This document presents a method for preparing ancient lead samples for metallographic study. Reliable polishing of lead is difficult due to smearing and distortion masking the microstructure. The method uses etch/polish reagents combined with colloidal silica for polishing. Final etching can be done with etch/polish methods or by immersing in acidified glycerol. The method is demonstrated by examining the microstructure of a Roman period lead sheet, which showed a worked and recrystallized grain structure.

1. A Note on the Metallographic Preparation of Ancient Lead
Author(s): David A. Scott
Source: Studies in Conservation, Vol. 41, No. 1 (1996), pp. 60-62
Published by: International Institute for Conservation of Historic and Artistic Works
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2. A NOTE ON THE METALLOGRAPHIC PREPARATION OF
ANCIENT LEAD
David A. Scott
Summary-Reliable and reproduciblepolishing of lead samplesfor metallographic study is difficult to achieve,
since smearing and distortion of the polished surface can easily mask the microstructureof the lead. A practi-
cal method is presented employing etch/polish reagents combined with colloidal silica. Final etching can be
achieved using either etch/polish methods or by immersion in acidified glycerol etchant. An example of the use
of this method in the examination of the microstructureof a lead sheet from the Roman period is discussed.
Introduction
Lead is a soft metal with a face-centred cubic struc- sample is removed with a fine jeweller's saw then
ture, having a melting point of 327?C and a Vickers this operation should be carried out slowly to avoid
Diamond Pyramid hardness (HV) of 4-5 [1]. Lead undue heat, or in a solvent such as ethanol. If
flows readily and recrystallizes so easily at room available, a metallographic cut-off saw such as the
temperature that mechanical alteration to the Buehler 'Isomet' can be used for removing samples;
microstructure can be anticipated unless precau- the oil or solvent-cooled bath helps to keep the dia-
tions are taken to avoid any local temperature mond impregnated copper wafering blade cool,
increase during sample removal and preparation. thereby reducing any heat transfer to the object.
Chemical or 'soft' lead is defined as containing The sample is then mounted in the usual way [3].
less than 0 1% of impurities, while ancient lead may The mounted sample must then be ground on sili-
be considerably less pure and typically contains con carbide papers before final polishing. A faster
1-2% of impurities. Lead is very sensitive to rate of removal of lead on the grinding papers can
changes in properties caused by the addition of be achieved by using glycerol instead of water as
even a few hundredths of one percent of an alloy- the lubricant, although it was not found that the
ing element, which may result in increased hardness change from water to glycerol was particularly use-
values or variation in grain size [2]. For example, ful in our laboratory work. The principal problem
the presence of 0.02% calcium, 0-02% magnesium is to prevent excessive embedding of silicon carbide
and 0.25% tin in a lead sheet, partially recrystal- particles in the lead surface during the grinding
lized after cold-rolling to 90% deformation, resulted stage. The lead sample should not be firmly pressed
in an ASTM grain size of less than eight, compared into the papers during the grinding, and the opera-
with pure lead, whose grain size was ASTM 1 [2]. tor should always be aware of this potential diffi-
The presence of common impurities, such as cop- culty.
per, arsenic, antimony, bismuth and tin, also affects Mechanical damage and excessive smearing of
lead hardness values, although the alloys are gener- the metal during polishing was avoided using a
ally still soft compared, for example, with copper, Buehler 'Chemomet I' polishing cloth which is
and continue to present problems in routine metal- designed to be chemically resistant. An etch/polish
lographic preparation of cross-sections. Satisfactory reagent consisting of diluted nitric acid (139ml of
polishing and etching of lead samples cannot be concentrated nitric acid mixed with 100ml of water)
achieved by conventional methods and some of the was applied to the Chemomet I cloth, together with
older suggestions in the literature are often not con- Buehler colloidal silica suspension of 0.06,m parti-
venient or practical. cle size. These two components must be mixed in a
suitable carrier on the cloth. In this case, a com-
mercially available automobile anti-freeze mixture
Metallographic preparation was used to suspend the etchant and polishing
agent. This worked well, removing the deformed
The object or sample to be mounted should be layer of lead crystals produced during the prelimi-
removed with care, to minimize the potential distur- nary grinding operation. It may be necessary to
bance of microstructure that might be caused by return to the final grinding stage on 600 grade sili-
excessive heating or mechanical deformation. If the con carbide roll, followed by further etch/polish
attack to eliminate the disturbed surface com-
Received 30 October 1995 pletely.
