Delayed setting and hygroscopic linear expansion of three gypsum products used for cast articulation



Delayed setting and hygroscopic linear expansion of three gypsum products used for cast articulation




Journal of Prosthetic Dentistry, 2009-11-01, Volume 102, Issue 5, Pages 313-318, Copyright © 2009 The Editorial Council of the Journal of Prosthetic Dentistry


Statement of problem

Wetting of the set gypsum product used for cast articulation may result in additional expansion.

Purpose

The purpose of this study was to evaluate the linear expansion of 2 type II and 1 type III gypsum products with and without addition of water after setting, over a time period of 120 hours.

Material and methods

The gypsum products tested in this study were Model Plaster, Lab Plaster, and Mounting Stone. All materials were hand mixed for 5 seconds and mechanically mixed under vacuum for another 15 seconds. The mixes were poured into a linear expansion-measuring instrument (n=20). Half of the specimens were wet with 25 ml of water immediately after the setting time recommended by the manufacturer, and at all time intervals, immediately after data collection. No treatment was performed on the remainder of the specimens. The expansion values were recorded with an expansion-measuring device over a period of 120 hours. Collected data were subjected to a 2-way repeated-measures analysis of variance (ANOVA) (α=.05).

Results

The highest expansion values for all gypsum products were recorded at 96 and 120 hours. No significant difference in setting expansion values was recorded between these 2 time intervals for any of the materials and treatment conditions (dry vs. wet). Mean expansion values ranged between 0.08% ±0.06% for dry Mounting Stone specimens at 15 minutes, to 0.57% ±0.02% for wet Lab Plaster specimens at 96 and 120 hours. Significant differences ( P <.001) between different mounting gypsum products and time were identified. A significant difference between wet and dry specimens was not observed.

Conclusions

The expansion of all gypsum products used for articulation purposes was complete at 96 hours. The type III gypsum product demonstrated lower mean expansion values than type II products. Time and material were more important factors than the dry/wet condition when measuring total expansion values. Type II gypsum products demonstrated 80% or more of the total expansion values in the first 45 minutes, while type III stone demonstrated the same expansion after 24 hours. (J Prosthet Dent 2009;102:313-318)

Clinical Implications

Articulating errors in the laboratory can be minimized by selecting a gypsum product that has low setting and hygroscopic expansion and by minimizing the time for the laboratory procedures.

Diagnosis, treatment planning, and accurate execution of planned restorative procedures require that the diagnostic and the definitive casts be mounted on an articulator. Apart from the techniques used for the articulation of the casts, the use of interocclusal recording media and gypsum products which demonstrate dimensional stability over time are important.

Plaster and stone products used in dentistry are made by calcining calcium sulfate dehydrate. The principal constituent of gypsum-based products is calcium sulfate hemihydrate, which is CaSO 4 . 1/2 H 2 O. Gypsum products are classified by ADA Specification 25 into 5 types : type I: impression plaster; type II: model plaster; type III: stone; type IV: high strength, low expansion stone; type V: high strength, high expansion stone. Of these, types II and III are usually used for articulation purposes.

The articulating technique for both diagnostic and definitive casts is essentially the same. A small inaccuracy in articulated diagnostic casts may provide enough information for diagnosis and treatment planning. However, articulation of the definitive casts is more crucial, since minute errors may cause occlusal discrepancies, which can require lengthy clinical appointments or additional laboratory procedures for correction.

Gypsum products exhibit dimensional changes during setting. A decrease in the true volume of the reactants can be observed in the early phase of setting, while the mix is still fluid. However, as the reaction advances, gypsum crystals start forming and an isotropic expansion is observed. Although ADA Specification 25 requires the linear expansion measurements to be made 2 hours after mixing, authors have reported that delayed linear setting expansion can continue for up to 120 hours. In addition to the setting expansion, hygroscopic expansion is observed when the gypsum products are exposed to additional water while setting. Several theories have been introduced to explain the mechanism for hygroscopic expansion. Nevertheless, the influence of water on set gypsum products used for cast articulation has not been investigated thoroughly. Since plaster and stone used for mounting the casts are often wet with water in the early stages that follow cast articulation, during plaster addition and polishing with wet sandpaper, further investigation is needed. Any dimensional changes that may occur in the mounting plaster or stone can affect the 3-dimensional occlusal contacts of the definitive prostheses.

