Chapter 15 - Amalgam

Question 1

describe amalgamation

Answer 1

When metal particles are mixed with mercury, the outer portion of the particles dis- solves into mercury. At the same time, mercury di uses into the metal particles. When the solubility of the metal in mercury is exceeded, crystals of mercury-containing com- pounds start to precipitate within the mercury. During this period of reaction the metal particles coexist with the liquid mercury, giving the mix a plastic consistency. is means that the mixture can be adapted to any shape with a light pressure. As the content of liquid mercury in the mixture decreases by the formation of precipitates, the mixture hardens.

Question 2

why is copper needed for Ag-Sn amalgams?

Answer 2

Antimicrobial, also needed to help blend Ag and Sn uniformly;

Question 3

why is Zn needed in Ag-Sn amalgams?

Answer 3

deoxidizer, oxygen scavenger; Alloys without zinc are more brittle, and their amalgams tend to be less plastic during condensation and carving.

Question 4

how are high copper alloys made into smaller particles?

Answer 4

e presence of the higher copper content makes mechanical cutting of ingots into par- ticles difficult. us, they are o en provided in a spherical form that is produced by an atomization process

Question 5

Spherical alloys require less mercury than typical lathe-cut alloys because spherical alloy powder has a smaller surface area per volume ratio than does the lathe-cut powder

Answer 5

ye

Question 6

what is the y1 phase?

Answer 6

Ag2Hg3

Question 7

what is the y2 phase?

Answer 7

Sn7Hg8

Question 8

e physical properties of the hardened amalgam depend on the relative percentages of each of the microstructural phases. how does each phase affect strength?

Answer 8

More unconsumed Ag-Sn, the stronger it will be

Sn-Hg is the weakest in corrosive environments

Question 9

what are the mercury/alloy ratios?

Answer 9

for lathe-cut: 50:50, for spherical: 42% Hg by weight

Question 10

why is trituration important?

Answer 10

an oxide layer of alloy surface hinders diffusion of Hg into alloy–oxide layer is removed by abrasion

Question 11

how does undertrituration affect teh Hg-Alloy mix?

Answer 11

too dusty/chalky–>will not be smooth when putting into cavity–>hard to make smooth after burnishing and will tarnish easier

Question 12

how can burnishing lead to corrosion?

Answer 12

burnish too hard–>heat created (above 60)–>Hg release; significant amounts of Hg in the margins–>accelerated corrosion (and/or fracture)

Question 13

How can burnishing prevent corrosion?

Answer 13

dull, unburnished surfaces are full of pits, scratches, etc–>areas allowed for concentrated cell corrosion; burnishing gets rids of those pits and scratches

Question 14

What does the ADA require dimensional change of amalgum to be?

Answer 14

range of 15 to 20 μm/cm mea- sured at 37 °C between 5 min and 24 h

Question 15

how does mercury content cause expansion?

Answer 15

intitial contraction of mix due to particles dissolving to create gamma-1 crystals; however, if sufficient Hg is present, as gamma-1 crystal grow and are lodged in Hg –> gamma crystals grow and push outwards, pushing on matrix–>expansion

Question 16

why is moisture bad for amalgam fillings?

Answer 16

causes a large expansion; this type of expansion is known as delayed expansion or secondary expansion. e e ect is caused by the hydrogen produced by electrolytic action involving zinc and water. e hydrogen does not combine with the amalgam but, rather, collects within the lling, increasing the internal pressure to levels high enough to cause the amalgam to creep,

Question 17

When an amalgam creeps, it is the γ1 phase that deforms plastically. Higher creep rates should be expected for low- copper amalgam with higher γ1 volume fractions and vice versa. e presence of the γ2 phase increases the creep rate. In addition to the absence of the γ2 phase, the very low creep rates in single-composition high-copper amalgams may be associated with η′ phase rods, which act as barriers to defor- mation of the γ1 phase. erefore, the manipulative factors discussed previously that maximize strength also minimize creep for any given type of amalgam.

Answer 17

ye

Question 18

what is tarnish?

Answer 18

a layer of silver sulfide on the surface of the amalgam

Question 19

why can high-Cu amalgam not be used with conventional low Cu amalgam?

Answer 19

High-Cu is cathodic relative to the low-Cu amalgam; will cause corrosion

Question 20

why can corrosion sometimes be good for amalgam?

Answer 20

can fill in any gaps between amalgam and tooth–bc amalgam does not adhere to tooth structure

Question 21

how is the ultimate lifetime of an amalgam restoration determined?

Answer 21

1) the material, (2) the dentist and the assistant, and (3) the patient’s environment