DENTAL MATERIALS Dr Waseem Mushtaha Al Azhar University Gaza Lama El Banna

1. Dental amalgam
Dr. Waseem Bahjat Mushtaha
Specialized in prosthodontics

2. Terminology
Amalgam : an alloy of mercury.
Amalgamation : the process of mixing liquid
mercury with one or more metals or alloys to
form an amalgam.
Creep : the time-dependent strain or deformation
that is produced by a stress. The creep process
can cause an amalgam restoration to extend out
of the cavity preparation, there by increasing its
susceptibility to marginal breakdown.

3. Delayed expansion: the gradual expansion of
a zinc-containing amalgam over a period of
weeks to months that is associated with
hydrogen gas development caused by
contamination of the plastic mass with
moisture during its manipulation in a cavity
preparation.
Dental amalgam: an alloys of mercury, silver,
copper, tin, which may also contain
palladium, zinc, and other elements to
improve handling characteristics and
clinical performance.

4. Dental amalgam alloy : an alloy of silver,
copper, tin and other elements that is
formulated and processed in the form of
powder particles or as a compressed pellet.
Trituration : the process of grinding powder,
especially within a liquid. In dentistry, the
term is used to describe the process of
mixing the amalgam alloy particles with
mercury in an amalgamator.

5. Trituration
Amalgamator

6. Alloys composition
American dental associated (ADA)
specification No. 1 requires that amalgam
alloys be predominantly silver and tin.
Unspecified amount of other elements, such
as copper, zinc, gold, and mercury, are
allowed in concentrations less than the
silver or tin content. Alloys containing zinc
in excess of 0.01% or less of zinc are
designated as nonzinc.

7. Metallurgic phase in dental amalgam
The setting reactions of alloys for dental
amalgam with mercury are usually described
by the metallurgic phases that are involved.
These phase are named with Greek letters that
correspond with the symbols found in phase
diagram for each alloy system

8. Symbols and stoichiometry of phases that are
involved in the setting of dental amalgam
Phases in amalgam stoichiometric formula
Alloys and set dental
Amalgam.
γ Ag3Sn
γ1 Ag2Hg3
γ2 Sn7-8Hg
ε Cu3Sn
ή Cu6Sn5
Silver-copper eutectic Ag-Cu
The Greek letter named as follows: γ(gamma);ε(epsilon);ή (eta).

9. Manufacture of alloys powder
Lathe-cut powder: to make lathe-cut powder,
an annealed ingot is placed in a milling
machine or in lathe and is fed in to cutting
tool or bit. The chips removed are often
needlelike. (irregular particles(.

10. Homogenizing Anneal: because of the rapid
cooling conditions form the as-cast state, an
ingot of an Ag-Sn alloy has a cord structure
and contains nonhomogeneous grains of
vary composition. A homogenizing heat
treatment is performed to re-establish the
equilibrium phase relationship. The ingot is
placed in an oven and heated at a
temperature below the solidus for sufficient
time to allow diffusion of the atoms to
occur and the phases to reach equilibrium.

11. Particle treatments: once the alloy ingot has been
reduced to cuttings, many manufactures performed
some type of surface treatment of the particles.
Treatment of the alloys particles with acid has
been a manufacturing practice for many years. The
exact function of this treatment is not entirely
understood, but it is probably related to the
preferential dissolution of specific components
from the alloy. Amalgams made from acid-washed
powder tend to be more reactive than those made
from unwashed powder.

12. Atomized powder: atomized powder is made by melting
together the desired elements. The liquid metal is
atomized into fine spherical droplets of metal. (spherical
powder(.
Particle size: maximum particle size and the distribution of
sizes within an alloy powder are controlled by the
manufacturer. The average particle sizes of modern
powders range between 15 and 35 µm. The most
significant influence on amalgam properties is the
distribution of sizes around the average. For example
very small particles (< 3 µm) greatly increase surface
area per unit volume of the powder. A powder
containing tiny particles requires a greater amount of
mercury to form an acceptable amalgam.