The purpose of this study was to measure the linear dimensional changes of 3 different gypsum products used for mounting procedures after exposing them to water treatment after setting, and to compare these changes with linear setting expansion values of specimens not exposed to water treatment, at 15 minutes, 45 minutes, and 2, 24, 72, 96, and 120 hours. The null hypothesis was that linear dimensional changes of the examined gypsum products would not be affected by the water treatment.


Material and methods

Three gypsum products used routinely for cast articulation were included in this study: Model Plaster (Heraeus Kulzer GmbH, Hanau, Germany), Lab Plaster (Dentsply Trubyte, York, Pa), and Mounting Stone (Whip Mix Corp, Louisville, Ky). The first 2 gypsum products are classified as plasters (type II) and require a powder (g) to water (ml) ratio of 100:47, while the third is classified as a dental stone (type III) and requires a powder (g) to water (ml) ratio of 100:26. Manufacturers' instructions were followed for the mixing procedures. Therefore, a 5-second hand mixing was followed by a 15-second mechanical mixing. An electronic scale (EC-411; Acculab Sartorius Group, Gotting, Germany) was used to measure the gypsum powder. Distilled water (Holyoke Distilled Water Co, Holyoke, Mass) was first measured and added in a vacuum bowl (Twister Pro; Renfert GmbH, Hilzingen, Germany). The gypsum product was then added. A spatula with a stiff blade (3R; Buffalo Dental Mfg Co, Syosset, NY) was used for a 5-second hand mixing to fully incorporate the powder into the water. Mechanical mixing under vacuum at 25 mm Hg for 15 seconds followed. The mixture was then poured into a V-shaped mold, which was part of an expansion-measuring device (EMI 100; SAM Präzisiontechnik GmbH, Munich, Germany). The mold was lined with a rubber dam (Hygenic Dental Dam; Coltène/Whaledent, Inc, Cuyahoga Falls, Ohio). The dial gauge attached to the device was calibrated to 0.01 mm. Twenty specimens of each material were fabricated to be 100 mm in length and triangular in cross-section (33 × 50 × 33 mm) ( Fig. 1 ). The guidelines of ADA Specification 25 and International Organization for Standardization (ISO) 6873 were followed for the fabrication of the specimens. Half of the specimens were wet with 25 ml of distilled water immediately after the setting time recommended by the manufacturer: after 8 minutes for the Model Plaster (Heraeus Kulzer GmbH) and the Lab Plaster (Dentsply Trubyte), and after 5 minutes for the Mounting Stone (Whip Mix Corp). The same quantity of water (25 ml) was also used for wetting the specimens at all time intervals immediately after data collection. Water was poured slowly over the treated specimens to facilitate full absorption. No treatment was rendered for the remaining specimens.

Linear setting expansion-measuring instrument with specimen in V-shaped tray.
Fig. 1
Linear setting expansion-measuring instrument with specimen in V-shaped tray.

Measurements for each specimen were made at 7 time intervals: 15 minutes, 45 minutes, and 2, 24, 72, 96, and 120 hours. Specimen treatment and data collection were accomplished by the same operator. The linear dimensional change raw data was converted to a percentage value by the following equation:

(L c /L o ) × 100% where L c represents the specimen's change in length, and L o represents the original specimen length in millimeters. The temperature of the distilled water was 23°C ±2°C. Room temperature (21°C ±1°C) and relative humidity (50% ±10%) were recorded each day throughout the experiment. Two-way repeated-measures analysis of variance (ANOVA) (α =.05) was used to determine statistically significant differences among different gypsum products and different treatment conditions over time.


