Gypsum

• records the position, shape and dimensions of teeth • aids visualisation/assessment of dentition • enables manufacture of dental prosthesis (RPDs)

• plaster (β-hemihydrate) • dental stone (α-hemihydrate) • densite (improved stone)

• cast (plaster/stone) • die (stone/improved stone) • mould material (stone) • investment binder (stone)

manufacture conditions determine type of gypsum produced

• dihydrate is heated in an open vessel with air readily available • creates a powder with a large porous and irregular crystal structure

• dihydrate is heated in an autoclave (closed vessel) • produces non-porous and regular shaped crystals

• dihydrate is heated in the presence of Ca and Mg (CaCl2, MgCl2) • produces a more compact, less porous material, with smoother particles

reverse of manufacturing process

• plaster requires more - 50-60ml • stone - 20-35ml

• hemihydrate dissolves in water and dihydrate forms • dihydrate crystals precipitate on impurities as crystals and increase in size until all hemihydrate is dissolved • during initial set, dihydrate crystals grow until they contact and expansion starts • during final set, crystals are large and different sizes • gypsum is strong and hard and strength continues to develop

• excess water allows the powder and water to be mixed easily and achieve the correct consistency to produce an effective cast or die • during setting, excess water is trapped in the powder • on completion of setting, excess water is evaporated and leaves voids (porosity) • evaporation may cause dissolved dihydrate crystals to cement together

• reproduction of surface detail - gypsum is porous and produces a rough surface, reducing accuracy :( • strength - plaster-stone-densite - compressive strength develops over 24 hour period (less water=stronger) :) • low surface hardness - easy to abrade :( • setting time - must be convenient, not too long or short • expansion on setting - should be low :) • compatability with impression materials - need to wet the IM to reduce formation of voids and bubbles :/ • brittle - fractures with small amount of strain :(

• decreased setting time • increased expansion • more nuclei of crystallisation per unit volume so crystals come into contact sooner

• decreased setting time • increased expansion • breaks down growing dihydrate crystals so there are more nuclei of crystallisation • more crystals can be created and come into contact sooner

• decreased setting time • increased expansion • more impurities for dihydrate crystals to precipitate on so more nuclei of crystallisation • more crystals can be created and come into contact sooner

• can increase or decrease setting time depending on specific temperature range • does not affect expansion • water temp from 10-40 degrees reduces setting time • water temp from 40-80 degrees increases setting time • water temp above 80 degrees greatly increases setting time • due to increased rate of diffusion of ions and decreased solubility of hemihydrate with increased temperature

• can increase or decrease setting time • decreases expansion • borax decreases setting time as deposits calcium borate on dihydrate crystals, restricting growth • potassium sulfate accelerates setting time as it encourages development of more crystals

• dimensionally accurate and stable • low expansion of stone/densite • good colour contast

• low tensile strength • poor abrasion resistance • very brittle • surface detail less than elastomer impression • poor ‘wetting’ of some impression materials