13. Lathe-cut compared with atomized alloys:
Amalgams made from lathe-cut powders, or admix
powder of blend of lathe-cut and spherical
powders, tend to resist condensation better than
amalgams made entirely from spherical powder.
Spherical alloys require less mercury than typical
lathe-cut alloys because spherical alloys have a
smaller surface area per volume than do the lathe-
cut alloys. Amalgam with a low mercury content
generally have better properties.

14. Amalgamation and resulting
structure
Low-copper alloys:
Amalgamation occurs when the mercury comes into
contact with the surface of the Ag-Sn alloy
particles. When the powder is triturated, the silver
and tin in the outer portion of the particles
dissolve in to mercury. At the same time, mercury
diffuse into alloy particles. The mercury has a
limited solubility for silver (0.035 wt%) and tin
(0.6 wt%(.

15. When that solubility is exceeded, crystal of two
binary metallic compounds precipitate in to the
mercury. These are the body-centered cubic
Ag2Hg3 compound (the γ phase) and the
hexagonal closed packed Sn7-8Hg compound
(the γ2 phase). Because the solubility of silver in
mercury is much lower than of tin, the γ1 phase
precipitates first, and the γ2 phase precipitates
later. Immediately after trituration, the alloy
powder coexists with the liquid mercury, giving
the mix a plastic consistency. As the remaining
mercury disappears, the amalgam hardens.

16. As the particles become covered with newly
formed crystals, mostly γ1, the reaction rate
decreases. The alloy is usually mixed with
mercury in approximately a 1:1 ratio. This
is insufficient mercury to completely
consume original alloy particles;
consequently, unconsumed particles are
present in the set amalgam. Alloys particles
( smaller now, because their surfaces have
dissolved in mercury) are surrounded and
bound together by solid γ1 and γ2 crystals.

17. Thus, atypical low-copper amalgam is a composite in which the
unconsumed particles are embedded in γ1 and γ2 phases. The
reaction can be conveniently expressed in terms of the phases that
form during amalgamation:
Alloy particles (β +γ) + Hg γ1+ γ2 + unconsumed alloy
particles (β +γ) .
The physical properties of the hardened amalgam depend on the
relative percentages of each of the microstructural phases. The
unconsumed Ag- Sn particles have a strong effect. The more of
this phase that is retained in the final structure, the stronger is the
amalgam. The weakest component is the γ 2 phase. The hardness
of γ2 is approximately 10% of the hardness of γ1.
γ2 phase is the least stable in a corrosive environment and
experience corrosion attack, especially in “cervices” of the
restorations. Pure γ1 phases are stable in an oral environment.
However, γ1 in amalgam does contain small amounts of tin,
which can be lost in a corrosive environment.

18. High –copper alloys:
High –copper alloys have become the materials of
choice because of their improved mechanical
properties, corrosion characteristics, and better
marginal integrity and performance in clinical
trials, as compared with traditional low-copper
alloys. Two different types of high-copper alloy
powders are available:
1(Admix alloy powder.
2(Single composition alloy powder.
Both types contain more than 6 wt% copper.

19. 1(Admix alloy powder
In 1963, Innes and Youdelis added apherical silver-copper
(Ag-Cu) eutectic alloy (71.9 wt% silver and 28.1 wt%
copper) particles to lathe-cut low-copper amalgam alloy
particles. This was the first major change in the
composition of alloy for dental amalgam since Black’s
work. Theses alloys are often termed admix alloys
because the final powder is a mixture of at least two
kinds of particles. An admix powder, showing lathe-cut
low-copper alloy particles and spherical Ag-Cu alloy
particles.