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Delayed setting and hygroscopic linear expansion of three gypsum products used for cast articulation Konstantinos X. Michalakis DDS, MSc, PhD , Athanasios Stratos DDS , Hiroshi Hirayama DDS, DMD, MS , Argiris L. Pissiotis DDS, MS, PhD and Foteini Touloumi DDS Journal of Prosthetic Dentistry, 2009-11-01, Volume 102, Issue 5, Pages 313-318, Copyright © 2009 The Editorial Council of the Journal of Prosthetic Dentistry Statement of problem Wetting of the set gypsum product used for cast articulation may result in additional expansion. Purpose The purpose of this study was to evaluate the linear expansion of 2 type II and 1 type III gypsum products with and without addition of water after setting, over a time period of 120 hours. Material and methods The gypsum products tested in this study were Model Plaster, Lab Plaster, and Mounting Stone. All materials were hand mixed for 5 seconds and mechanically mixed under vacuum for another 15 seconds. The mixes were poured into a linear expansion-measuring instrument (n=20). Half of the specimens were wet with 25 ml of water immediately after the setting time recommended by the manufacturer, and at all time intervals, immediately after data collection. No treatment was performed on the remainder of the specimens. The expansion values were recorded with an expansion-measuring device over a period of 120 hours. Collected data were subjected to a 2-way repeated-measures analysis of variance (ANOVA) (α=.05). Results The highest expansion values for all gypsum products were recorded at 96 and 120 hours. No significant difference in setting expansion values was recorded between these 2 time intervals for any of the materials and treatment conditions (dry vs. wet). Mean expansion values ranged between 0.08% ±0.06% for dry Mounting Stone specimens at 15 minutes, to 0.57% ±0.02% for wet Lab Plaster specimens at 96 and 120 hours. Significant differences ( P <.001) between different mounting gypsum products and time were identified. A significant difference between wet and dry specimens was not observed. Conclusions The expansion of all gypsum products used for articulation purposes was complete at 96 hours. The type III gypsum product demonstrated lower mean expansion values than type II products. Time and material were more important factors than the dry/wet condition when measuring total expansion values. Type II gypsum products demonstrated 80% or more of the total expansion values in the first 45 minutes, while type III stone demonstrated the same expansion after 24 hours. (J Prosthet Dent 2009;102:313-318) Clinical Implications Articulating errors in the laboratory can be minimized by selecting a gypsum product that has low setting and hygroscopic expansion and by minimizing the time for the laboratory procedures. Diagnosis, treatment planning, and accurate execution of planned restorative procedures require that the diagnostic and the definitive casts be mounted on an articulator. Apart from the techniques used for the articulation of the casts, the use of interocclusal recording media and gypsum products which demonstrate dimensional stability over time are important. Plaster and stone products used in dentistry are made by calcining calcium sulfate dehydrate. The principal constituent of gypsum-based products is calcium sulfate hemihydrate, which is CaSO 4 . 1/2 H 2 O. Gypsum products are classified by ADA Specification 25 into 5 types : type I: impression plaster; type II: model plaster; type III: stone; type IV: high strength, low expansion stone; type V: high strength, high expansion stone. Of these, types II and III are usually used for articulation purposes. The articulating technique for both diagnostic and definitive casts is essentially the same. A small inaccuracy in articulated diagnostic casts may provide enough information for diagnosis and treatment planning. However, articulation of the definitive casts is more crucial, since minute errors may cause occlusal discrepancies, which can require lengthy clinical appointments or additional laboratory procedures for correction. Gypsum products exhibit dimensional changes during setting. A decrease in the true volume of the reactants can be observed in the early phase of setting, while the mix is still fluid. However, as the reaction advances, gypsum crystals start forming and an isotropic expansion is observed. Although ADA Specification 25 requires the linear expansion measurements to be made 2 hours after mixing, authors have reported that delayed linear setting expansion can continue for up to 120 hours. In addition to the setting expansion, hygroscopic expansion is observed when the gypsum products are exposed to additional water while setting. Several