stainless steel cylinder was fabricated according to ADA Specification No. 18 for alginate impression material. The dimensions of the cylindrical model were 25 mm in width and 15 mm in length with three reference lines in surface. The references lines were 75 µm (labeled A); 50 µm (labeled A'); and 25 µm (labeled A") in width and 2.5 mm apart from each other. These lines were used for estimating surface detail. Two bisecting lines (x and x') were also marked on the surface to test the dimensional stability by measuring the distance between them (Figure 1).

Ten special trays were fabricated from chemical-cure acrylic resin (Superacryl Plus, SpofaDental, To ensure equal thickness of the impression material, two sheets of wax, about 2.8 mm in width each, were added around the cylinder. At the dough stage, the acrylic resin was adapted over the wax. After the acrylic set, the special trays were finished and perforated. All impressions were made using high-precision chromatic alginate (Alginmax®, Major Prodotti Dentari, S.p.A., The manufacturer’s instructions were followed regarding mixing and working time, as well as powder and water ratio.

Type III dental stone was used for pouring the alginate impressions (Gyproc, Prevest DenPro, Sixty-seven cast models were obtained from impressions after two interval times. The models were divided into two groups. Group A, which had 37 impressions, was the control group and the casts were removed after 1 hour. Group B, which had 30 impressions, was the study group and the casts were removed after 6 hours. All casts were preserved for 48 hours in a well-sealed plastic bag until testing.

The properties tested were dimensional stability, surface details, and hardness. Dimensional stability was examined by measuring the distance on the cast between x and x' along the 50-µm line using a digital caliper (Digital Caliper, Hornady, Surface details were evaluated by reproduction of A, A', and A", according to the scores index (Table 1). Evaluation was performed under 4X magnification microscope and 100-watt artificial light (VanGuard, VEE GEE Scientific, Hardness was assessed with the digital caliper by measuring the depth of the indentation made by 997 Newton in 15 seconds. The casts were mounted on the testing machine (Avery's Brinell Hardness Tester, W&T Avery, Birmingham, England) to accommodate the hard ball indenter along the 50-µm line.

One examiner carried out all the tests. Three measurements were taken and the mean was calculated and used for the final analysis.


The data were entered and analyzed using SPSS software V22 (Statistical Package for Social Sciences, IBM, Descriptive statistics for dimensional stability and hardness are shown in Table 2. The mean value of dimensional stability of the control group (Group A, 1-hour contact) along the 50-µm line was 17.45 mm ± 1.66 mm, ranging from 17.64 mm to 17.99 mm. Similarly, the study group (Group B, 6-hour contact) showed a mean value of 17.66 mm ± 0.26 mm, ranging from 17.13 mm to 18.09 mm. No statistical significance was found between the two groups (P = .507) (Table 3).

With regard to hardness readings, the mean value of the control group was 0.52 ± 0.24 with a range between 0.20 and 1.17. However, the mean value of the study group was less, 0.28 ± 0.13, ranging from 0.09 to 0.53. Student’s t test showed statistical significant differences between the two groups (P = .001) (Table 3).

All lines used for surface detail evaluation (Table 1) were clearly duplicated by the two groups. No further test was used to explore the significance of differences between the groups.


Although alginate impression material has dimensional stability problems, it can produce an accurate stone cast if handled properly.6,17,21,22 Separation of the cast after a short period of contact time will produce a cast with good properties in terms of surface details, dimensional stability, and hardness. Some previous studies investigated the effect of 12-hour6 and 9-hour19 contact times on these properties. To the best of the authors’ knowledge, this study is the first to investigate the effect of 6-hour contact time on cast properties. The study group (6-hour contact) results showed no significant changes in detail reproduction and dimensional stability when compared to the control group (1-hour contact). However, there were significant differences regarding hardness of the resultant casts.