20. Amalgam made from these powder is stronger than
amalgam made from lathe-cut low-copper
(composite materials {materials that consist of a
matrix and filler} can be strengthened by the
addition of strong fillers) and the Ag-Cu particles
probably act as strong fillers, strengthening the
amalgam matrix.
Several classic studies have shown that restorations
made with this prototype admixed amalgam were
clinically superior to low-copper amalgam
restorations when they were evaluated for
resistance to marginal improved clinical
performance.

21. Admix alloy powders usually contain 30 wt% to 55 wt
% spherical high-copper powder. The total copper
content in admixed alloys ranges from approximately
9 wt% to 20 wt%. The phases present in the copper-
containing particles depend on their composition. The
Ag-Cu alloy consists of mixtures of two phases-silver
rich and copper rich-with the crystal structures of
pure silver and pure copper, respectively. Each phase
contains a small amount of the other element. In the
atomized powder (which is fast cooled), the eutectic
two-phase mixture from very fine lamellae.
Compositions on either side of the eutectic from
relatively large grains of copper-rich phase or silver-
rich phase amide the eutectic mixture.

22. When the mercury reacts with an admixed powder, silver
dissolves into the mercury from Ag-Cu alloy particles,
and both silver and tin dissolve into the mercury from
Ag- Sn alloy particles. The tin in solution diffuses to the
surfaces of the Ag-Cu alloy particles and reacts with the
copper phase to form the ή phase (Cu6Sn5). A layer of ή
crystals form around unconsumed Ag-Cu alloy particles.
The ή layer on Ag-Cu alloy particles also contains some
γ1 crystals. the γ1 phase form simultaneously with the ή
phase and surrounds both the ή covered Ag-Cu alloy
particles and the Ag- Sn alloy particles. As in the low-
copper amalgams, γ1 is the matrix phase, that is, the
phase that binds the unconsumed alloy particles together

23. The reaction of the admixed alloy powder with
mercury can be summarized as follows:
Alloy particles (β + γ) + Ag-Cu eutectic + Hg
γ1 + ή + unconsumed alloy of both types of
particles.
N.B γ2 has been eliminated in this reaction.

24. Single composition alloys
Unlike admixed alloy powders, each particle of these
alloy powders has the same chemical composition.
Therefore, they are called single-composition
alloys. The major components of the particles are
usually silver, copper, and tin. The first alloy of
this type contained 60 wt% silver, 27 wt% tin, and
13 wt% copper. The copper content in various
single composition alloys rang from 13 wt% to 30
wt%. In addition, small amounts of indium or
palladium are also found in some of the currently
marked single-composition alloys.

25. A number of phases are found in each single-composition
alloy particle, including β (Ag- Sn) γ(Ag3Sn), and ε
(Cu3Sn). Some of the alloys may also contain some ή
phase (Cu6Sn5).
When triturated with mercury, silver and tin from the Ag-
Sn phases dissolve in mercury, little copper dissolves in
mercury. The γ1 crystals grow, forming a matrix that
bind together the partially dissolved alloy particles. the
ή crystals are found as meshes of rod crystals at the
surface of alloy particles, as well as, dispersed in the
matrix. Theses are much larger than the ή crystals found
in the reaction layers surrounding Ag-Cu particles in
admix amalgams.

26. To summarized the reaction of the single-
composition alloy powder with mercury is
as follows:
Ag-Sn-Cu alloy particles + Hg
γ1 + ή + unconsumed alloy particles.

27. Manipulation
1(proportioning
2(Trituration
3(Condensation
4(Trimming and carving
5(Polishing
6(Some precautions

28. Proportioning
a) Mercury : the required quantity can be
obtained by weighing or by using a volume
dispenser. Clearly the latter method is
quicker. It is important to use pure clean
mercury.

29. b) Alloy : this can be proportional by:
1(Weighing
2(Using tables of alloy, particularly with mechanical
mixing.
3(Having envelopes with pr-weighed quantities.
4(Using a volume dispenser.
Two disadvantages of a volume dispenser are:
1(It is difficult to measure any powder accurately by
volume, as the weight of material per volume depends
on the efficiency with which the particles are packed
together.
2(Alloy can cling to the walls of the dispenser.

