Porcelains

Question 1

Porcelains high or low strength

Answer 1

Low strength, but can be improved with bonding

Question 2

What is the minimal thickness required for porcelain to be reliable restoration

Answer 2

0.7mm

Question 3

What are traditional feldspathics used for

Answer 3

Artificial teeth for dentures, full porcelain crown, metal ceramic crown, inlays, veneers

Question 4

Do traditional feldspathics have high or low melting point

Answer 4

High

Question 5

What is the thermal and electrical conductivity of dental ceramics

Answer 5

Low

Question 6

Is strength of feldspathics high or low

Answer 6

Compressive strength high, very stiff

Low tensile strength

Question 7

Why does traditional feldspathic have good biocompatibility

Answer 7

No conversion or leeching of monomer, very inert

Question 8

Is porcelain ductile

Answer 8

No. Very stiff

Question 9

Function of leucite in traditional feldspathics (3)

Answer 9

Leucite is a glass ceramic. It is a filler to reduce polymerisation shrinkage. It strengthens and toughens the ceramic. It increases the coefficient of thermal expansion.

Question 10

Describe process of producing traditional feldspathics

Answer 10

Melt the feldspar. Alkali unite with aluminium and silica to form sodium or potassium silicate. Glassy phase and free crystalline silica phase formed. Ground blocks into porcelain powders. Filter to separate glassy from leucite particles. Firing

Question 11

Describe porcelain at low bisque stage of firing

Answer 11

Very porous. Minimal shrinkage. Fired body is weak and brittle. Grains of porcelain have started to soften and lense at certain points

Question 12

Describe porcelain at medium bisque of firing

Answer 12

Surface porous but increased flow of glass grains. Any entrapped furnace atmosphere that has not escaped grain boundaries may become trapped in porcelain. Definite shrinkage has taken place

Question 13

Describe porcelain at high bisque of firing

Answer 13

Surface of porcelain completely sealed. Much smoother. Porcelain is strong.

Question 14

What happens if too much leucite crystal is artificially added to the porcelain

Answer 14

Too opaque

Question 15

What type of structure is porcelain

Answer 15

Vitreous (formed by vitrification). meaning it is not well structured, this random organisation with increased space between atoms allows light to pass through

Question 16

How to adjust colour of porcelain

Answer 16

Add concentrated colour frit. Add high temperature resistant pigments such as metallic oxides

Question 17

Why are opacifying agents needed in porcelain

Answer 17

Porcelain is too translucent

Question 18

What is fluorescence

Answer 18

Absorb radiation of wavelength and re emit as radiation of longer wave length

Question 19

How to make porcelain fluoresce

Answer 19

Small percentage of sodium diuranate added into the frit

Question 20

Why is it important that porcelain crown is well adjusted

Answer 20

If there is space between ceramic and tooth, ceramic is unsupported. Not ductile at all, any deformity will cause fracture

Question 21

Why do traditional feldspathics have low tensile strength

Answer 21

Surface defects eg minute cracks, porosities, unevenness

Question 22

Leucite vs lithium disilicate as porcelain filler

Answer 22

Leucite take shorter time to break as crack path less tortuous

Question 23

Compare effects of porcelain surface defects

Answer 23

Cracks are very detrimental to mechanical properties especially surface cracks

Bubbles are not much a concern due to rounded surface

Inclusions mean porcelain not homogenous any more

Question 24

How do surface defects cause brittle fracture in traditional feldspathics

Answer 24

Surface defects cause a concentration of stresses. Under tensile stresses, concentration of stresses exceed strength of ceramic body. Not ductile unable to relieve stress

Question 25

Polycrystalline solids vs alumina ceramics

Answer 25

Polycrystalline solids are twice as strong and tough

Crack propagation triggers tetragonal to monoclinic transformation with accompanying 4,5% increase in volume to stop further propagation