Gypsum Flashcards
model
a + likeness of an object
How gypsum is made
calcination-process of heating a solid will drive off volatile chemically combined components (i.e. water)
varying amts of heat
different products
Dihydrate
starting product2CaSo4.2H20
Beta Hemihydrate
plaster, irregular shape, spongy, more water
Alpha Hemihydrates
regular shape, dense, less water than beta
die stone
modified alpha hemihydrate, shorter and thicker crystals, less water than stone for mixing, higher strength than stone
mixing time mechanical
20-30 s
hand mix time
1 min
setting time, how to know intitial set
when the light-Gilmore needle can barely penetrate mixture
final setting determiner
barely a mark left by heavy gilmore needle
increase water/powder ratio
increased setting time, decreased strength, decreased setting expansion
longer vigorous mix means
shorter setting time
warm water
shorter setting time-erratic
chemical accelerants
Potassion S, NaCL, NaS, Borax
Borax
chemical accelerant which accelerates at low c and retards at high c
slurry water
provides additonal seeds of nucleation
setting expansion mech
crystal of dihydrate is an outgrowth, can intermesh and obstruct growth of adj crystals, excess water during drying –> porous
excess water loss during drying
porous structure
hydroscopic setting expansion
expansion that occurs when gypsum sets while immersed in water, 2x normal expansion, crystals grow more freely
wet strength
=green strength, less
dry strength
st. when all excess water is used, 2-3 times of wet st.
increase W/P ration
decrease st. (strength)-fewer hydrate crystals per unit V
spatulation
increaed mixing time increases st.
overmixing
dec st.-dec crystal interlocking
inc mixing time
inc. st.
type I gypsum
impression plaster, impressions and occlusal records-OBSOLETE-Runny
Type II
Model plaster, study models, denture flasking, not used much
Type III
dental stone, working casts and mounting on articulators
IV
High st. dental stone and low expansion-for wax patterns-used often
V
High st high expansion dental stone-dies for cast restirations w/ high casting shrinkage
Mounting Stone
faster setting, low expansion, lower strength, altered type III
Manipulation of Gypsum
storage, mL water, and stone powder in weight (grams)
mixing by hand
parabolic bowl, stiff spatula, water into bowl, powder sifter onto water, mix 1 min (longer mix dec working time), reduce air entrapment (bubbles) by high frequency/low amplitude vibration
longer mixing time for gypsum
dec working time
mechanical mix
mix water and powder, stir, mix in vacuum, vibrate, attach hose and mix, take of lid, vibrate with lid just barely on, RINSE LID!! Vibrate on smaller machine while you put in each tooth slowly
mechanical mix/pouring of an impression
mix water and powder, stir, mix in vacuum, vibrate, attach hose and mix, take of lid, vibrate with lid just barely on, RINSE LID!! Vibrate on smaller machine while you put in each tooth slowly, clean bowl out in garbage and sink
T/F can use normal infetion control on cast?
True
store above 130?
no
gypsum composoition
Calcium sulfate dihydrate
Add hardness to home water
gypsum
Gypsum product uses in dentistry
diagnostic/models, working models, dies, mounting casts, denture processing, metal casting
impression
a negative replica of the oral cavity
a negative replica of the oral cavity
impression
a positive likeness of an object
model
Diagnostic study casts are useful for
Evaluating dentition
Treatment planning
Patient education
Tracking Progress of otho/pros
a reproduction of a prepared tooth made from gypsum, metal, or refractory material
die
110 to 130 degrees calcination product
Plasters and stone (calcium sulfate hemihydrate)
130 to 200 degrees C calcination product
hexagonal anhydrite
200 to 1000 degrees calcination product
orthothrombic anhydrite
