Gypsum Science
This is the common chemical name for the mineral known as gypsum
What is Calcium Sulfate Dihydrate?
This type of medium-strength plaster is processed by autoclaving gypsum under pressure and is primarily used for diagnostic and denture working casts.
What is dental stone?
To prevent lumps and excessive air entrapment, you must always add this component to the mixing bowl first.
What is water? (Powder must always be added to water) .
Surface pitting and erosion can be noticed on a stone cast if it is mistakenly submerged in tap water for as short as this many minutes.
What is 10 minutes?
Less water, more mixing/spatulation, accelerators
What shortens setting time?
Leaving a gypsum specimen to dry for 24 hours will cause its compressive strength to undergo this dramatic change.
What is doubling? (Its dry strength doubles) .
Because they cannot spring around undercuts when withdrawn from the mouth, impression plasters must purposely possess these two brittle physical traits.
What are being weak and brittle?
While borax and vinegar act as chemical retarders, these two common household items act as chemical accelerators.
What are potassium sulfate and table salt?
When completely soaking a cast in an SDS bath, a technician must keep the fluid level below this specific anatomical surface to allow trapped air to escape.
What is the tissue surface?
Potassium sulfate and table salt
Name two accelerators.
This is the stage of setting time that ends when the gypsum becomes solid enough to remove from a tray and trim without distorting.
What is the initial set?
To make die stone so dense and hard, manufacturers boil the raw gypsum in a 30-percent solution of this chemical before autoclaving it.
What is calcium chloride?
If an investment material takes longer than this specific timeframe to reach its initial set, the properties have deteriorated and the batch must be discarded.
What is 20 minutes?
A saturated SDS solution contains exactly this many grams of dihydrate per 100 cc of water.
What is 0.2 grams?
Why shouldn't accelerators be used in casting investments?
They alter expansion and strength, making results unpredictable
This term describes the chemical liberation of heat that naturally happens while all gypsum products are setting.
What is an exothermic reaction?
According to ANSI/ADA Specification No. 25, this is the maximum allowable setting expansion percentage for model plaster.
What is 0.3 percent?
This type of water—harvested from cast trimming machines—is full of sedimentary particles that dramatically accelerate setting times.
What is slurry water?
This chemical acronym represents a clear solution of water and a maximum amount of dissolved dihydrate gypsum used to safely soak casts.
What is SDS? (Saturated Calcium Sulfate Dihydrate Solution).
More water, retarders
What lengthens setting time?
This specific type of expansion occurs when a gypsum material is allowed to solidify entirely underwater, more than doubling its normal expansion.
What is hygroscopic setting expansion?
These ultra-high-strength materials, used for creating crown and inlay dies, have the lowest setting expansion range at 0.08 to 0.18 percent.
What are die stones (or improved stones)?
If a technician increases the water-to-powder ratio to create a thinner mix, the resulting gypsum's crushing strength undergoes this change.
Answer: What is lowering/decreasing? (Thinner mixes increase porosity and lower strength) .
If a cast is allowed to dry with this trimmer-generated buildup on its surface, it becomes highly difficult to remove and may cause cast damage.
What is a slush layer (or gypsum slurry)?
Borax, vinegar, potassium citrate
Name two retarders.