Ceramics for the Archaeologist

447 Pages·1965·3.71 MB·English

by  Anna O. Shepard

CERAMICS FOR THE
ARCHAEOLOGIST
ANNA O. SHEPARD
Publication 609
CARNEGIE INSTITUTION OF WASHINGTON
WASHINGTON, D. C.
Manuscript submitted November 1954
Published 1956
Reprinted 1957,1961, 1963, 1965, 1968,1971, 1974, 1976, 1977,1980
/f
1985 reprinting: Braun-Brumfield, Inc., Ann Arbor
ISBN 0-87279-620-5
LC 56-4818
Foreword to Fifth Printing
Ceramic Studies, 1954 to 1964
MANY CHANGES in ceramic studies have occurred since Ceramics for the Archae-
ologist was written in 1954. The physicists have introduced sensitive and
rapid methods for compositional analysis; the availability of computers has given
statistical studies great impetus; there has been renewed interest in the potter's
natural resources; the classification of pottery has been pursued with fresh enthu-
siasm, taking different directions in different regions; ethnologists have been
inquiring into the place of pottery in the lives of village people; and the archaeol-
ogist, in his turn, has been challenged to think about the cultural significance of
pottery rather than content himself with its use for relative dating. A review of
these developments will point toward the fundamentals in ceramic studies and
suggest how the archaeologist can best obtain the aid he needs. Although this re-
view is not seasoned with either humor or satire, it is brief, and the points are car-
ried by illustration rather than by precept.
The physicist's contribution to ceramic studies. Physics has wrought and is
working a remarkable revolution in chemical analysis by the development of new
instrumental methods. In contrast to classic procedures, these methods are based
on the measurement of characteristic properties, especially atomic and electronic
properties. The methods are so varied that it is difficult to generalize about them,
but a number require relatively little material, and some are nondestructive—con-
ditions that are important to the archaeologist. They vary widely in sensitivity
and specificity; some are especially useful for the identification of impurities and
trace elements because of their extreme sensitivity; and some give a complete ele-
mental analysis whereas others can be used only for certain elements. A number
employ standards that must be prepared by classic methods, and their accuracy is
therefore dependent on the accuracy of those methods. .Most of them require ex-
pensive instruments, but in routine industrial testing the cost may be offset by the
reduced time for analysis. On the other hand, since standardization is required
when unknown material is tested, more time may be needed by the new than by
the standard method. In other words, the instrumental methods have great poten-
tialities but must be used with judgment. Reference to a few that have been ap-
plied in archaeology will illustrate their potentialities and limitations.
IV FOREWORD TO FIFTH PRINTING
Emission spectroscopy is a method that has been throughly tested over many
years; it offers a quick means of obtaining complete qualitative data, and its value
for quantitative analysis is well established for both routine work and research.
Its earlier applications in archaeological ceramics were mainly for pigment anal-
ysis, for which it is particularly well adapted. As an example, qualitative analyses
of Rio Grande glazes were performed in the laboratories of the Massachusetts In-
stitute of Technology to learn (a) whether production of glaze paint ware awaited
discovery of local sources of ore or whether lead ores were initially imported from
the western glaze paint centers; (b) how many sources of lead ore were known in
the Rio Grande Valley, and the influence of their location on the centers of produc-
tion and on trade (Shepard, 1942, p. 258).
