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Gypsum (2)


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  • 1. DEPARTMENT OF ORTHODONTICS Government dental college Trivandrum INDIAN DENTAL ACADEMY Leader in continuing dental education
  • 3. Gypsum products and its orthodontic applications
  • 4. Gypsum products History of gypsum. Chemistry. Manufacturing. Setting process. Properties of gypsum products.
  • 5. Gypsum products Classification of gypsum products. Recent developments. Orthodontic applications. Construction of a model. Disposal of gypsum.
  • 6. History of Gypsum Gypsos – Greek word – chalk. Oldest traces of plaster – Anatolia and Syria – 9000 years old. Egypt's – CHEOPS PYRAMID – 5000 years old. Windows made up of transparent gypsum – the SELENITE gypsum.
  • 7. Alabaster – king Solomon's temple. Phillips Pfaff introduced it to dentistry. Plaster of PARIS – MONTEMATRE – near to Paris - large manufacture of gypsum. King of France – all walls covered with plaster to prevent fire – after the big London fire destroyed the city, in 1966.
  • 8. Gypsum Gypsum – naturally occurring mineral (sedimentary rock) coloured from white, grey to brown according to the nature of impurities. Chemically – calcium sulfate usually the dihydrate form (caso4.2 H2o). Structurally – monoclinic space lattice.
  • 9. Gypsum products Refers to various forms of calcium sulfate, hydrous and anhydrous manufactured by calcinations of gypsum, the dihydrate. Also obtained from the byproducts of phosphoric acid manufacture. All these products are generally called gypsum plasters.
  • 10. Chemistry In the temperature ranges 20˚-700˚ C, three phase transformation occurs in the CaSO4.2H2O system. First two stages represent stages in dehydration of gypsum, & these are followed by a further transformation into anhydrous CaSO4.
  • 11. 1. CaSo4.2H2o  CaSo4.1/2H2o + H2o. (dihydrate) 40-45oc (hemihydrate) Gypsum Plaster or Stone 2. CaSo4.1/2H2o  γ CaSo4 90-100oc (hexagonal anhydrite) [soluble form] 3. γ CaSo4  β CaSo4 300-400oc (orthorhombic anhydrite) [insoluble form]
  • 12. Hexagonal anhydrate is unstable below 80˚c & if cooled to a lower temperature, it rapidly rehydrates in the presence of water vapor, to hemi hydrate. β CaSo4 is the stable anhydrous form in this system & exists as the mineral insoluble anhydrite .
  • 13. Theoretically hemi hydrate is the stable form of CaSo4only in the temperature range 45-90˚ C .It exists as the metastable phase under dry condition at lower temp. It has been shown that hydration can occur if particles are exposed to atmosphere under conditions where the water vapor pressure is high.
  • 14. Manufacturing Commercial manufacturing. Methods of calcining. 1.Dry calcination. 2.Wet calcination. Difference between POP and STONE.
  • 15. Commercial manufacturing At 45o-90o – more than 12 hrs.— to complete – not feasible. Higher temperatures used commercially. Single step conversion to hexagonal anhydrite – cooled down to temp below 80o – hemihydrates obtained.
  • 16. Methods of calcining OPEN AIR, DRY CALCINATION – no water for crystal reorganization – rough porous hemihydrate – α form – POP. Loss of water under dry conditions leaves parallel channels. so hemihydrates produced by dry calcinations are powders with low apparent densities, high relative surface area & poor packing abilities.
  • 17. WET CALCINATION -- β hemihydrate. (STONE) Sufficient water -- to allow reorganization, so prismatic crystals of hexagonal CaSO4.
  • 18. They rehydrate to hemihydrate on cooling to the air , but this secondary conversion cannot be accompanied by recrystallization . The final particles are pseudomorphic, monoclinic crystal structure of hemihydrate but retaining the hexagonal crystal habit of anhydrous CaSO4 precursor.
