1. Dental replicas: 2materials --> gypsum and epoxy
   1. desirable qualities
      1. Accuracy, dimensional stability, strength, reproduction of fine detail, resistance to abrasion, ease of adaptation
2. define term of replicas
   1. study models
      1. used for observation of pt's oral structures
   2. casts
      1. working model
   3. dies
      1. highly accurate replicas of single tooth,eg. for crown
3. 5 types of gypsum products (MODEL MATERIALS)
   1. impression plaster (articulating plaster)
   2. model plaster
   3. dental stone
   4. high-strength/ low-expansion stone (die stone)
   5. high-strength/high-expansion stone
4. Properties of gypsum materials
   1. Setting time
      1. Initial setting time (working time)
         1. freshly mixed material loses its gloss
            1. because hemihydrate uses up the water & surface water drawn into material bulk
         2. 8-16min
         3. Should not be manipulated
            1. because still weak & often fracture
      2. Final Setting time
         1. Convertion of hemihydrate to dihydrate is complete
         2. Can be removed from impression & manipulated without distortion & fracture
         3. release some heat
            1. allow set for few more minutes as arbitrary time
   2. Reproduction of detail
      1. Depends on
         1. Compatibility & interaction of model material & impression material
      2. Wetting ability of gypsum
         1. Reduced wetting limits the gypsum to flow into all details of impression
            1. Bubble formation
         2. to improve wetting
            1. use surfactant
            2. reduce contact angle of gypsum on impression
   3. Strength
      1. Compressive strength
         1. contributes to hardness & abrasion ressistance of materials
      2. Tensile strength
         1. ability to resist fracture during bending (when remove impression from model)
      3. When model/die is wet
         1. compressive & tensile strength is half of its strength when dry
            1. excess water evaporated
   4. Hardness & Abrasion resistance
   5. Dimensional accuracy
      1. ideally should neither expand nor contract
      2. but gypsum expand slightly on setting
         1. amount of expansion depends on types of gypsum materials
         2. expansion is not uniform in every directions
            1. buccal-lingual expansion may not equal vertical
         3. Addition of water to gypsum materials after initial mixing will increase setting expansion
            1. HYGROSCOPIC expansion
5. Chemical & physical nature
   1. physical form
      1. The physical forms of model plaster, dental stone & high-stength dental stone are different
         1. BUT, same chemical : Hemihydrate (CaSO4 . 1/2H20)
            1. Plaster
            2. small, irregular hemihydrate crystal (beta-calcium sulfate hemihydrate)
            3. dental stone
            4. larger, denser, more regular crystal (alpha-calcium sulfate hemihydrate)
            5. high stength stone
            6. least porous, most regular & largest hemihydrate (gypsum boiled under 30% calcium chloride)
         2. difference of these model materials & gypsum is the amount of water in the crystal
   2. chemical reaction & excess water
      1. calcium sulfate hemihydrate react with water to form dihydrate
         1. CaSo4 . 1/2H2O + 1 1/2H2O -->CaSo4. 2H2O + heat
      2. In practice, excess water must be added to produce workable mass
         1. increase setting time, reduce strength, reduce expansion & reduce hardness of set gypsum
      3. difference in crystal size and porosity
         1. plaster requires the most excess water, high-strength stone the least
         2. excess water do not react, but trapped in the mass when it sets
            1. will eventually evaporates
            2. leaves voids in the mass
            3. reduce the overall density of material
            4. Plaster has the least density (most porous)
   3. accelerators & Retarders
      1. Accelerators
         1. Potassium sulfate
         2. terra alba
            1. slurry water containing particles of dihydrate (from model trimmer)
      2. retarders
         1. Borax
         2. Colloidal particles
            1. blood, saliva, agar, unset alginate
   4. Water-powder ratio
      1. All model materials needs 19ml of water for every 100g powder to convert hemihydrate to dihydrate
         1. excess water
            1. model plaster
            2. 18-31ml
            3. dental stone
            4. 9-13ml
            5. high-strength stone
            6. 0-5ml
   5. Temperature & Humidity
      1. Slow setting if water temp increases
      2. Gypsum materials are hygroscopic(absorb water from air)
         1. if left in open air, surface particles will convert to dihydrate
6. Manipulation
   1. Measurement & combination of powder & water
      1. Water-powder ratio
      2. in mixing bowl
         1. water should be added first
            1. powder sprinkled into water slowly over 30sec
            2. allows air entrapped in powder to escape
            3. reduce bubbles
   2. Spatulation
      1. mixing powder & water
         1. Hand
         2. mechanical mixer
   3. Pouring the model
   4. Disinfection