theories have been introduced to explain the mechanism for hygroscopic expansion. Nevertheless, the influence of water on set gypsum products used for cast articulation has not been investigated thoroughly. Since plaster and stone used for mounting the casts are often wet with water in the early stages that follow cast articulation, during plaster addition and polishing with wet sandpaper, further investigation is needed. Any dimensional changes that may occur in the mounting plaster or stone can affect the 3-dimensional occlusal contacts of the definitive prostheses. The purpose of this study was to measure the linear dimensional changes of 3 different gypsum products used for mounting procedures after exposing them to water treatment after setting, and to compare these changes with linear setting expansion values of specimens not exposed to water treatment, at 15 minutes, 45 minutes, and 2, 24, 72, 96, and 120 hours. The null hypothesis was that linear dimensional changes of the examined gypsum products would not be affected by the water treatment. Material and methods Three gypsum products used routinely for cast articulation were included in this study: Model Plaster (Heraeus Kulzer GmbH, Hanau, Germany), Lab Plaster (Dentsply Trubyte, York, Pa), and Mounting Stone (Whip Mix Corp, Louisville, Ky). The first 2 gypsum products are classified as plasters (type II) and require a powder (g) to water (ml) ratio of 100:47, while the third is classified as a dental stone (type III) and requires a powder (g) to water (ml) ratio of 100:26. Manufacturers' instructions were followed for the mixing procedures. Therefore, a 5-second hand mixing was followed by a 15-second mechanical mixing. An electronic scale (EC-411; Acculab Sartorius Group, Gotting, Germany) was used to measure the gypsum powder. Distilled water (Holyoke Distilled Water Co, Holyoke, Mass) was first measured and added in a vacuum bowl (Twister Pro; Renfert GmbH, Hilzingen, Germany). The gypsum product was then added. A spatula with a stiff blade (3R; Buffalo Dental Mfg Co, Syosset, NY) was used for a 5-second hand mixing to fully incorporate the powder into the water. Mechanical mixing under vacuum at 25 mm Hg for 15 seconds followed. The mixture was then poured into a V-shaped mold, which was part of an expansion-measuring device (EMI 100; SAM Präzisiontechnik GmbH, Munich, Germany). The mold was lined with a rubber dam (Hygenic Dental Dam; Coltène/Whaledent, Inc, Cuyahoga Falls, Ohio). The dial gauge attached to the device was calibrated to 0.01 mm. Twenty specimens of each material were fabricated to be 100 mm in length and triangular in cross-section (33 × 50 × 33 mm) ( Fig. 1 ). The guidelines of ADA Specification 25 and International Organization for Standardization (ISO) 6873 were followed for the fabrication of the specimens. Half of the specimens were wet with 25 ml of distilled water immediately after the setting time recommended by the manufacturer: after 8 minutes for the Model Plaster (Heraeus Kulzer GmbH) and the Lab Plaster (Dentsply Trubyte), and after 5 minutes for the Mounting Stone (Whip Mix Corp). The same quantity of water (25 ml) was also used for wetting the specimens at all time intervals immediately after data collection. Water was poured slowly over the treated specimens to facilitate full absorption. No treatment was rendered for the remaining specimens. Fig. 1 Linear setting expansion-measuring instrument with specimen in V-shaped tray. Measurements for each specimen were made at 7 time intervals: 15 minutes, 45 minutes, and 2, 24, 72, 96, and 120 hours. Specimen treatment and data collection were accomplished by the same operator. The linear dimensional change raw data was converted to a percentage value by the following equation: (L c /L o ) × 100% where L c represents the specimen's change in length, and L o represents the original specimen length in millimeters. The temperature of the distilled water was 23°C ±2°C. Room temperature (21°C ±1°C) and relative humidity (50% ±10%) were recorded each day throughout the experiment. Two-way repeated-measures analysis of variance (ANOVA) (α =.05) was used to determine statistically significant differences among different gypsum products and different treatment conditions over time. Results The results of the descriptive statistics for mean setting expansion and standard deviation values at different time intervals for dry and wet specimens are summarized in Table I . The highest expansion values for all gypsum products were recorded at 96 and 120 hours. No significant difference in setting expansion values was recorded between these 2 time intervals, for any of the materials and treatment conditions ( Table I ). Mean expansion values ranged between 0.08% ±0.06% for