  • 19. Wet calcination Autoclaving 117Kpa 123oc 5-7 hrs Hydrocal Stone Investment material Boiling 30%Cacl2 Densite Improved stone Die stone Autoclaving 1%sodium succinate 130-140oc Crystacal
  • 20. Plaster Stone Die stone
  • 21. Difference between POP and STONE plaster (β-hemi hydrate) stone (α-hemi hydrate) Porous, irregular Prismatic, regular Very low apparent density High apparent density Needs more water to float particles (mix) Needs less water Low strength and hardness High strength and
  • 22. Setting process Setting mechanism. Theories of setting. Stages in setting. Water requirements (w:p ratio). Working time/setting times. Setting expansion.
  • 23. Setting mechanism Reversal of calcining. CaSO41/2H2O + 3/2H2O  CaSO42H20+∆ When 1 g mole of CaSO41/2H2O is made to react with 1.5 g mole of water 1g mole of CaSO42H2O is produced with evolution of 3900 cal of heat.
  • 24.
  • 25. Theories of setting 1.Crystalline theory -- Henry louis le chatelier. 2.Colloidal gel theory – Mahaelis.
  • 26. CRYSTALLINE THEORY (1887) : -- due to difference in solubility of dihydrate and hemihydrate. --hemihydrate --- 6.5g/l --dihydrate --- 2.4g/l at 20oc. Hemihydrate + water  two phase suspension is formed hemihydrate dissolves to form a super saturated solution  dihydrate crystallises out of it.
  • 27. Since dihydrate is less soluble and hemihydrate is used up, hemihydrate continues to dissolve. Two centers are there— precipitation centre - site of dihydrate formation dissolution centre - site of hemihydrate dissolution
  • 28. COLLOIDAL GEL THEORY (1893) : 1. The CaSo4 exists as the dispersed phase in the colloidal system. 2. There is a small delay before the dihydrate crystals precipitate out of the colloid – induction period. 3. As crystals grow the mass thickens and the consistency increases.
  • 29. Stages in setting FLUID STAGE : continuous aqueous phase present -- viscous liquid, exhibiting pseudo plasticity -- flows under vibration -- glossy surface. PLASTIC STAGE : gypsum crystals continue to grow at the expense of aqueous & viscosity of mix increases - glossy surface disappears.
  • 30. FRIABLE STAGE : Continuous crystal growth converts the plastic mass into a rigid solid, initially weak & friable. CARVABLE STAGE : Slowly continuous growth converts mix into solid mass which can be carved.
  • 31. Water requirements & W/P ratio Reaction of 1g mole of plaster with 1.5g mole of water produces 1g mole of gypsum material. 145g of plaster +27g of water  172 g of gypsum 100 g of plaster requires 18.6 g of water (ideal w:p ratio = 0.18).
  • 32. In practice -- Usual to mix 100 g of plaster 45 g of water to make it of usable consistency. Only 18.6 g of water takes part in the reaction & rest is distributed as free water in the set mass . Differing water requirements of plaster, stone, high strength stone are the result mainly of diff. in apparent densities of powder.
  • 33. Working & setting times Mixing time : time from the addition of powder to the water until mixing is completed. Mechanical mixing -- 20-30 sec. Hand spatulation -- 1 min.
  • 34. WORKING TIME : Time available to use a workable mix -- even consistency that may be manipulated. 3 min working time is generally adequate. SETTING TIME : The time elapsed from the beginning of mixing until the material hardens is known as the setting time. Usually measured by some kind of penetration tests.
  • 35.
  • 36. Tests to find setting times TESTS FOR INITIAL SETTING : (a) Loss of gloss: (b) Gillmore test: (c) Vicat test: TESTS TO FIND OUT FINAL SETTING TIME: (a) Gillmore test: (c) Vicat test:
  • 37. TESTS FOR INITIAL SETTING : (a) Loss of gloss: As the reaction proceeds -- excess water is taken up -- so that the mix loses its gloss. It occurs at approximately 9 min.
