Gypsum Products (properties) Quiz 2 Flashcards
tensile and compressive strengths increase rapidly as?
the gypsum hardens following the initial set
Why is there wet strength and dry strength?
Because free water decreases strength. When dry, small crystals anchor larger ones. When wet, small crystals go on a watery holiday
tensile strength is less affected by high W/P ratios than?
compressive strength
tensile and compressive strengths decrease with?
high W/P ratios
tensile strength of gypsum is lower than ________ strength? What percent lower?
compressive (10-25% of the value for compressive strength)
increased spatula time above one minute causes strength to?
decrease
List gypsum material in order of weakest to strongest
plaster < stone < die stone < high strength die stone
CaO and gum arabic are used to?
enhance particle wettability and reduce the amount of gauging water required
How many categories of gypsum are there?
5
Type 1
impression plaster -modifiers are added to control setting time
Type 2
model plaster - low strength, used for flasking dentures
Type 3
dental stone- used for study models and master casts for fabricating full dentures
Type 4
high strength dental stone -due stone
Type 5
high strength, high expansion dental stone -high expansion is to compensate for shrinkage of base metal alloys; “Maryland” bridges
How should you wet a gypsum cast?
by soaking it in a water bath with gypsum debris on the bottom. AVOID running water
What temp should you store gympsum casts in?
high temps. 190-230F
How should you store gypsum powder?
sealed containers in cool dry places.
don’t store gypsum powder in high humidity, why?
can cause plaster to initiate a setting reaction. *initially as a decrease in setting time, after a while hemihydrate will participate causing further retardness :)
surfactants are often in what form? Do what?
a liquid or a spray used on hydrophobic impression materials in order to enhance the wetting of gypsum product during pouring
Why is disinfection of casts important?
prevent cross-contamination among patients, dental office staff, lab personal (disinfected casts compare physically to non, but improvements still needed)
disinfection policy at UNMC
Remove contamination, spray with phenol, set for 10 minutes in bag, rinse in water
Why do you need a clean bowl and spatula?
dirt provides seeds for nucleation of crystals and increases the setting rate