dry Mounting Stone (Whip Mix Corp) specimens at 15 minutes, to 0.57% ±0.02% for wet Lab Plaster (Dentsply Trubyte) specimens at 96 and 120 hours. Mounting Stone and Lab Plaster specimens did not present different expansion values between the 45-minute and 2-hour time intervals, for both treatment conditions. Two-way repeated-measures ANOVA revealed significant differences ( P <.001) between different mounting gypsum products and time ( Table II ). Gypsum products that were treated with additional water demonstrated more setting expansion at all time intervals. There was no significant difference between wet and dry specimens (interaction material × condition) ( P =.922) ( Table II ). Type II gypsum products demonstrated 50% of the total expansion in the first 15 minutes, and about 80% or more of the total expansion in the first 45 minutes. However, type III stone presented 35%-44% of the total expansion in the first 15 minutes, depending on dry or wet condition, 65%-72% in the first 45 minutes, and 74%-80% after 24 hours ( Table III ). There was no significant difference between dry and wet specimens of Model Plaster (Heraeus Kulzer GmbH) and Mounting Stone (Whip Mix Corp) between 45 minutes and 2 hours. TABLE I Mean setting expansion values (standard deviation) at different time intervals for dry and wet specimens (%) Time Model Plaster (Dry) Model Plaster (Wet) Lab Plaster (Dry) Lab Plaster (Wet) Mounting Stone (Dry) Mounting Stone (Wet) 15 min 0.18 (0.14) 0.20 (0.02) 0.26 (0.02) 0.27 (0.01) 0.08 (0.06) 0.11 (0.09) 45 min 0.29 (0.02) 0.31 (0.02) 0.47 (0.02) 0.49 (0.02) 0.15 (0.01) 0.18 (0.01) 2 h 0.30 (0.01) 0.32 (0.01) 0.47 (0.02) 0.49 (0.02) 0.15 (0.01) 0.18 (0.01) 24 h 0.32 (0.01) 0.33 (0.14) 0.49 (0.02) 0.52 (0.02) 0.17 (0.01) 0.20 (0.01) 48 h 0.33 (0.01) 0.35 (0.02) 0.51 (0.02) 0.53 (0.02) 0.19 (0.01) 0.22 (0.02) 72 h 0.35 (0.01) 0.38 (0.01) 0.53 (0.02) 0.55 (0.02) 0.21 (0.01) 0.24 (0.02) 96 h 0.36 (0.02) 0.40 (0.01) 0.54 (0.02) 0.57 (0.02) 0.23 (0.01) 0.25 (0.02) 120 h 0.36 (0.02) 0.40 (0.01) 0.54 (0.02) 0.57 (0.02) 0.23 (0.01) 0.25 (0.02) TABLE II Two-way repeated-measures ANOVA ( P <.05) Source Type IV Squares df Mean Square F P Tests of within-subjects; effects Time Greenhouse-Geisser 1.968 2.712 0.726 2389.776 <.001 Time × material Greenhouse-Geisser 0.191 5.424 0.035 116.069 <.001 Time × condition Greenhouse-Geisser 0.002 2.712 0.001 2.549 .064 Time × material × condition Greenhouse-Geisser 0.003 5.424 0.000 1.541 .175 Error (time) 0.044 146.451 0.000 Tests of between-subjects effects Intercept 53.881 1 53.881 44704.550 <.001 Material 7.266 2 3.633 3014.351 <.001 Condition 0.062 1 0.062 051.530 <.001 Material × condition 0.000 2 9.750 ×10 −5 0.081 .922 Error 0.065 54 0.001 TABLE III Percentage of total mean expansion values for all time intervals Time Model Plaster (Dry) Model Plaster (Wet) Lab Plaster (Dry) Lab Plaster (Wet) Mounting Stone (Dry) Mounting Stone (Wet) 15 min 50 50 48 47.5 35 44 45 min 80 77.5 87 86 65 72 2 h 83 80 87 86 65 72 24 h 88 82.5 90 91 74 80 48 h 91 87.5 94.5 93 83 88 72 h 97 95 98 96.5 91 96 96 h 100 100 100 100 100 100 Discussion The present in vitro study investigated the effect of water treatment of different mounting gypsum products on setting expansion. The results of this study indicate that water treatment does not significantly affect the setting expansion values. Therefore, the null hypothesis is not rejected. Only the linear setting expansion of gypsum products used for cast articulation over time was measured in this study. These measurements provide only a partial indication as to what occurs 3-dimensionally. As expected, type II gypsum products demonstrated higher expansion values than type III stone. All gypsum products included in the present study showed continuous setting expansion for all time intervals up to 96 hours. These results are in agreement with those of Heshmati et al, who reported that at 96 hours, gypsum expansion was complete. It should be mentioned, however, that type IV and V dental stones were used, and the specimens were not wet with water after their initial setting in that study. The results of the present study indicate that the addition of water to initially set gypsum products has some positive effect on final setting expansion values. As previously mentioned, published literature has focused on the effect that water has on the setting material. Hygroscopic expansion of dental gypsum products is observed when additional water is brought into contact with gypsum while the material is setting. Several theories have been proposed for the explanation of this phenomenon. The most well-accepted theory for the increased expansion observed when the hemihydrate reacts to water contact is that free crystal growth occurs, since water