In the study conducted by Marquezan et al6 (12-hour contact), only 60% of the casts reproduced the 75-µm line, 33.33% reproduced the 50-µm line, and 6.66% reproduced the 25-µm line. These results were not compliant with ADA Specification Nos. 18 and 25. However, in the study conducted by Ibrahim et al19 (9-hour contact) 100% of the casts reproduced the 75-µm and 50-µm lines, and only 10% of the casts did not reproduce the 25-µm line. These results were compliant with ADA Specification Nos. 18 and 25. In the present study (6-hour contact), the three lines (75-µm, 50-µm, and 25-µm) were reproduced clearly on the casts, which is considered satisfactory according to both ADA Specification No. 18 for alginate impression material and ADA Specification No. 25 for gypsum products.

(Video) Taking an Awesome Alginate Impression

The dimensional stability was unaffected in this study (P = .507) between the two groups. This result is consistent with previous studies6,19 of 12- and 9-hour contact times, which means that increasing contact time has no effect on the dimensional stability of the resultant casts.

Finally, the hardness readings showed significant decrease with increased contact time (P = .001). This result is in agreement with the previous studies. Thus, the resultant casts after 6 hours of contact with alginate impression material appear to be more brittle than those that had 1 hour of contact.

Cast models and dies made of dental stone and used for cast dental restorations must provide dimensional stability, strength, and resistance to abrasion and must reproduce surface details.23 Alginate impressions, however, have dimensional stability problems over time because of water loss, which leads to surface roughness of the gypsum cast model.5 Good properties of cast models can be achieved by following the recommendations of pouring the dental stone immediately after the impression has been made or anytime within 60 minutes if the impression is kept in 100% humidity.


Though no significant changes in dimension or surface details were observed in the study, the hardness of the dental casts was significantly lower after 6 hours of contact time with the alginate impression compared to 1 hour of contact time. This may affect a cast’s strength and resistance to abrasion.

About the Authors

Amna Adam Ibrahim, BDS, MSc

Department of Oral Rehabilitation, Faculty of Dentistry, Khartoum University, Khartoum, Sudan; Department of Prosthodontics, Faculty of Dentistry, Al-Gezira University, Wad Madani, Sudan

Mohammed Nasser Alhajj, BDS, MSc

Department of Oral Rehabilitation, Faculty of Dentistry, Khartoum University, Khartoum, Sudan; Department of Prosthodontics, Faculty of Dentistry, Thamar University, Dhamar, Yemen

Nadia Khalifa, BDS, MSc, PhD

Department of General and Specialist Dental Practice, College of Dental Medicine, University of Sharjah, Sharjah, United Arab Emirates

Magdi Wadie Gilada, BDS, MSc

(Video) Alginate Impressions

Department of Oral Rehabilitation, Faculty of Dentistry, Khartoum University, Khartoum, Sudan


1. Powers JM, Wataha JC. Dental Materials: Properties and Manipulation. 10th ed. Philadelphia, PA: Elsevier Health Sciences; 2014.

2. Habib F, Fleischmann LdA, Gama SKC, et al. Obtenção de modelos ortodônticos. R Dental Press Ortodon Orthop Facial. 2007;12(3):146-156.

3. Imbery TA, Nehring J, Janus C, et al. Accuracy and dimensional stability of extended-pour and conventional alginate impression materials. J Am Dent Assoc. 2010;141(1):32-39.

4. Van Noort R. Introduction to Dental Materials. 4th ed. Philadelphia, PA: Elsevier Health Sciences; 2013.

5. Faria AC, Rodrigues RC, Macedo AP, et al. Accuracy of stone casts obtained by different impression materials. Braz Oral Res. 2008;22(4):293-298.

6. Marquezan M, Jurach EM, Guimaraes VD, et al. Does the contact time of alginate with plaster cast influence its properties? Braz Oral Res. 2012;26(3):197-201.

7. Nallamuthu NA, Braden M, Patel MP. Some aspects of the formulation of alginate dental impression materials—Setting characteristics and mechanical properties. Dent Mater. 2012;28(7):756-762.