60 Studies in Conservation41 (1996) 60-62

3. < A
A note on the metallographicpreparation of ancient lead
Following chemical polishing on the Chemomet I E..:,
cloth, the lead can be etched in the usual acidified ? '";.::"
??
*'?
":-
.4...~'~. .":. ...'
... . '.'..
acetic (ethanoic) acid/glycerol etchant (84ml glyc- !',' .''- _ .
*^ e
^
'
F'~.x',s',%,';<':-" ,,'
erol, 8ml acetic acid and 8ml of concentrated nitric ,e. i:.':..
I?'. '1. -.*'. -...a .c
acid). If final etching is still unsatisfactory, the
etchant can be applied to a clean Chemomet I cloth
1:*
and the sample gently etch/polished. ' '
m
...S .
.. X
; *'
' A;?
'- *
-.
4 ? ^.<
-^aa^l^. '^ . -'
,
.... ,ok?~:,:~ g* ' , ?,
'~.,
-;: .~ :..I . .'.? . . :
aU. :,:* . .
Example of the method ,~.~':~..~!
"".'},~" ' -' . '" ' 1?
.-?.
*,
:~'.".' ~ :':~-." .. ,.;
?"~"'' " ~' ?..i
To illustrate the application of this procedure, a
r
.~..
.~;~
?ai~- !) X Z
...
'~,
. lf.-~, . 4 ,.
.?: ?
b
fragment of a Roman lead tablet from Britain, dat-
ing to about the first century A.D., was cut and --r L*:
mounted for metallographic examination. The
resulting photomicrographs are shown in Figures 1 Figure 2 Photomicrograph of the sample shown in
and 2. The microstructure can be clearly distin- Figure 1, at higher magnification. Twinned and
guished: both the grains and grain boundaries are recrystallized grain structure can be seen, with rea-
visible. The etching and preparation of the mounted sonable grain contrast. Twinned regions appear well
sample has been successful. The lead sheet has a defined in this preparation. The variable grain size,
worked and recrystallized grain structure, with vari- approximately ASTM 2-3, can still be gauged.
ation in size from ASTM 3 to ASTM 6. The Magnification 45 x.
twinned structure of some grains is evident, show-
ing that annealing of the worked sheet has practically impossible to reveal a grain or crystal
occurred, and that the sheet has been worked into structure. Instead, the segregation which may have
shape. Pure lead is effectively hot-worked at room occurred on casting can be revealed by differential
temperature, which means that plastic deformation etching attack using this technique, resulting in the
can occur even at 20?C, producing microstructures
which show a worked and recrystallized matrix. development of surface holes, which may be den-
dritic in outline, which accompany the darkening of
Cast lead objects, such as lead slingshot, are very some regions due to casting segregation.
difficult to etch regardless of what technique is Ancient lead objects frequently retain, within the
employed since, with many cast lead objects, it is patina or corrosion layers, information regarding
surface shape and morphology. The lead minerals
in this corrosion crust may be attacked during the
final stage of preparation of the lead sample,
although they will not be completely removed. If
;?
i?:3
;f'f the lead object is heavily mineralized it should first
???,tri"t:
gt.
;.b:t?ijf??
.*Zt4.1: be examined in the polished condition resulting
*1?5 from the etch/polish stage before final etching with
r:.cai
?*?rrl.??- the glycerol reagent.
.?p?
Equipment
* CL Ira:
.Y ?" ??:''
r*
.r.?? '* *' -?- ,,:t
;*dr?,,
-? All metallographic equipment was obtained from
: :P. :*.i** .r
.-?*1?
f *":r.'.*?
