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Metallographic examination

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					                                        Metallographic examination

Metallography requires the            Because of this heterogeneity,        to give a significantly harder,
removal of small samples, which       bulk chemical analyses,               tougher alloy. Steel: The dark
are then mounted in a resin or        especially of the surface, may be     etching regions, are known as
bakelite block, polished and          misleading. Metallography,            pearlite (and contain 0.8 %
etched in dilute acid, before         especially when combined with         carbon). Steel is not only harder
examination under a                   micro-analysis, investigates          and tougher than iron but allows
metallurgical microscope. This        rather than ignores the               further hardening by heating and
reveals the crystal structure of      microstructure of the metal and       quenching.
the metal, from which an              provides a greater insight into
assessment of the type of alloy       the technological and post            Heat treatment:
and its mechanical and heat           depositional history of the           An acicular structure, martensite,
treatment history can be made.        artefact.                             results from the rapid cooling
Metallography thus provides a                                               (quenching) of steel without
good measure of the quality of        Ferrous alloys                        subsequent tempering. This is a
the metal and its suitability for a   It is with iron alloys that           very hard material but with a
particular application. Scott         metallography can extract the         tendency to brittleness.
(1991) provides a good                most detailed information. The        Tempered martensite appears
introduction to the structure of      following notes show the sort of      less distinct and although not as
metals, metallography and the         features which can be                 hard provides a much tougher
phase diagrams which help             determined.                           metal.
explain the microstructures it                                              Metals worked at low or ambient
reveals.                              Slag content:                         temperatures and not
                                      Examination of the unetched           subsequently annealed show
The structure of many ancient         sample allows the slag inclusions     deformed, elongated grains. This
alloys is, typically,                 to be measured, both by volume,       cold working increases the
heterogeneous. Ferrous alloys         and by shape. The shape of the        hardness of both iron and steel.
were never heated to melting          inclusions show the way the
temperature and therefore retain      artefact has been wrought.            Composite Artefacts:
quantities of slag as well as an                                            To capitalise on the hardness of
uneven distribution of alloying       Alloy identification:                 steel and the ductility (and
elements, particularly carbon and     Prior to the introduction of the      cheapness) of iron the two alloys
phosphorus.                           blast furnace three main ferrous      were welded together to form
                                      alloys were in use.                   composite artefacts. Such
Non-ferrous alloys may show           Ferritic iron: The grains of          structures are frequently found in
segregation of certain elements       ferrite appear white and contain      edged tools and weapons. A
towards the centre or surface of      no impurity elements; it is a soft,   weld is generally seen as a sharp
the artefact. All alloys can show     ductile metal.                        division between two metal types
depletion of elements at the          Phosphoric iron: Indicated by         (although carbon may diffuse
surface following oxidation,          grain enlargement and 'ghosting'.     across a boundary). Scale and
either during manufacture or          Phosphorus contents in the            silicate inclusions may also be
after post-depositional corrosion.    region of 0.1-0.2% are sufficient     trapped at the interface.

   The Historical Metallurgy Society: Archaeology Datasheet No. 11 Downloaded from hist-met.org
Techniques for combining              composition to the object's parts    Metallography can be a very
different alloys may have             or an alloy chosen because is has    informative investigative
important cultural implications.      a lower melting point (or melting    technique but programmes need
For example, in many Saxon            range) than the material to be       to be well thought out to ensure
knife blades a steel edge was butt    joined.                              that valid conclusions can be
welded to an iron back, whilst                                             reached.
Anglo-Scandinavian smiths             Surface treatments such as
favoured 'sandwiching' the steel      gilding, silvering and tinning       Reference
between two low carbon sides.         were all used in antiquity.          Scott, D A (1991)
                                      Metallography can identify the          Metallography and
Carburization was an alternative      method used to apply the coating        microstructure of ancient and
means of creating a hardenable        by examination of compounds             historic metals. Getty
surface. The iron was heated          formed between the two                  Conservation Institute.
with a carbon-rich material in a      dissimilar metals.
reducing atmosphere so carbon
diffused into the surface.            Alternative approaches
Unfortunately this thin layer may     Occasionally it is possible to
be lost to corrosion in all but the   polish a small area of the surface
best preserved artefacts.             of a well preserved artefact, and
                                      examine it without the removal
Non-ferrous alloys                    of a specimen. This will be of
Identification of phases present      more limited value, because it
allows the nature of the alloy to     does not allow the examination
be determined. For instance, in       of a complete section, however it
leaded copper alloys the lead is      may be the only method
immiscible and will be visible as     considered acceptable.
dark grey areas at the grain
(crystal) boundaries.                 Associated methods: Both
                                      micro-hardness and analysis in                       David Starley
The shape of the metal crystals       the SEM or microprobe (see            Ancient Monuments Laboratory
will show how the object was          Datasheet 12) are extremely                        English Heritage
produced. Cast alloys generally       useful in extending studies of
have the characteristic dendritic     Metallographic sections because                            April 1995
structure, but this is broken         they operate on a scale
down by subsequent annealing          that allows individual phases to
which produces equiaxed               be examined. A micro-hardness
crystals. Cold working is             tester measures the indentation
indicated by deformation of these     produced by a diamond under a
crystals and subsequent               set load and provides a direct
annealing produces a twinned          measure of the
crystal structure.                    hardness of the metal.

Fabrication techniques                Justification: Most opposition
Whilst ancient iron could be fire     to metallography arises because
welded by heating and                 of the need to 'destructively'
hammering, most non-ferrous           remove samples. This must
alloys required additional liquid     always be balanced against the
metal to bond the parts together.     potential evidence which could
This may be of similar                be obtained by the technique.

   The Historical Metallurgy Society: Archaeology Datasheet No. 11 Downloaded from hist-met.org

				
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