I think that your supervisor is referring to the use of a Nital solution (1- 2% nitric acid in ethanol) that is applied briefly (10-20 seconds) to the surface of a previously polished steel surface.
After rinsing and drying, the surface is examined under the microscope.
The surface will be found to have been preferentially etched depending on presence of carbon.
See, below paper; I have only copied the first page, because it has many photographs, and the references.
Regards,
Motoball
Ann Feuerbach iams 25 for 2005, 27-43
Introduction
In 1999, IAMS generously provided the author with a grant
for travel to Russia to study the microstructure of blades
from the Russian Northern Caucasus. Thirty-seven blades
were studied including knives, daggers, spearheads, sabres,
single-edged and double-edged swords, ranging in date from
the 3rd to the 12th century AD (Figures 1-35). Vickers hardness tests were only performed on a few samples because
the original purpose of the study was solely to determine if
any of the blades were made of crucible steel. However, it
was later decided that it was important to publish the results
of the metallographic investigations of all the samples due
to the fact that there are few published reports on blades
from this region.
The first group of eighteen blades were excavated
in the Kislovodsk depression, and are now housed in the
Kislovodsk Museum, Russia. These were dated by associated finds or by stylistic attributes. The second group is
comprised of nineteen blades from the Upper Kuban River
region, and are now in the Jewish University Museum in
Moscow, Russia. These blades were not uncovered during
controlled excavations and therefore have no associated
archaeological contexts or firm date; however their style
indicates that they are late Medieval.
Metallographic Examination of the Blades
A section of each blade was removed using a Plas-plug tile
cutting saw with a wafer thin diamond impregnated blade.
The Kislovodsk samples (KIS) were often taken from less
corroded areas or where the blades would not be disfigured.
All the samples of the Jewish University Museum blades
(JUM) were taken from the area near the handle so as not
to disfigure the blade. The reader should be aware that in
using this method of sampling, the sample taken may not
always be representative of the entire blade. For example,
if the edge near the tip was carburised, or if a piece of steel
was forge welded to the tip, the sample will not necessarily
exhibit these features, and therefore will not be characteristic of the blade as a whole. However, any disfiguration to
the blade is minimal and easily disguised by filling the hole
with resin, if desired, or by placing the mount fixtures over
the hole.
The samples were mounted in resin with the transverse
side positioned for examination. The mounted samples were
ground and polished using standard metallographic procedures (e.g. Scott 1991, 63-66). All the samples, except those
that were completely corroded, were examined before and
after etching in 3% Nital. The examination of the samples
indicated the use of iron, carburised iron, crucible steel,
forge welded or piled iron and steel (Table 1). The samples
are now housed at the Ashmolean Museum, Oxford, UK.
Iron Blades
Nine blades seem to be composed solely of iron: three from
Kislovodsk (KIS #4, #5, #14) and six from the Upper Kuban
River region (JUM #5, #10, #11, #12, #13, and #16). The
microstructures were composed of ferrite grains with varying amounts of slag inclusions. It seems possible that at least
some of the blades had either a carburised steel edge or an
edge of steel forge welded onto an iron core. A purely iron
blade would not have held its shape or an edge during use,
unless the purpose of the blade was for display or ceremony;
however, blades composed solely of iron are known from
numerous time periods, such as those from Celtic contexts.
It is more likely that the evidence for steel may have corroded away or is not observed because the sample is not representative of the entire blade. In the iron sword KIS #4, the
occurrence of intergranular corrosion is clear. The possible
exception to this is blade KIS #14, which was ritually bent.
Some of the ferrite grains are 0.5 mm in length, indicating a
very soft blade that would be beneficial in bending the sword
to such a twisted shape. It may have been deliberately made
for a ritual and/or burial and therefore there was not necessarily a need to give the blade a sharp edge and the presence
of carbon would have impeded bending.
The microstructure of the sabres reveals iron and not
steel. It may be that the area near the handle was made from
a softer metal to provide some flexibility to the blade, but
the edge of the blade may have been made of steel. Although
the samples are composed of iron rather than steel, three
blades show evidence of quenching (JUM #11, #13 and
#16). JUM #13 appears to have carbide or nitride etch pits
which, according to Samuels (1980, 69), often appear in
quenched ferrous objects. JUM #11 and JUM #16 have laths
inside the ferrite grains. These may also be a form of carbides or nitrides that sometimes appear as laths in ferrite (see
Tylecote and Gilmour 1986, 5). Quenching is usually associated with steel and not iron, as quenching has a dramatic
hardening effect on steel, but virtually no effect on iron. The
process involves heating steel to between 700 - 850 °C and
then suddenly cooling it in water or another substance such
as oil. The evidence of quenching suggests that either part
of the blade was composed of steel, and the smith may have
quenched the blade as a matter of course, or the sample did
not reflect the composition of the entire blade.
An investigation of the varied technology found in swords,
sabres and blades from the Russian Northern Caucasus...
Bibliography
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