  • 38. (b) Gillmore test: The mix is spread out in a shallow container. The needle is lowered on to the surface and the time at which it no longer leaves an impression is called initial set. The needle used is ¼ pound or 113.4g weight with a 2.13 mm point.
  • 39. (c) Vicat test: A rod carrying a needle of 300g weight, 5cm length, tip cross section area 1mm is used. The time elapsed until the needle no longer penetrates to the bottom is the initial setting time.
  • 40. TESTS TO FIND OUT FINAL SETTING TIME: (a) Gillmore test: Here one pound or 453.6g needle with 1.06 mm diameter point is used. The time elapsed until the needle leaves only a barely perceptible mark on the surface is called the final setting time. (b) Vicat test is also used to find out the final setting time.
  • 41. Ready for use criterion Subjective measures of the time at which the set material may be safely used. Technically it may be considered the time when compressive strength is at least 80% of that attained at 1 hr. Most modern products reach the ready for use stage in 30 minutes.
  • 42. Control of setting time. Theoretically there are at least 3 methods by which setting time can be controlled. Increasing or decreasing the solubility of hemihydrate. Increasing or decreasing the nuclei of crystallization. Increasing or decreasing the rate of crystal growth.
  • 43. Variables affecting setting time Impurities -- ↑nuclei -- ↓ setting time. Fineness -- ↑ nuclei -- ↓ setting time. W : P -- > water -- ↓ nuclei -- ↑ setting time Mixing – increased mixing within limits -- ↑ nuclei -- ↓ setting time. Temperature – 2 effects. 1. as temp increases solubility ↓. 2. mobility increased.
  • 44. After 30oc – solubility is reduced – setting time increased. At 100oc -- solubility of hemihydrate and dihydrate are equal, so there is no reaction and the material does not set. Between temp range of 20o – 30oc – setting reaction is fastened– increase in mobility of Ca++ -- so setting time is reduced.
  • 45. Humidity : ↓ solubility of hemihydrate. --↑ setting time. Colloidal systems : agar, alginate, biological fluids (blood, saliva) decrease the setting time by adsorbing on the hemihydrate and dihydrate particles. Adsorption on the nucleation sites reduce the setting time more than on the hemihydrate. Accelerators and retarders : most effective way.
  • 46. Accelerators and retarders Accelerator : influences the rate of dissolution of the hemihydrate. without increasing the solubility of the dihydrate
  • 47. Accelerator : - 2% NaCl (above 2% act as a retarder). - 3.4% Na2SO4 (above 3.4% act as a retarder). - 2% K2SO4- Commonly used. Syngenite [K2 (CaSO4) 2H2O] formed which crystallizes rapidly. -Tera alba (ground gypsum) 1%. - Lime (CaO) .1% & Gum Arabic 1%. - Potassium sodium tartrate (Rochelle salt).
  • 48. Retarder : forms an adsorbed layer on the hemihydrates to reduce the solubility, and on the gypsum crystals to inhibit its growth. Citrates Acetates Borates Ortho rhombic anhydrite (very stable. Reacts less with water. So act as a retarder).
  • 49. Setting expansion Mechanism. Types of setting expansion 1.Normal setting expansion, 2.Hygroscopic setting expansion. Factors affecting setting expansion.
  • 50. Setting expansion Volumetrically if the equivalent volumes of hemihydrate water and the reaction product are compared the volume of dihydrate formed is less than the equivalent volume of hemihydrate and water.
  • 51. CaSO41/2H2O + 3/2H2O  CaSO42H20+∆ vol → (159.767) → (148.405) Change in volume = -7.11% (contraction ???) However a setting expansion is actually observed. This can be rationalized on the basis of crystallization mechanism.
  • 52. Crystallization process is pictured as an out growth of crystals from nuclei of crystallization. These crystals intermesh with and obstruct the growth of the adjacent crystals. An out ward thrust develops that produces an expansion of the entire mass. During the induction period a contraction of the mix occurs. Visible setting expansion is evident only after initial set.