In recent years spectrographic analysis has been used in the study of pastes,
with the aim of differentiating pottery from different sources. A study of Myce-
naean and Minoan pottery conducted by the Oxford Laboratory for Archaeology
and the History of Art is the most extensive and successful of these. Archaeolo-
gists of the British School of Archaeology in Athens and the Ashmolean Museum
outlined a sound and challenging problem of trade in the eastern Mediterranean
in Late Bronze Age times (Catling, 1961), and they collected an excellent sample
from sites considered centers of production in the Mycenaean-Minoan world and
from sites in the eastern Mediterranean where pottery, especially Mycenaean,
seemed to have been traded. Nine elements were chosen for quantitative spectro-
graphic anah sis because they had boon found useful in comparing and contrasting
various ty pes of pot ten (Blin-Stoyle and Richards, 1961). Initially, analysis of
10 sherds each of Mycenaean and knossian pottery showed that the composition
patterns (plot of percentages of elements) of the two were sufficiently distinct to
permit assignment of origin of sherds to one or the other if there seemed to be only
the two alternatives. Both the advantages and the limitations of the study are in-
dicated in the second report (Catling, Richards, and Blin-Stoyle, 1963) and the
review of archaeological interpretations (Catling, 1963). Eleven composition pat-
terns were recognized from the analysis of more than 500 sherds. In addition, there
were a few odd patterns that were not comparable to any of the others or to one
another. The largest group (composition pattern \) was represented by all except
a few of the- Peloponnesian sherds tested. Mycenaean-like pottery from three sites
in C\ [mis. one in north Syria, and Tell \marna, Kgypt, as well as pottery from
various sites in the \egean, had the same pattern. The second important group
'composition pattern B> occurred in most but not all the Minoan sherds. It was
much less common outside Crete than I lie Mycenaean pattern, although there are
occurrences within the Vegean area. Most of these may represenl trade, hut the
proportion of sherds with this composition pattern in Thebes in central (Jreece is
surprisingly high, making up nearly two-thirds of the sample analyzed. Catling
FOREWORD TO FIFTH PRINTING V
comments that an intensive trade between Thebes and Crete is highly improbable,
and that the occurrence in the two regions of clays indistinguishable by the ana-
lytical method employed is the more likely explanation (op. cit., p. 6). Catling,
Richards, and Blin-Stoyle generalize that differences in composition pattern in-
dicate different sources but the same pattern does not necessarily prove identity
of source (op. cit., p. 103). The sherds with the eleven other composition patterns
are more limited in distribution: two occur in Thessaly, one in a site in Attica, and
two were found in each of four islands, Melos, Rhodes, and Cyprus, and the east-
ern end of Crete.
A disappointment in the study was the similarity of all pattern A Pelopon-
nesian pottery analyzed. Although the sites did not give a good coverage, the area
was nevertheless considered too extensive to have been dependent on the Myce-
naean region alone; similarity of clays within the area is proposed as the probable
explanation.
The Mycenaean-Minoan study was conducted without reference to geological
and geochemical conditions. Geological formations in which the sites are located
are not mentioned. It is interesting to note that the Minoan sites with type B com-
position patterns are in the Pliocene, which is not present in the eastern end of
Crete where the principal formation is Cretaceous, and where type B pattern is
rare, but two distinct patterns were found.
Although the geology of the Peloponnesus is complex, including many different
formations, the four sites on which the study was based all appear from a geologic
map (Renz, Liatsikas, and Paraskevaidis, 1954) to be in alluvial formations. Oc-
currences on the island of Melos. an important staging port for ships, is singled out
by Catling as especially interesting. In addition to group A sherds, presumably im-
ports from the Mycenaean region, two distinct composition patterns were found.
One, described as "very distinct," is low in calcium (2.4 per cent); in the other
group calcium (16.2 per cent) is within its range for most patterns reported. The
low-calcium sherd is classed as trade of unknown source because it is indistinguish-
able from the Mycenaean sherds in general appearance. The high-calcium sherd,
an imitation of Minoan pottery, "was certainly made on Melos" (Catling, 1963,
p. 7). But the formation of Melos is Tertiary volcanic of acid composition, and
consequently the local clay should be low in calcium. This observation illustrates
a situation that often arises in archaeological ceramic investigations: finds from
one method of study suggest re-evaluation of those obtained from an independent
approach; there-evaluation, in turn, may suggest further investigation by the first
method; and so problems are solved by a repeated interplay between different-
methods. The question that immediately arises about Melos is the occurrence of
some sedimentary formations. The geologic map shows only a small section of
Cretaceous formations in the sou!liens part, far from Phylakopi, where the sherds
VI FOREWORD TO FIFTH PRINTING
were collected, and shows some undifferentiated Miocene-Pliocene on the north side.
The formations plotted on a geologic map are broad guides and offer many
helpful hints, but of course it would be extremely naive to expect clays of the same
composition pattern to occur throughout a formation. Aside from the necessary
broad generalizations of mapping, there are many factors that affect the compo-
sition of clays: composition of the parent material, the conditions prevailing dur-
ing formation of the clay, and, if the clay is sedimentary rather than residual,
the conditions during transport, during deposition, and subsequent to deposition.
Localized conditions, such as action of hydrothermal solutions, may also have a
marked effect.
Analysis of clays from production centers is a direct approach in the study of
sources of fine-textured pottery. Catling, Richards, and Blin-Stoyle (1963, p. 95)
dismissed this possibility because the raw clays had presumably been refined and
they were dehydrated in firing. Dehydration is simply performed in analysis. Re-
finement of clay, which would affect relative values of some elements, would be
carried out by a process of sedimentation, and the relative degree of refinement of
clay and pottery could be compared optically. Since Pliocene sedimentary clay oc-
curs in many parts of the Peloponnesus and in the Megara it would be interesting
to compare the composition of clays in some of these areas with clays from the
Pliocene of Minoan settlements of Crete. This would be an independent and more
direct approach to the problem raised by the B pattern sherds in Thebes than re-
liance on pottery analyses and relative distances alone, granting that knowledge of
style in the various centers is also essential.