8. Nandini VV, Venkatesh KV, Nair KC. Alginate impressions: a practical perspective. J Conserv Dent. 2008;11(1):37-41.

9. Revised American National Standards Institute/American Dental Association Specification No. 18. Alginate Impression Materials. 1992.

10. Saniour S, El-Ghaffar A, El-Bab F, et al. Effect of composition of alginate impression material on “recovery from deformation.” Journal of American Science. 2011;7(9).

(Video) How To Take Perfect Primary Dental Impressions and Make Perfect Stone Models (Pour Up Alignate)

11. Anusavice KJ, Shen C, Rawls HR. Phillips’ Science of Dental Materials. 12th ed. Philadelphia, PA: Elsevier Health Sciences; 2013.

12. Alberto N, Carvalho L, Lima H, et al. Characterization of different water/powder ratios of dental gypsum using fiber Bragg grating sensors. Dent Mater J. 2011;30(5):700-706.

13. He LH, van Vuuren LJ, Planitz N, et al. A micro-mechanical evaluation of the effects of die hardener on die stone. Dent Mater J. 2010;29(4):433-437.

14. Revised American National Standards Institute / American Dental Association Specification No. 25 for Dental Gypsum Products. J Am Dent Assoc. 1981;102(3):351.

15. Winkler MM, Monaghan P, Gilbert JL, Lautenschlager EP. Freeze-drying and scanning electron microscopy of setting dental gypsum. Dent Mater. 1995;11(4):226-230.

16. Lewry AJ, Williamson J. The setting of gypsum plaster. J Mater Sci. 1994;29(23):6085-6090.

17. Hamedi Rad F, Ghaffari T, Safavi SH. In vitro evaluation of dimensional stability of alginate impressions after disinfection by spray and immersion methods. J Dent Res Dent Clin Dent Prospects. 2010;4(4):130-135.

18. Chen SY, Liang WM, Chen FN. Factors affecting the accuracy of elastometric impression materials. J Dent. 2004;32(8):603-609.

19. Ibrahim AA, Alhajj MN, Gilada MW. Effect of contact time between alginate impression and type III dental stone on cast model properties. RSBO. 2015;12(3):252-257.

20. Myers HL, Myers LB. ‘It’s difficult being a dentist’: stress and health in the general dental practitioner. Br Dent J. 2004;197(2):89-93.

21. Walker MP, Burckhard J, Mitts DA, et al. Dimensional change over time of extended-storage alginate impression materials. Angle Orthod. 2010;80(6):1110-1115.

(Video) Orthodontic Alginate Impression Techniques

22. Nassar U, Aziz T, Flores-Mir C. Dimensional stability of irreversible hydrocolloid impression materials as a function of pouring time: a systematic review. J Prosthet Dent. 2011;106(2):126-133.

23. Ahila S, Subramaniam E. Comparative evaluation of dimensional stability and surface quality of gypsum casts retrieved from disinfected addition silicone impressions at various time intervals: an in vitro study. J Dent Oral Hyg. 2012;4(4):34-43.


Should alginate impression material remain in contact with the gypsum cast overnight why? ›

The 12-hour time of contact between alginate and the plaster cast is not recommended because it influences the quality of the plaster cast.

How long can you leave alginate impression? ›

Numerous studies have evaluated the dimensional accuracy of conventional alginates [2,7–15]. Some studies show that it is possible to store a hydrocolloid impression for up to 1, 2 and even 4 hours [8–10].

What will affect the setting time of alginate impression material? ›

Mixing the alginate.

Usual times are set at approx. 20 – 22 C (70 -73 F). Colder water retards setting, while warmer water hastens setting. Use cold water if you need to increase working and setting time.

What is the main risk of handling alginate? ›

: Handling and/or processing of this material may generate a dust which can cause mechanical irritation of the eyes, skin, nose and throat.