  • 53.
  • 54. Hygroscopic expansion If the setting process is allowed to occur under water the setting expansion may be double in magnitude. (Mahler D.B, and Ady A.B ). This is because the setting takes place under water, water of hydration is replaced and the distance between the particles remains the same.
  • 55. Normal linear setting expansion = 0.15%. Hygroscopic setting expansion = 0.30% (twice).
  • 56. Factors affecting setting expansion Composition : finer the particles more the expansion (α hemihydrate > βhemihydrate). w/p ratio : the higher the w:p ratio lesser is the expansion. (less entanglement of crsytals). Spatulation : expansion is more with more spatulation. (more crystals/ unit volume). Accelerators and retarders : they decrease the setting expansion.
  • 57. Properties of gypsum products strength (compressive and tensile) 1.Wet strength, 2.Dry strength, 3.Factors affecting strength. Surface hardness and abrasion resistance. Dimensional stability. Surface detail reproduction.
  • 58. Strength usually expressed as compressive strength. The water content of the set product affects its strength. The strength is inversely related to the w/p ratio. 1 hr compressive strength values are Model plaster – 12 MPa, Dental stone – 31 MPa, High strength stone – 45
  • 59. There are two types of strength for the set plaster. Wet strength (Green strength), Dry strength. Wet strength : This is also known as green strength. This is obtained when the water in excess of that required for hydration of the hemihydrate is left in the test specimen.
  • 60. Dry strength : When the specimen has been dried of the excess water the strength obtained is the dry strength. Dry strength is 2 or more times than the wet strength. As the last traces of water leaves fine crystals of gypsum precipitate. If water is added the small crystals are the first one to dissolve and the reinforcing anchors are lost.
  • 61. Tensile strength Important because bending tends to occur because of lateral force applications, such as removal of cast from the impressions. 1hr wet tensile strength = ½ dry strength.
  • 62. Model plaster = ½ high strength stone. Model plaster = 1/5th of its compressive strength The high strength stone has a tensile strength of 8 MPa and compressive strength of 80 MPa under dry conditions ( 1/10 th).
  • 63. Factors affecting strength W : P ratio = indirectly proportional, (tensile strength is less affected by variations in w:p than compressive strength) Mixing time : within limits it is directly proportional, but over mixing leads to breakage of already formed crystals and less interlocking – less strength.
  • 64. effect of loss of water on compressive strength of dental stone :The hardened mass of stone contains 8.8% excess water. On 7.5% water loss there is a sharp increase in the compressive strength. And when all 8.8% water is lost the compressive strength is 55 MPa. Incorporation of accelerator & retarders decrease the strength. -- adulteration + decrease in intercrystalline cohesion.
  • 65. Material w/p ratio Comp strength Model plaster 0.45 12.5 0.50 11.0 0.55 9.0 Dental stone 0.27 31.0 0.30 20.5 0.50 10.5 High strength stone 0.24 38.0 0.30 21.5 0.50 10.5 w/p ratio Mixing time Compressive strength MPa psi 0.45 0.5 23.4 3400 0.45 1 26.2 3800 0.60 1 17.9 2600 0.60 2 13.8 2000 0.80 1 11.0 1600 W:P on strength Mixing time & strength
  • 66. Surface hardness and abrasion resistance related to the compressive strength (directly prop). Surface hardness increases at a faster rate than the comp strength and is maximum when dry strength is obtained.
  • 67. can be increased by impregnating gypsum with epoxy resins, and light cured dimethacrylate resins. Another method is to mix the dental stone with 30% colloidal silica solution . Measured using Rockwell hardness number, (RHN). (92 for type IV and 82 for Type III).
  • 68. Dimensional stability All gypsum products have good dimensional stability. Following setting the changes are immeasurable and this material is rigid to resist deformation when work is being carried out on them.