30. c) Alloy/mercury ratio. In the final set amalgam it is
desirable to have less than 50% mercury.
Two techniques have been recommended:
1(The use of an alloy/mercury ratio of 5/7 or 5/8. the
excess mercury makes the trituration easier, giving a
smooth plastic mix of material. Before insertion into the
cavity, excess mercury is removed from the mix by
squeezing it in a dental napkin.
2(Minimal mercury techniques, where about equal weights
of alloy and mercury are used and no mercury is
squeezed out of the mix before condensation.
d) Many materials are supplied in capsules with per-
proportioned alloy and mercury.

31. Amalgam

32. Amalgam capsules

33. Amalgam capsules

34. Trituration
a) Hand mixing by mortar and pestle. A glass
mortar and pestle are used. The mortar has
its inner surface roughened to increase the
friction between the amalgam and the
surface. A rough surface can be maintained
by occasionally grinding with carborandium
paste. The pestle is a glass rod with a
rounded end.

36. b) Mechanical mixing : the proportioned alloy and mercury
can be mixed mechanically in a capsule, either with or
without a stainless steel or plastic pestle. A pestle, which
should be of considerably smaller diameter than the
capsule, should be used with tablet alloys, to help break up
the material. The mechanical amalgamators have time
switches to ensure a correct mixing time. A number of
these materials are available in an encapsulated form, each
capsule containing a controlled weight of alloy, and having
the right quantity of mercury sealed in its lid. The choice
of trituration time is important, and will depend both on
the type of alloy and the speed of mixer. In particular rich
copper alloys require precise control of trituration
conditions. Some products require high energy mixing to
break up the oxide coating which forms on copper rich
particles.

37. Amalgamator

38. Condensation
1(Each portion is properly adapted by a condenser
of suitable size.
2(A load of up to 4-5 kg is applied to each
increment.
3(As the mix is condensed, some mercury- rich
material rises to the surface. Some of this can be
removed, to reduce the final mercury content, and
improve the mechanical properties. The remainder
will assist bonding with the next increment, to
avoid the production of a weak laminated
restoration

39. A material should be condensed as soon as
possible after mixing. If it is left too long,
and has begun to set:
1(Proper adaptation to the cavity will be
impossible
2(Elimination of excess mercury will be
difficult
3(Bonding between increments will be poor
4(Lower strength values will result.

41. Trimming and curving
When the cavity is overfilled, the top
mercury-rich layer can be trimmed away
and the filling carved to the correct
contours. The amalgam prepared from a
coarser grain alloy may be more difficult to
carve, as the instrument may pull out large
pieces of alloy from the surface. Spherical
alloys are used where ease of carving is
desired.

43. Polishing
Conventional amalgams are polished not less
than 24 hours after insertion that is, when
the material has gianed considerable
strength. Since amalgams from rich copper
alloys gain strength rapidly.

44. Some precautions
a) Mercury is toxic, so free mercury should not be
allowed to enter the atmosphere. This hazard can
arise during trituration, and condensation and
finishing of restorations, and also during the
removal of old restorations at high speed.
b) Skin contact with mercury should be avoided, as it
can be absorbed by the skin.
c) Any excess mercury should not be allowed to get
into sinks, as it can react with some of the alloys
used in plumbing.
d) Contamination of the amalgam by moisture must
be avoided.

45. Properties
1(Toxicity
2(Corrosion reactions
3(Marginal leakage
4(Strength
5(Marginal failure
6(Thermal diffusivity
7(Dimensional changes

46. Toxicity
a) The wisdom of using a restorative material
containing mercury has often been questioned.
b) The potential danger of any form of mercury is
related to:
1-The form in witch the mercury is present
2-The quantity and frequency of exposure
c) There is no evidence of harmful effect of the
amalgam.