is drawn into pores forming in the setting gypsum mass, and provides more space for longer crystal formation. However, when gypsum products set in air, the amount of water is reduced to a film, and there is an increased surface tension which restricts crystal growth. Since the experimental data demonstrate that the setting process is not complete until 96 hours have elapsed, it can be assumed that water penetrates into the micropores of the material and further decreases the surface tension, allowing the gypsum crystals to grow more. The setting expansion difference between the specimens to which water was added and those to which water was not added was not significant. Thus, it can be assumed that water addition during some laboratory procedures, such as wetting of the set gypsum product before addition of a small quantity of mounting stone or use of wet sandpaper for stone polishing, will not clinically affect the restorative procedures. However, the results of the statistical analysis may indicate the possibility that the sample size used in the present study was not large enough to reveal whether the treatment of the specimens can affect the final outcome or not. Thus, further investigation is needed. The results of this study suggest that linear setting and hygroscopic expansion are influenced more by the material used and by time than by the addition of water. Based on these findings, a material which presents a low setting and hygroscopic expansion should be selected to minimize articulating errors in the laboratory. In addition, the laboratory procedures, such as waxing, framework finishing, and porcelain application, should be completed as soon as possible. It should be mentioned, however, that in most commercial laboratories, completion of these procedures in a short time may not be feasible. Conclusions Within the limitations of this in vitro study, the following conclusions were drawn: 1. The expansion of all gypsum products used for articulation purposes was complete at 96 hours. 2. The type III gypsum product demonstrated significantly lower mean expansion values than type II products. 3. Time and material are more important factors than the dry/wet condition when measuring total expansion values. 4. Type II gypsum products demonstrated 80% or more of their total expansion values within the first 45 minutes, while type III gypsum demonstrated the same expansion (80%) after 24 hours. References 1. Millstein PL, Hsu CC: Differential accuracy of elastomeric recording materials and associated weight change. J Prosthet Dent 1994; 71: pp. 400-403. 2. Heshmati RH, Nagy WW, Wirth CG, Dhuru VB: Delayed linear expansion of improved dental stone. J Prosthet Dent 2002; 88: pp. 26-31. 3. Anusavice KJ: Phillips' science of dental materials.2003.ElsevierSt. Louis:pp. 255-281. 4. O'Brien WJ: Dental materials and their selection.2008.QuintessenceChicago:pp. 38-61. 5. American National Standards/American Dental Association Specification No. 25 : Dental gypsum products.2000.American National Standards InstituteNew York:pp. 244-253. 6. Rosenstiel SF, Land MF, Fujimoto J: Contemporary fixed prosthodontics.2006.ElsevierSt. Louis:pp. 544-547. 7. Sweeney WT, Taylor DF: Dimensional changes in dental stone and plaster. J Dent Res 1950; 29: pp. 749-755. 8. Lautenschlager EP, Harcourt JK, Ploszaj LC: Setting reactions of gypsum materials investigated by x-ray diffraction. J Dent Res 1969; 48: pp. 43-48. 9. Lautenschlager EP, Corbin F: Investigation on the expansion of dental stone. J Dent Res 1969; 48: pp. 206-210. 10. Harcourt JK, Lautenschlager EP: Accelerated and retarded dental plaster setting investigated by x-ray diffraction. J Dent Res 1970; 49: pp. 502-507. 11. Winkler MM, Monaghan P, Gilbert JL, Lautenschlager EP: Comparison of four techniques for monitoring the setting kinetics of gypsum. J Prosthet Dent 1998; 79: pp. 532-536. 12. Santos JF, Ballester RY: Delayed “hygroscopic” expansion of gypsum products. J Prosthet Dent 1984; 52: pp. 366-370. 13. Lyon HW, Dickson G, Schoonover IC: The mechanism of hygroscopic expansion in dental casting investments. J Dent Res 1955; 34: pp. 44-50. 14. Ryge G, Fairhurst CW: Hygroscopic expansion. J Dent Res 1956; 35: pp. 499-508. 15. Mahler DB, Ady AB: An explanation for the hygroscopic setting expansion of dental gypsum products. J Dent Res 1960; 39: pp. 578-589. 16. Holst K: Unintended hygroscopic expansion of dental stone and investment. Acta Odontol Scand 1962; 20: pp. 197-203. 17. International Organization for Standardization (ISO) : 1998.International Organization for StandardizationGeneva Available at: http://www.iso.org/iso/search.htm?qt=ISO+6873%3A1998&searchSubmit=Search&sort=rel&type=simple&published=on

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