  • 69. Surface detail reproduction The surface detail reproduction is not as great as epoxy dies or electro plated dies because the surface of the set gypsum is porous at the microscopic level. Type I & II can reproduce a groove of width 75µ, where as Type III, IV & V can reproduce a groove of width 50 µ.
  • 70. The presence of air bubbles interfere with the fine reproduction of the detail. Contamination of the impression with saliva can also affect the surface detail reproduction. Rinsing the impression and blowing away the excess water can overcome this problem.
  • 71. Gypsum products History of gypsum. Chemistry. Manufacturing. Setting process. Properties of gypsum products.
  • 72. Gypsum products Classification of gypsum products. Recent developments. Orthodontic applications. Construction of a model. Disposal of gypsum.
  • 73. Classification of gypsum products A. International Standards Organizations – Classified gypsum in 1983 into 4 types ISO Type 1 -Impression plaster ISO Type 2 - Plaster ISO Type 3 - Stone ISO Type 4 -High strength stone
  • 74. ADA classification Impression Plaster Type I : Composed of plaster of Paris modifiers added are : 1.K2SO4 is added to ↓ the setting expansion. 2. Borax is added to counteract the acceleration in setting time caused by K2SO4.
  • 75. 3. Alizarin red -- to impart a pink color. ( 1+2+3 supplied as an aqueous solution which is called anti expansion or AE solution). 4. Gum tragacanth -- improve the cohesiveness. 5. Starch -- disintegrate the plaster when kept in boiling water facilitating the removal of cast. 6. Rarely used to make final or wash impression in complete
  • 76. Model plaster Type II : It is now used preferably to fill a flask in denture construction where setting expansion is not critical and the strength is adequate.
  • 77. Dental stone Type III [Balanced stone] [Hydrocal] [Class I stone] : Available in colored form, usually in green color used for fabricating casts. This is called balanced stone because it contains both accelerators.(K2SO4 / rock salt) and retarders (Borax/Sodium tetra borate).
  • 78. High strength stone Type IV [Densite] [Class II stone] : It is called die stone. Principal requisition for a die stone are strength hardness and minimum setting expansion. The particles are cubical in shape.
  • 79. High strength high expansion Type V : Has high compressive strength than type 4 stone. Improved strength is attained by making it possible to mix at a even lower W/P ratio. In addition setting exp has been increased from a max. of 0.10% to 0.30% to compensate for greater solidification shrinkage of newer base metal alloys.
  • 80. Type Set time (min) Set expansion At 2hrs Comp strength at 1 hr w/p ratio Min Max Kg/cm2 Psi Type I 4 1 00.00 00.15 40 20 580 290 0.5-0.25 Type II 12 4 0.00 0.30 90 1300 0.45-0.50 Type III 12 4 0.00 0.20 210 3000 0.28-0.30 Type IV 12 4 0.00 0.10 350 5000 0.22-0.24 Type V 12 4 0.00 0.30 490 2000
  • 81. Recent developments Impregnation of gypsum with a polymer like polystyrene, polyester, acrylics and epoxy resins -- occupy the porosities – increased strength and toughness.
  • 82. Incorporation of wetting agents such as lignosulphonates -- reduces water requirement of stone -- stronger and denser stone. These additives -- retard the setting time and increase the setting expansion. Overcome by the addition of K2SO4.
  • 83. Some fast setting stones are developed and are ready to use in 5 min. It has a little working time. Another type of stone which changes color when it is ready for use. Plastics and resins are added to decrease the brittleness and improve the resistance to scratching during carving of wax pattern in newer products.
  • 84. synthetic gypsum It is possible to make α hemihydrate and β hemihydrate from the byproducts or waste products of the manufacture of phosphoric acid -- more expensive -- but the properties exceed those of the latter. Power plants with lime or lime stone scrubbers convert sulfur dioxide to synthetic gypsum.
