5Gypsum

Question 1

WHAT IS GYPSUM???

Answer 1

CALCIUM SULFATE DIHYDRATE (Mineral Gypsum)

Question 2

What is the POWDER form of Gypsum?

Answer 2

Calcium Sulfate HEMIHYDRATE!!

Question 3

What are the three types of material derived from the dehydration of Gypsum rock? (WITH FANCY NAMES) WHATS THE DIFFERENCE?

Answer 3

Plaster(PLASTER), Stone(HYDROCAL), and High Strength Stone(DENSITE)…Only difference is how they are produced! All the same Calcium Sulfate Hemihydrate!

Question 4

What is this describing? Nucleated recrystallization, characterized by a continuous solution of hemihydrate,
diffusion of calcium and sulfate ions to nucleation centers, and precipitation of microscopic gypsum crystals.

Answer 4

The Setting Reaction of Gypsum

Question 5

When hemihydrate dissolves in water, ________ (less soluble) is formed and precipitates out of solution from centers of _______ to form needle like crystals.

Answer 5

dihydrate…..nucleation

Question 6

Nucleation centers may be ________ (residual gypsum particles) or gypsum particles added as _____ (terra alba).

Answer 6

impurities….seeds

Question 7

Less water = ____ porosity; less porosity = _____ strength

Answer 7

Less…..More

Question 8

Thick mixes (low W/ P ratio) hardens more quickly because nucleation centers are _______ in a _____volume. The interaction of the growing solid occurs ______ and is more effective.

Answer 8

concentrated..smaller..Earlier

Question 9

Increase? or Decrease? Water/Powder Ratio = Denser Cast

Answer 9

Decrease Water/Powder Ratio = Denser Cast

Question 10

Increase W/P Ratio = ____ Strength = _____ porosity

Answer 10

Increase W/P Ratio = Decreased Strength = More Porosity

Question 11

Gypsum is used as a _______ in materials commonly employed in the casting of dental gold alloys and
is generally not heated above ______ Degrees C. *Not used for __________ or other high melting alloys.

Answer 11

BINDER….700 Degrees C…..Base Metals

Question 12

________ is present in ALL dental investments and provides EXPANSION to compensate for gold shrinkage.

Answer 12

REFRACTORY

Question 13

_______ forms a coherent solid mass, provides STRENGTH for the investment, and is the Calcium Sulfate Hemihydrate for _____ and either Phosphate or ethyl silicate for _________.

Answer 13

BINDER….gold…high temp alloys

Question 14

________ is used to control dimensional changes on heating. _______ contracts on heating – silica overcomes this to give a net + expansion on heating.

Answer 14

REFRACTORY….Gypsum

Question 15

______ exists in a low temperature (alpha) phase and a high temperature (beta) phase.

Answer 15

Silica

Question 16

What are the 4 effects of increasing the amount of silica(refractory)?

Answer 16

1. Increased Manipulation Time 2.Increased Setting Expansion 3.Increased Thermal Expansion 4.Decreased Compressive Strength(Inc silica=Dec Stone)