Can you leave alginate overnight? ›

An alginate impression should therefore never be left on the model overnight.

What is the longest time an alginate should be left in the wound? ›

Typically alginate dressings are changed every 1-3 days, however the dressing may be left in place for up to 7 days depending on the condition of the wound. Dressing change frequency will depend on wound characteristics and the amount of drainage.

What is the working time of normal alginate impression material? ›

Generally, 45 seconds to 60 seconds is sufficient depending on the brand and type of alginate impression material taken.

How long do impressions have to stay in your mouth? ›

In order for the impression to set, it must remain in a patient's mouth for two to three minutes. Once completed, the material hardens and creates a mold of the teeth.

Can you leave alginate impression overnight and pour the next day? ›

The recommendation is to immediately pour the alginate impression with dental stone to avoid dimensional changes; however, pouring can be delayed up to 60 minutes if the impression is kept in 100% humidity. Few studies have discussed the effect of contact time.

What is the control setting time of alginate? ›

American Dental Association (ADA) Specification No. 18 divides alginate into two types by its setting time. They are type I fast set with 75 s working time and 1-2 minutes setting time, type II normal set with 2 minutes working time and 2-4.5 minutes setting time [1,5, and 7].

How long do you have to pour an alginate impression before it starts to distort? ›

When working with alginate impressions, the model should be poured within minutes of when the impression is completed. This is necessary because alginate materials can start to distort after 5 minutes due to dimensional change.

What are the factors that affect the setting time of dental plaster? ›

A. Water, powder ratio: the more water used for mixing (thin mix), the fewer nuclei will be per unit volume; consequently, the setting time will be prolonged. B. Mixing time and rate: the longer and the more rapidly the gypsum is mixed, the shorter is the setting time.

What is a disadvantage of alginate? ›

Their disadvantages include less accurate reproduction of details as compared with elastomeric impression materials, poor dimensional stability, and that they are messy to work with.

When should you not use alginate? ›

They are to be avoided in dry or mildly exuding wounds because they may dry out these types of wounds. Their use in deep narrow sinuses is also contraindicated because removal may be difficult. As well as their absorptive ability, alginates also have hemostatic properties.

How long should you wait before pouring an alginate impression explain why? ›

Alginate impressions should generally be poured immediately or until 12 minutes in 100% humidity at room temperature. Alginate impression could be poured until 45 minutes if stored in 100% humidity at 4°C (SD=1).

How long is alginate stable for? ›

Conclusion: Alginate impressions should be cast within 10 minutes although, casts obtained from impressions stored by covering with wet gauze as is commonly done, for not more than 30 minutes will be clinically usable.

Is dental alginate hazardous? ›

The product is not classified as dangerous according to Directive 1999/45/EC and its amendments.

How long can alginate last? ›

Properly stored at room temperature (73°F/ 23°C) in a low humidity environment (below 50% RH), these products have a shelf life of 18 months from date of manufacture in unopened containers.

How long can Alginate Dressing stay on a wound? ›

Secure with an appropriate non‑occlusive cover dressing to help manage the wound drainage. its absorbent capacity or per facility protocol. Initially it may be necessary to change the dressing every 24 hours. The dressing remains effective for up to 14 days.

How long does calcium alginate stay on a wound? ›

In general, the life of an alginate dressing is 3 days, although some dressings can be left in place for as long as 7 days. If the wound is highly exudative, the life of the dressing will be shortened. If the alginate is being used for an infected wound, it should be changed frequently.

What are the pros and cons of alginate impressions? ›


It is inexpensive and can produce impressions with reasonable surface detail. Disadvantages: poor dimensional stability, poor tear strength, distortion if unsupported, requires good mixing otherwise air bubbles, minimum 3mm thickness required.

What is the setting time of impression material? ›

Elastomeric impression materials have a working time of approximately 2 minutes and a setting time of between 2 and 6 minutes (i.e., fast vs. regular set).

