Dental Stones

Question 1

what can stone be used for?

Answer 1

-Replicas of the hard and soft tissues used for the diagnosis and treatment of oral diseases
-study mode
-working cast
-design mouth guard

Question 2

What is investing in stone?

Answer 2

• Investing captures all details of the wax pattern
• Once the wax is burned away, provides a defined space into which the molten alloy or ceramic is cast

Question 3

what is the gypsum model-die

Answer 3

It is a soft sulfate mineral composed of calcium sulfate dihydrate
Most common

Question 4

what is the epoxy resin stone?

Answer 4

Resins are used as a die material to overcome the low strength and abrasion resistance of die stone

Question 5

what are the 3d printed resins?

Answer 5

–Model based on CAD/CAM
–UV curing resins Thermoset polymers

Question 6

what are the types of gypsum?

Answer 6

1- impression plaster
2- model plaster
3- dental stone
4- high strength/low dental stone
5- high expansion/high expansion

Question 7

what is impression paster used for?

Answer 7

To mount casts in articulator

Question 8

what is Model plaster used for?

Answer 8

For typical study models, diagnostics, art portion

Question 9

what is Dental stone used for?

Answer 9

• For full/partial denture model
• Less water and stronger than Type II
• “Hydrocal”

Question 10

what is High-strength/low-expansion used for?

Answer 10

For dies, which require high strength/high abrasion resistance
densite

Question 11

what is High-strength/high-expansion used for?

Answer 11

High expansion to compensate shrinkage of casting alloys

Question 12

what is gypsum made of?

Answer 12

Calcium Sulfate Dihydrate

Question 13

what is dental stone made of?

Answer 13

Calcium Sulfate Hemihydrate

Question 14

what is the cast/die made of?

Answer 14

Calcium Sulfate Dihydrate

Question 15

what is the process from dehydrate to hemihydrate?

Answer 15

dihydrate, calcination, hemihydrate

Question 16

what is the difference between beta and alpha hemihydrate?

Answer 16

Size and porosity of the crystals require different amount of excess of water to produce workable mass

Porosity: Plaster (35%) > Stone (20%) > Die Stone (10%)

Question 17

From Hemi- to Di-hydrate is whay type of rxn?

Answer 17

exothermic

Question 18

what is the difference between the dehydrate and hemihydrate?

Answer 18

dehydrate is more water
hemi is not much water

Question 19

what is the Water–Powder Ratio?

Answer 19

WATER/POWDER
Where powder is aways 100

Question 20

how can you best the the types of stones apart?

Answer 20

differences in the w/p ratio
high strength is lowest ratio, mode plaster is highest

Question 21

what happens as the w/p ratio increases?

Answer 21

• Setting time increases
• Strength of gypsum product decreases
• Setting expansion increases

Question 22

what effect does excessive water have?

Answer 22

• Increases setting time
• Increases porosity
• Decreases strength

Question 23

how does temperature effect the setting time?

Answer 23

• If water temperature is raised above >37.5C, the setting time
  will be delayed (slower reaction)
• If water temperature below body temperature (<37.5C), the
  setting time is faster (accelerates)

Question 24

how does spatulation change setting time?

Answer 24

Increase in speed and time, shortens setting time

Question 25

how does humidity change the setting time?

Answer 25

• Exposed powder (hemihydrate) will be dihydrate
• During mixing, increased setting time

Question 26

what do accelerators do?

Answer 26

• Increase the rate of setting (faster setting time)
• Accelerators increase the number of crystal nuclei available for growth

Question 27

what is an example of a Accelerators?

Answer 27

Potassium Sulfate (K2SO4) or Sodium Chloride (increases expansion but reduces set time from 10 to 4 mins)

Question 28

what do retarders do?

Answer 28

• Decreases rate of setting (slower setting time)
• Make hemihydrate (powder) slightly less soluble than dihydrate (final)

Question 29

what is an example of a retarder?

Answer 29

Borax (Na2B 4O 7·10H2O) or Sodium citrate
* 2% powder addition increases setting time to several hours

Question 30

what is the initial setting time?

Answer 30

• Interval between mixing with water and the mix can’t be poured into mold
• Similar to working time of a material
• Material losses gloss
• Between 8-16 min

Question 31

what makes up the final setting time?

Answer 31

• Finalize the hemi- to di-hydrate conversion
• Gypsum can be removed from the impression
• No heat
• 20-60 min

Question 32

what are the strongest stones?

Answer 32

type 5 and 4

Question 33

what is the weakest stone?

Answer 33

1

Question 34

what is dimensional accuracy?

Answer 34

Gypsum materials expand on setting: 0.06% to 0.5%

Question 35

what stone has the worst expansion?

Answer 35

Plaster > Dental stone > High-strength stone

Question 36

what is hygroscopic expansion?

Answer 36

after initial mixing
* Adding water to the surface
* Humidity
NOT uniform expansion

Question 37

what are hardening solutions?

Answer 37

Solutions of water + 30% colloidal silica + chemical modifiers

Question 38

how do hardening solutions change the properties of the gypsum?

Answer 38

• Harden the gypsum and also increase its abrasion resistance

Question 39

what is something to considerer when adding a hardening solution?

Answer 39

changes the W/P ratio

Question 40

what do you need to consider when selecting an impression material and stone?

Answer 40

picking ones that are compatible for good wettability and contact angle