Chapter 9 Quiz 3 Flashcards
a mineral ground and heated to drive out water particles through a process called calcination
gypsum
what is gypsum in a powder form
calcium sulfate hemihydrate
what is gypsum powder mixed with to form a slurry that can be poured and shaped and will harden into a rigid mass
water
gypsum is mainly used for ___________ reproductions of oral structures that are obtained from a negative mold
positive
used to treatment plan and to observe treatment progress
study model
replica of a restoration or applicance
cast
a working replica of a single tooth
die
what are the three types of gypsum used in dentistry
plaster
stone
high strength/improved stone
gypsum rock is made of __________
calcium sulfate dihydrate
when gypsum rock is heated or __________, it becomes calcium sulfate _________ in its powdered form
dehydrated
hemihydrate
calcium sulfate hemihydrate can be turned into
alpha calcium sulfate hemihydrate or beta calcium sulfate hemihydrate
porous; weakest; least wear resistant; cheapest; white in color
plaster type II
what are plasters often used for
preliminary casts
what can plasters be attached to to simulate occlusion
articulator
what is the composition of plaster
B-hemihydrate (requires more H2O=weaker)
stronger and more expensive; light tan color
stone (type III)
what is the composition of stone
a-hemihydrate (requires some water=pretty strong)
stones are used for what
diagnostic casts,
casts for denture construction,
retainers,
aligners, and
custom trays
dense; strongest; most wear resistant; most expensive
high-strength/improved stone (type IV)
what is the composition of high-strength/improved stones
a-hemihydrate
what are high-strength/improved stones used for
casts/dies,
crown,
bridge,
and inlay fabrication
the more water needed in gypsum, the ________ the setting expansion and the ___________ the compressive strength of the final product
greater
lower
because beta-calcium sulfate hemihydrate requires _______ water to reconstitute than the alpha-hemi-hydrate, it undergoes a __________ expansion and therefore has a _______ compressive strength compared to the alpha
more
greater
lower
the proportion of water to powder used to make a workable mix of a particular gypsum product
water-powder ratio
the water-powder ration depends on the type of _______
powder
plaster requires more or less water
more = thinner
improved stones require more or less water
less = thicker
water-powder ratio of plaster
45-50mL/100g
water-powder ration of stone
28-30mL/100g
water-powder ratio of improved stone
19-24mL/100g
which gypsum material is not too thin and not too thick
stone
length of time from the start of the mix until the setting mass reaches a semi-hard stage
working time (initial setting time)
length of time from the start of the mix until the setting mass becomes rigid and can be separated from the impression
final setting time
how long is final setting time
30-45 minutes
increase setting time so that it takes longer for the product to set
retarders
what increases working time
retarders
decrease setting time so that the product sets up faster
accelerators
what will decrease the working time
accelerators
all gypsum products expand ______ when they set
externally
expansion is controlled by the addition of ________ and by proper mixing techniques
chemicals
strength depends on the ______ of the material and is developed in the first 30-45 minutes of setting
porosity
set gypsum is stable under normal conditions, but highly soluble in ________
water
how long should you spatulate stone mixtures
30 seconds
what does the vibrator do
remove air bubbles
where should you place the stone from on the impression
distal to the last molar
pile the remainder of material on top of the impression to cover all areas by about ________ inches
0.25
allow gypsum mix to reach initial set which takes about _________ minutes and is evident by a loss of gloss
10
how many mL of gypsum mix go into the base of the model
36 mL
how thick should the paddy of the base be
1 inch
where should the stone material NOT touch
on the plastic or metal tray
how long should you wait before removing the cast from the impression
45-60 minutes
_________ mL H2O per bag of stone to fill impression
38-40
________ mL H2O per bag of stone for paddy
36-38