47. Corrosion reaction
a) Tarnish : amalgam can tarnish in the
presence of sulpher, to give layer of
sulphides on the surface of restoration.
b) Corrosion of conventional amalgams: the
set material is heterogeneous stimulate
corrosion. Of the three phases present, the
γ2 is the most active electrochemically,
being anodic in relation to both the γ and γ1
phases.

48. As γ2 corrodes, essentially two products result:
1)Ionic tin produced: in the presence of saliva,
corrosion products such as SnO2 and
Sn(OH)6Cl are found.
2)Hg is produced, which can react with some of
the remaining hitherto unreacted γ phase.
c) Corrosion of rich copper amalgam:
1(No γ2 is present.
2)However, the corrosion currents associated with
those with conventional amalgam.

49. 3(The volume of corrosion products is less
than with conventional amalgam.
4(No mercury is produced as a result of the
corrosion.
d) Practical considerations:
1(Corrosion resistance is greatly improved if
the amalgam is polished. This process
removes pits and voids on the surface,
which aid concentration cell corrosion

50. 2(If amalgam comes in contact with a gold
restoration, an electrolytic cell may be set
up leading to corrosion of the amalgam and
incorporation of mercury on the gold
restoration.
3(Corrosion of conventional amalgam can
have a significant effect on long-term
mechanical properties. It has been shown,
fore example, the tensile strength is reduced
by 30% when the network of γ2 has
corroded.

51. Marginal leakage
The initial marginal leakage of an amalgam
restoration, reduces with time, because of
sealing of the micro-fissures by products of
corrosion breakdown.

52. Strength
The following factors can lead to the
production of a weak restoration:
1(undertrituration
2(Too high a mercury content
3(Too low condensation pressure
4(Slow rate of packing
5(Corrosion

53. The rate of development of strength of an
amalgam is of importance. With amalgams
which develop strength slowly, there is
danger of early fracture of such a
restoration. Generally, spherical and rich
copper amalgams have high early strengths.
Of the phases present in conventional
amalgams, the γ2 is the weakest and softest.

54. Marginal failure
Ditching of the margins of amalgam is a common
occurrence. Clinical trials have shown that higher
copper alloy formulations show much less
marginal breakdown than conventional materials.
The following observations are relevant:
a) Poor technique can cause breakdown, e.g. an
unsupported ledge of amalgam extending over the
enamel may fracture during mastication

55. b) A theory has been propounded which links
marginal breakdown with corrosion
characteristics. It has been suggested that the
mercury corrosion product (from conventional
materials) reacts to form more γ1 and γ2
material, with associated expansion, termed
“mercuroscopic expansion”. The expanded
material, weakened by corrosion, protrudes away
from the supporting tooth structure, and
fractures.

56. Thermal diffusivity
Dental amalgam is a conductor of heat,
whereas the enamel and dentin it replaces is
a thermal insulator. Consequently, large
amalgam restorations are usually lined with
a thermal insulating cement to product the
pulp from temperature changes in the mouth
caused by hot and cold foods and liquids.

57. Dimensional changes
Ideally there should be little or no contraction
on setting of a dental amalgam, otherwise a
gap between filling and cavity walls may
result, enhancing the possibility of further
decay. Too great an expansion should also
be avoided, as this will cause the filling to
protrude from the cavity.

58. In laboratory experiments, where free
expansion is measured, it has been shown
that a greater expansion on setting will
result if:
1(A higher alloy/mercury ratio is used.
2(There is a shorter trituration time
3(Lower pressure during condensation is
used
4(The alloy has a larger particle size
5(There is contamination by water before
setting in zinc-containing materials.

59. An electrolytic reaction between zinc (the
anode) and the other metals which are
cathodic and the water as an electrolyte.
Hydrogen is evolved as a result of this
reaction.
The pressure of the evolved hydrogen may
cause the amalgam to flow.
This causes an expansion, which may not
appear within the first 24 hours, but may
become evidence some days after insertion
of the restoration.