How long should dental impressions be disinfected before the impression is poured? ›

The most effective means of disinfecting impressions is to immerse them in disinfectant solution for 30 minutes.

How long after impressions can you get dentures? ›

How Long Does It Take For The Dentures To Be Ready? After your dentist takes your mouth impressions, they send the impression to a dental lab. At the dental lab, a dental technician carefully makes the dentures. The dentures should be ready in a day or two.

Are impressions contaminated? ›

Background: Dental impressions are a common source for transmission of infection between dental clinics and dental labs. Dental impressions can be cross-contaminated by patient's saliva and blood, which then cross-infect the dental casts poured from the impressions.

What is the best way to avoid distortion of an alginate impression? ›

The best way to avoid air bubbles on the occlusal surfaces in an impression is to: Rub some alginate onto the occlusal surfaces prior to placing the tray into the patient's mouth.

What is the best method for delaying the setting of alginate? ›

The set time of alginate is inversely proportional to the temperature of the water, so the colder the water the longer it takes to set, and you gain lots of working time. If you are looking to speed up the set, make the water warmer.

What are the two main factors that affect dental health? ›

The main risk factors for periodontal disease are poor oral hygiene and tobacco use.

What faults can occur during dental casting procedures? ›

Casting defects by Loss (Incomplete Casting)
  • Inadequate heating of the alloy.
  • Incomplete wax elimination.
  • Excessive cooling of the mold.
  • Insufficient casting force.
  • Inadequate amount of alloy.
  • Accessory sprues not used or not enough.
Aug 19, 2018

What are the factors affecting time of set? ›

A. The major factors affecting the time of setting are cement composition, water-cementitious material ratio (w/cm), temperature, and admixtures. When cement hydrates faster, the time of setting is reduced. Increases in the w/cm result in increased time of setting.

What might happen if alginate is not mixed thoroughly enough? ›

Improper mixing can distort the impression which in turn will cause distortion in the model.

What are the ideal properties of alginate? ›

The following are characteristics of an ideal alginate impression material:
  • cost effective,
  • easy to mix,
  • adequate flow properties,
  • non‐toxic or irritating,
  • sufficient strength to avoid tearing material upon removal from the mouth, and.
  • acceptable working and setting times.
Jul 18, 2020

What happens if you inhale alginate? ›

Inhalation: May cause respiratory irritation. May cause cancer by inhalation. Skin Contact: Causes skin irritation. Eye Contact: Causes serious eye damage.

Should you pour the gypsum immediately after taking impression Why? ›

The impression should be poured immediately after mixing. Pouring the impression should be completed within the working time. The gypsum begins to crystallize at the end of the working stage. No further work can be carried out because fine details can no longer be accurately reproduced after setting begins.

Why it is not recommended to place the alginate impression in the water? ›

[10] The impression should then be covered in a damp gauze/napkin to prevent syneresis (generally, not recommended to place in water, which would cause imbibition-expansion).

What term is used when an alginate impression shrinks from being out on the counter overnight? ›

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.

What is the working time of alginate material? ›

Generally, 45 seconds to 60 seconds is sufficient depending on the brand and type of alginate impression material taken.

How do you control the time of alginate? ›

The set time of alginate is inversely proportional to the temperature of the water, so the colder the water the longer it takes to set, and you gain lots of working time. If you are looking to speed up the set, make the water warmer.

What happens if you store the impression for after an hour before you pour the cast stone? ›

However, if you allow the impression material to remain in contact with the stone for more than a few hours, it becomes chalky, and as the alginate dries out, it becomes stiff and will tend to break off teeth as well. Bottom line: The casts should be removed, ideally, between 1-3 hours after pour-up.

How do you prevent air bubbles in alginate impression? ›

Smear the impression material (ex: alginate) onto the occlusal surfaces of the teeth and into undercuts by finger pressure before the impression tray is placed. This prevents the occurrence of trapping bubbles in the impression.


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