U?
.???f
1
r? ?? Buehler Inc., 41 Waukegan Road, Lake Bluff, IL
f-. ;?c *'* .+p ?'I:r
?13*;r
-? :
*?
yLE:
?.CC ''.. i
f*jr', ? t..?*"
'.?c?*':I;;.?q?"Sj:
.'.
jl.?*
60044, USA. The part number for the Chemomet I
.. I
.?..? *
.??v
'' P cloth is 40-7918.
Figure 1 Photomicrograph of a polished cross-sec-
tion of a fragment of Roman lead prepared by References
etch/polishing and then etched in acidified glycerol.
The section shows variable grain size with adequate 1 O'NEILL, H., Hardness Measurements of Metals
definition between twinned crystal boundaries. The and Alloys, 2nd edn, Chapman and Hall,
grain structure shows the lead sheet to be worked London (1967).
and recrystallized with straight lines. Magnification 2 SAMANS, C.H., Metallic Materials in
25 x. Engineering, Macmillian, New York (1963).
Studies in Conservation41 (1996) 60-62 61

4. David A. Scott
3 SCorr, D.A., Metallography and Microstructure tion, Scientific Department, Getty Conservation
of Ancient and Historic Metals, Getty Con- Institute. His principal research interests are in
servation Institute/Getty Museum, Malibu the conservation and scientific examination of
(1991). metals, the archaeometallurgy of ancient South
America, and the examination of ancient pigments
Author from rock art and museum objects. Address. GCI
Museum Services, Ranch House Laboratory, J. Paul
DAVIDA. SCOTT, BA, BSc, PhD, C.Chem, FRSC, Getty Museum, 17985 Pacific Coast Highway,
FIIC, is currently head of the museum services sec- Malibu, CA 90265, USA.
Resume-Un polissage friable et reproductibledes echantillons de plomb pour l'etude metallographiqueest dif-
ficile a effectuer, car les traces de frottement et la deformation de la surface polie peuvent facilement masquer
la microstructuredu plomb. On presente une methode pratique utilisant les reactifs de polissage ou d'attaque
avec de la silice colloidale. L 'attaquefinale peut e?treterminee soit en utilisant des methodes de polissage, soit
par attaque par le glycerol acidifie. On montre un exemple de l'emploi de cette methode dans l'examen de la
microstructured'unefeuille de plomb datant de la periode romaine.
Zusammenfassung-Die Herstellung von Bleipolituren als Proben fur metallografische Untersuchungenberei-
tet gewdhnlich Schwierigkeiten, da die durch die Politur hervorgerufenen Verdnderungender Oberfldche die
Mikrostrukturen des Blei verunkldren. Die Verfasser dieser Arbeit beschreiben hierfiir eine neuartige
Methode, die eine Kombination von Atz-/Polierreagenzien mit amorpher Kieselerde nutzt. Eine abschlie,iende
Atzung erfolgt entweder durch den Einsatz von Atz-/Polierverfahren oder durch ein Tauchbad in einer
Atzfliissigkeit auf der Basis von angesaiertem Glycerin. Als praktisches Beispiel wird die Untersuchungder
Mikrostruktur einer Bleifolie der romischen Antike vergestellt, bei der das neue Verfahren zur Anwendung
kam.
Resumen-Es dificil lograr un pulido fiable y reproduciblede muestras de plomo para su estudio metalogra-
fico, dado que el embadumamientoy la distorsi6n de la superficiepulida pueden enmascarar la microestructura
del plomo fdcilmente. Se presenta un metodo prtctico que emplea reactivos mordientes/pulidoresen combi-
nacion con silice coloidal. Se puede lograr un mordido final utilizando o metodos mordientes/pulidoreso
inmersion en mordiente de glicerina acidificada. Se trata un ejemplo del uso de este metodo en el examen de
la microestructurade una lamina de plomo del periodo romano.
62 Studies in Conservation41 (1996) 60-62