The analysts1 selection of nine elements on which to base their study was em-
pirical; they did not consider the place of these elements in clays. Many different
roles are played by chemical elements: they may be major constituents of the clay
minerals, impurities in the clay, adsorbed cations, proxying atoms, and additives.
The meaning of variations in amounts of elements is judged by their place in the
clay. Once chemical data are studied in the light of the geological nature of
ceramic materials, complementary and supplementary means of analysis will be-
come apparent.
It is important first to have an estimate of the impurities in a sample. The
calcium, magnesium, and iron are much too high in these analyses for clay min-
erals. Petrographic thin sections of the pottery would permit estimation of the
major impurities in the pastes even though the paste is fine and presumably the
clay was sedimented. I have observed particles of carbonates, iron oxides, and
quartz in thin sections of classic Greek pottery. The presence of dolomite could
be checked by X-ray diffraction. The significance of migratory elements, such as
sodium, should also be critically evaluated. The possibility of its presence as an
adsorbed cation can be checked by identification of the clay mineral; the structure
FOREWORD TO FIFTH PRINTING Ml
of montmorillonites leads to adsorption of cations, whereas the structure of ka-
olinite does not. These two clay minerals can be differentiated by their silica-to-
alumina ratio. Granted that silica may be present as an impurity in the form of
quartz, its amount can be estimated in thin sections. Alumina was included in the
analyses, but silica was omitted. The minor and trace elements may occur in the
clay minerals or in the impurities of clays. The ratio of pairs of elements that are
often associated may be more significant than the amounts of the elements con-
sidered individually. The values reported for nickel, chromium, manganese, and
titanium are very high for clays, but they are not reported as absolute values; they
are estimates conditioned by the method of analysis and are adequate for compar-
isons because the evaluations were consistent.
The nine elements selected for analysis are not necessarily always the most
useful for differentiation. Knowledge of the geology of a region might suggest other
elements that would be more significant -for instance, if alteration had taken
place in areas of mineralization or in the vicinity of a hot spring, or if the forma-
tion were marine. These comments may suggest the role of geochemistry in the
study of archaeological ceramics. It is a new field of application, and the theoretical
frame of reference is still to be developed; but the importance of understanding the
meaning of chemical composition in terms of the genesis of clays is clear.
I have commented on the Mycenaean-Minoan study in detail not only because
of its importance but also because it illustrates the problems about the sources of
pottery that arise when inferences are based on chemical composition alone. These
comments apply equally well to results obtained by the newer methods of instru-
mental analysis, because they too yield chemical data. It will be sufficient to men-
tion a few of the newer methods to suggest the difference in sample requirement,
the number of elements detectable, and the differences in sensitivity.
X-ray fluorescence is a newer method than emission spectroscopy that has the
advantage of adaptation to nondestructive analysis. The fact that a paint or glaze
need not be removed from the surface eliminates risk of contamination and saves
time as well; but quantitative results cannot be obtained if the pigment is thin or
uneven. The method has been used effectively for the analysis of manganese in
Greek glazes and pigments (Farnsworth and Simmons, 1963). Another application
was for identification of the red color that trails beyond the lines of black paint on
Urfimis pottery (Greek Neolithic). Superficially the paint appears to change from
black to red, but there is no corresponding change in surrounding surface color that
would accompany a localized oxidizing effect. My experience with pottery of the
American southwest led me to expect that a medium in the black mineral pigment
had extended beyond it and had reacted with the clay to liberate iron. If this hy-
pothesis is correct there should be the same percentage of iron in the red line and
in the light orange body. A test of red lines and body surface with an X-ray fluo-
VIII FORK WORD TO FIFTH PRINTING
rescent probe proved that the amounts of iron were the same in both. (Test made
by courtesy of the U.S. Geological Survey, results unpublished.)
