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BDS2 CLINICAL Dental Materials > Amalgam > Flashcards

Flashcards in Amalgam Deck (71)
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What is amalgam

an alloy formed by the reaction of mercury (liquid) and silver, tin, copper & other metals (powder)


how can amalgam be classified

particle shape and size


what are the two amalgams with differing compositions

copper enriched


what does the particle shape and size determine

handling and final properties
it was originally applied to traditional but now also to copper enriched


what is the composition of the powder

silver, tin
copper (copper enriched)
mercury (sometimes)


what is the function of zinc

Most amalgams nowadays don’t have zinc in them, they used to have a small amount in them and its purpose was to act as a scavenger as it was preferentially oxidized during the manufacturing process which prevented the other metals in the alloy being oxidized


what is the function of silver, tin

silver is the main component, present in combination with tin as the intermetallic compound Ag3Sn known commonly as the γ-phase. It reacts with mercury liquid to form amalgam


what is the function of copper

present to increase the strength and hardness of the amalgam


what is the function of mercury in the powder

mercury is sometimes added to provide a more rapid reaction in what is referred to as pre-amalgamation


what is in the liquid

mercury (50% by weight)


what is the function of mercury

it is triple distilled (very pure) and reacts with other metals. The mercury has to be very pure otherwise a surface layer of contaminants is formed that interferes with the setting reaction.


what do the particle types apply to

The alloy is used in the form of a powder and the size and shape of the particles in this powder are critical to the handling characteristics and the final properties of the restoration.


what are the two types of particles

lathe cut


how are lathe cut particles produced

by machining a solid ingot of the alloy on a lathe. The chippings that are produced are graded and those in the right size range are used in the powder to be amalgamated with mercury.


what are the different types of lathe cut particles

The alloy is available as coarse, medium or fine grained powder and each will handle slightly differently.


what should be done to lathe cut amalgam and why

The individual chippings will become highly stressed during the machining, and this makes their surface very reactive to mercury. A consequence of this is that the setting reaction is far too rapid unless heat treatment (which relieves internal stresses) is applied. The heat treatment is usually carried out by placing the powder in boiling water


what are spherical particles

the various ingredients of the allow are melted together and then sprayed into an inert atmosphere, where the droplets solidify as small, spherical pellets of various sizes. This method of manufacture has the advantage that no further machining processes are required and that the composition of the alloy can be readily altered.


how is the setting reaction for amalgam initiated

vigorous mixing of the two ingredients. This mixing causes the other layer of the alloy particles to dissolve into the mercury, forming two new phases which are solid at room temperature


what is the setting reaction for amalgam

Ag3Sn + Hg > Ag3Sn + Ag2Hg3 +Sn7Hg
y + mercury > y + y1 + y2
powder + liquid > unreacted alloy + amalgam matrix


what is the set structure for amalgam

Not all of the alloy particles dissolve in mercury but rather a considerable amount remains so that the final structure is one of a core of gamma held together by a matrix of predominantly y1 which is interspersed with y2


what is the function of gamma

provides good strength and corrosion resistance


what is the function of gamma 1

provides good corrosion resistance


what is the function of gamma 2

y2 is weak and poor corrosion resistance


what do voids do

voids decrease strength and increase corrosion


what has the strongest tensile strength from the amalgam components



what amalgam component has the lowest tensile strength

gamma 2


describe the setting dimensional changes in traditional amalgam

there is initial contraction (0.2%) as there is a solution of alloy particles in the mercury then there is expansion due to the crystallization of y1 (0.4%)


describe the setting dimensional changes in modern amalgam

there is a small contraction (0.2%) and the end result is a solid solution of mercury in Ag3Sn


why was zinc removed from amalgam

if some of the zinc is unreacted then it may react with water in saliva/blood to produce bubbles of hydrogen


what can hydrogen bubbles forming in the amalgam result in

• Pressure build up which causes expansion
• Downward pressure causing pulpal pain
• The material being forced to expand over the surface of the tooth