  • 85. Orthodontic applications model plaster : 1. used to make the base of orthodontic casts & study models, 2. mounting the cast on the articulator, and 3. for flasking for acrylization.
  • 86. dental stone , (hydrocal or orth.stone, quick stone): for preparing casts, study models. gypsum bonded investment: Used as soldering investment
  • 87. SOLDERING INVESTMENT: The investment for soldering contains quartz and hemihydrate binder -- designed to have lower setting and thermal expansions than casting investments, so that the assembled parts do not shift in position during setting and heating of the investment.
  • 88. Special gypsum products (A) Orthodontic stone [ADA Type III] : This is hard accurate stone of super white color for optimal aesthetics. Has ample working time -- multiple models with one mix. Has a glossy surface when the finished models are treated with model glow or soap.
  • 89. (B) Quick stone [ADA Type III] : It is an economic dental stone with consistent handling properties and fast setting time. Its very smooth mix makes it ideal for a variety of lab procedures.
  • 90. Properties Ortho. Stone Quick stone w/p ratio 28:100 28:100 Working time 5-7 min 3-5 min Setting time 11 min 10 min Comp. strength (wet) 31MPa 27MPa Comp. strength (dry) 59MPa 55MPa
  • 91. Construction of a model Proportioning. Mixing and pouring. 1. manual. 2. manual + mechanical. 3. power driven mechanical spatulator under vacuum. Finishing and caring of the cast.
  • 92. Proportioning. 1.Strict adherence to the stated W/p ratios is recommended. 2. Pre weighed envelops have become popular because they promote accuracy, reduce waste and save time. Powder should always be measured by a weighing balance and not by vol. (use of scoop), since the powder does not pack
  • 93. Mixing and pouring. Hand mixing : Flexible rubber bowl, Water is placed into the bowl first and then powder is sifted through, A stiff-bladed spatula is used to strop (push hard against the side-wall) against the mixing bowl while rotating the bowl in one’s hand. After about 1 minute, a smooth glossy mixture is produced
  • 94. Hand & Mechanical spatulation : Powder is incorporated during approx. 15 sec. of mixing by the hand, followed by 20-30 seconds of mechanical mixing under vacuum by a mixer. PROPERTIES HAND MIX POWER DRIVEN MIXED UNDER VACUUM SETTING TIME 8.0 7.3 COMPRESSIVE STRENGTH AT 24 HRS 43.1 45.5 SETTING EXP. AT 2 HR (%) .045 .037 VISCOSITY (CENTIPOISE) 54000 43000
  • 95.
  • 96. Disinfection of the impression The disinfectants can be mixed with water when the casts are poured. Mc Gill in 1988 used 5% phenol and Ivanovski et al 1995 used 2% gluteraldehyde. The casts and dies can be immersed in sodium hypochlorite solution (1:10 dilution) for 30 min after each clinical stage.
  • 97. iodophor can also be sprayed over the impression. Autoclave sterilization of the casts has been suggested by White and Brochhurt in 1996. Some loss of strength and surface hardness and an increase in dimension may occur.
  • 98. Pouring of the casts After disinfecting the impression -- dried with a paper towel or a gentle spray of compressed air. SURFACTANT is also sprayed for better wetting. Using the wax spatula a little bit of material is added while it is on the vibrator. It should be made sure that each tooth is full before we proceed to the next tooth.
  • 99. Once the impression is poured, it should be either placed in a humidor or 2%potassium sulfate while the stone hardens. A rough stone surface results if excess water remains in the cast before pouring. However the impression should not be completely dried – gel adheres to the cast on removal – also syneresis and distortion of the impression takes place.
  • 100. Separating the models Setting process goes through 3 stages: 1.Loss of gloss: When the mix becomes less shiny. Do not manipulate plaster beyond this point. 2.Gypsum feels warm to touch 3.Gypsum reaches room temperature Alginate can be removed from the stone after the stone has reached the third stage. This usually takes an hour.