At present the possibilities of analysis by neutron activation are arousing
interest. Spectacular results have been obtained with this method in certain spe-
cialized fields outside of archaeology. Its primary limitation lies in the number of
elements it is practicable to determine. In archaeology the method has not had ex-
tensive tests comparable to those of emission spectroscopy. It was used by research
workers at the Oxford Laboratory in some of their studies of differences in paste
composition (Emeleus, 1958, 19606; Simpson, 1960); they later substituted emis-
sion spectroscopy for paste analysis because it is more comprehensive and less
expensive. Some pilot work in the application of neutron activation for paste anal-
ysis was undertaken in this country by E. V. Say re and his associates (Sayre and
Dodson, 1957; Sayre, Murrenhoff, and Weick, 1958). With one exception the pot-
tery analyzed had no added nonplastics; therefore the comparisons were of clays.
The exception was the only test of American Indian pottery made by Sayre; it in-
volved comparison of tempered and untempered pastes, although allowance was
not made for this condition. The most recent published report of the neutron acti-
vation method deals with tempered pottery (Benny hoff and Heizer, 1964). The
conclusions drawn from this study, which support the authors' hypothesis, are
difficult to judge because of several unknown factors: the reliability of the mega-
scopic identification of rock temper in 9 of the 11 sherds; justification for the
assumption that the manganese was derived from the clay alone; and the possibil-
ity of postdepositional alteration of the sherds that might affect the percentage of
the single element upon which judgment was based.
Beta-ray backseatter is a quick and inexpensive analytical method but has
limited applications. It has been used effectively for the determination of lead in
glazes (Emeleus, 1960a; Aitken, 1961, pp. 169-171). There are a number of other
instrumental methods of chemical analysis, some of which are so new that they
have not yet been used in archaeology. Those I have mentioned will suggest the
wide differences in applicability and sensitivity.
Fortunately, the archaeologist does not have to rely on trial-and-error tests
of new methods; he can learn from specialists in the different fields of the physical
sciences the kind of results their methods give—their accuracy, their cost, the size
of sample required, and whether the test is nondestructive. But first the problem
must be outlined, and the archaeologist must decide which class or classes of data
give the best promise of solution. This review of chemical applications in archaeo-
logical ceramics should be balanced by reference to other analytical fields.
Mineralogical analysis, the other important method of archaeological ceramic
analysis, is made by means of optical mineralogy and X-ray diffraction. There are
supplementary methods, morphological (electron microscope) and thermal (DTA),
FOREWORD TO FIFTH PRINTING IX
for example, but for our immediate purpose it is not necessary to consider them.
Comparison of chemical and mineralogical methods will illustrate the necessity of
evaluation and the frequent need to combine different methods in a complementary
or supplementary way. The petrographic microscope has long been used for the
identification of nonplastics in pottery, though far more extensively in the Amer-
icas than in the Old World. The applications of X-ray diffraction are much more
recent, as is the development of the method itself. Optical mineralogy and X-ray
diffraction are complementary methods because X-ray diffraction is an extremely
efficient and powerful means of determining the mineralogical composition of sam-
ples too fine-grained for optical analysis. It is limited to the identification of crys-
talline materials, but it has shown that many materials supposed to be amorphous
are actually crystalline—such as many "limonites" and most clays- but it gives no
record if the material is actually amorphous. It is the essential instrument for the
study of clays, and clay mineralogy has made tremendous strides since its intro-
duction, leading to a basic classification of days, the establishment of the relation
of their properties to structure and composition, and the determination of the con-
ditions of formation and alteration. During this period the method has been refined
and elaborated. The value of X-ray diffraction in the study of clay is not matched
by its applications in the study of pottery because the crystalline structure of clay
is often, though not always, destroyed in firing. The problem of the identification
of clay minerals in fired pottery is being investigated by a combination of methods.
Although optical crystallography is a very old method in comparison with
X-ray diffraction, it, too, has undergone developments and refinements, some of
which are especially advantageous in the study of pottery.
The differences between chemical and mineralogical data suggest their primary
applications in archaeological ceramics: chemical methods for the analysis of paints
and glazes, and fine amorphous materials; optical crystallography and X-ray dif-
fraction for pottery with nonplastie inclusions, the natural mineral inclusions in
clays, and raw clays. In the earlier years of ceramic technological investigation in
the American field this distinction in application was gene rally followed. In recent
years chemical analysis has been used more often in the analysis of pottery itself.
This trend has been influenced by the physicists* interest in testing the applications
of instrumental methods of analysis.
The fact that a chemical analysis, whatever its method, identifies the elements
of a substance, whereas mineralogical analysis identifies minerals and rocks, means
that each has its particular advantages and limitations. There is the difference in
scope: all substances can be analyzed chemically, but only minerals and rocks can
he identified mineralogically which is not a serious limitation for the ceramist
inasmuch as minerals and rocks arc* the potter's principal materials.
In contrast to scope of application is the number of distinctions that can be