  • 101. Compatibility with hydrocolloids With hydrocolloid impressions, the surface of the model may remain soft or peel off while removing the set cast from the impression -- due to the retarding effect which hydrocolloids have on setting of gypsum products.
  • 102. This can be overcome in two ways. By immersing the impression in a solution containing an accelerator for the setting of gypsum product before pouring the impression with the gypsum. By incorporating a plaster hardener in the impression material.
  • 103. The hardening solution act in 2 ways: (a) act as an accelerator overcoming the retarding action of the gel (b) reacts with the gel to produce a surface layer that reduces or prevents syneresis and retarding action of gel. Various hardening chemicals employed are potassium sulfate, zinc sulfate, manganese sulfate, and potash alum. The most effective is 2% potassium sulfate solution.
  • 104. Finishing the cast The models are soap polished. The soap solution is boiled for about 30 min and the models are kept in the solution for about 10 min. The casts are then dried and wiped with soft silk or nylon cloth or chamois cloth.
  • 105. When soap reacts with gypsum calcium sulfonate is formed which gives the shining surface. Nowadays commercial model glows are available which can be applied on the model surface.
  • 106. Caring the cast [a] Solubility If the cast immersed in running water the linear dimension may decrease approx 0.1% for every 20 minute. The safest method is to place it in a water bath in which plaster debris is allowed to remain constantly at the bottom to provide a solution of calcium sulfate.
  • 107. [b] Temperature If the storage temp is raised between 90- 110˚ C shrinkage occurs as the water of crystallization is removed and the dehydrate returns to the hemihydrate. The contraction of the plaster is greater than that of stones. So it is not safe to keep stone cast or heat a stone cast in air at temp higher than 55˚C.
  • 108. Caring for the gypsum products Gypsum products are sensitive to the changes in the relative humidity of the environment. It has been shown by Torrance and Darwell in 1990 that if the particles are exposed to the atmosphere where the water vapor pressure is high, hydration of the hemihydrate crystals can occur.
  • 109. Gypsum products made with thinner mix appear to be affected more than those with low w/p ratio. Skinner and farmer in 1942 proposed that it is safe to store gypsum products at a relative humidity of 70%. First manifestation of deterioration of plaster is increase in setting time. Best method is to store it in closed metal containers.
  • 110. Disposal of gypsum products The finer particles of gypsum when inhaled can cause respiratory problems. Gypsum products are considered as hazardous materials by the national board of gypsum. These products are kept in special polythene packs and containers which are taken away by the authorities.
  • 111. These products can be recycled and the dihydrate is converted to hemihydrate -- can be used again. The recycled gypsum can be used for agricultural purposes, in power plants, can be used in the manufacturing of cements.
  • 112. Scientists from Cairo university Ibrahim R M, Seniour S H, and Sheehab G T, have tried to prepare the dental stone from previously fabricated dental casts. Reheating at diff. conditions were tried by them in an autoclave. Results indicated that calcium sulfate dihydrate can be reproduced using previously fabricated casts.
  • 113. Advantages Inexpensive. Easy to use . Good accuracy and dimensional stability. Has the ability to reproduce details.
  • 114. Disadvantages Poor mechanical properties. Brittle, tensile strength. Poor abrasion resistance. Incompatibility with hydrocolloids.
  • 115. CONCLUSION Alternatives to gypsum do exist but are rarely used. They are generally less stable, difficult to use and are more expensive. Certainly the advantages and uses of gypsum out weigh its disadvantages. Gypsum products are inexpensive mold- making materials that have been used in dentistry since the 1850s because of their low cost and ease of mixing and are still used widely.
  • 116. Thank you
  • 117.
  • 118. Reference Philips science of dental materials – Anusavice Dental materials a programmed review of selected topics – William. J. O’Brien Restorative dental materials – Craig Applied dental materials – J. F. Mc Cabe Notes on dental materials – Combe Applied dental materials – John. A. Anderson
  • 119. Thank you For more details please visit