SHELL COLORANTS

ANALYSIS OF COLORANTS ON 3 PREHISTORIC SHELLS FROM CASAS GRANDES, MEXICO
AND 2 MODERN SERI INDIAN BEADS FROM BAJA, MEXICO
By Paul T. Kay
Studies done in 1992/1993

PAINTED SHELLS by Paul T. Kay

Several spectrometric techniques, Scanning Electron Microscope/Energy Dispersive Spectra (SEM/EDS),
X-Ray Powder Diffraction (XRD), and Fourier Transform Mid-InfraRed (FT-IR) were employed in an effort to
partially 'describe the composition of colorants on the surface of five marine shell specimens. These
methods provided data which assists in developing a partial characterization; a description in terms of
constituent elemental chemistry of the colorant. In one case mineralogical identification of primarily Atacamite
was made, and from all five samples information was obtained on organic materials used in the artisan's
mixtures. Highlighting the FT-IR observations is a positive identification of a natural dye, indogotin or indigo,
which requires some cultural processing.

These five shells, obtained from Ronna J. Bradley, were identified as both modern and prehistoric. Two
specimens, one red and one blue, are Nassarius sp. beads obtained from the Seri who occupy portions of
coastal Sonora. Three prehistoric specimens were among the samples obtained from the Casas Grandes
(Paquime) collections for chemical testing. One shell was blue, another black/red/bronze, and one
green. Difficulty was encountered in most attempts to describe these hues using the Munsell-1929, while
The Dictionary of Color (Maerz and Paul-1930) had fewer limitations, but is not readily available to most
researchers. Ultimately, the Pantone system was used by the author to approximate the color, although the
determination is, therefore, somewhat subjective.

In their natural and beach-worn state Nassarius sp. exhibit an ivory colored surface, and any natural
coloration is in the matrix of' the shell not on the surface of the mollusca. The coloration upon these colored
specimens are, relatively opaque and uniform and occur on the crest of anatomical features as well as in the
crevasses, indicting deliberate cultural application of a colorant or artisan's mixture (Kay and Phagan 1993).
Felger and Moser (1985) describe in detail a number of different plants and minerals used in paints and
dyes.

A comprehensive survey of this genus can be studied in Keen (1971). An excellent photo image of one such
nearly identical to CG 1929 Blue is shown in De Paso et al (1974). Further, the shell have been modified;
holes were generated through the shell material to perhaps use as ornamentation (see figures 16, 17a and
17b and discussion). Ethnographic studies by Folger
and Moser (1985) depicts artisans stringing Nassarius along with other items to create a necklace.

Table 1. SEM/EDS analysis results.

Specimen
SEM/EDS of Chemical Elements and Occurrence

                                C O Na Mg Al Si P   S   Cl K Ca Fe Cu
Seri Red                   X   X    X    X    X   X   X   X    X   X    X    X    -
Seri Blue                   X   X    X    X    X   X   X   X    X   X    X    X    -
CG 1929 Blackish    X   X    X    X    X   X   X   X    X   X    X     -    -
CG 1929 Blue          X   X    ?     ?   X   X   X   X    X   ?    X     -    -
CG 1929 Green       -    X     -     -    X   X   -   X    X   X    X     -   X

    X= present, ?= uncertain presence, - = not present

Results of SEM/EDS

The elemental chemistry of both the modern Seri and the pre-Columbian coatings are intriguingly similar
(see Table 1 and spectra 1,2,3, and 5), with the exception of the presence of Fe and Cu. Iron (Fe) in the
Seri shell suggests that it may act as a mordant. Copper (Cu) is confined to the pre¬Columbian green 1929
which lacks carbon (C), indicting that this material is probably inorganic. The presence of carbon(C) in the
two Seri and Casas Grandes blue and black/red/bronze specimens is an indication of the organic nature of
the coatings.
. Interestingly, the presence of chlorine (Cl), phosphorus (P), and possibly sulfur (S) is suggestive of a
sulfur substituted Apatite which interpretation is somewhat supported by the FT-IR microscopic examination
results seen in Figures 25, 31, 33, and 13, in the broad area of 1050 cm-1. Low power microscopic
observations of these four specimens, together with spalling resulting from immersion in solvents, have
exhibited raised grains of mineral appearance. Conversely, examination of the raw shells show their
surfaces to be very smooth and shiny (see left field of Figure 7 for example). Material has flaked from the
surface of the CG 1929 black/red/bronze specimen, while the other three are dull in appearance. A granular
coating can be seen even with a hand lens. When opportunity presents, further observations wil be
undertaken.

EDS spectra do not establish mineralogy. Coupled with FT-IR, however, the rare and unusual combination
of these elements along with the functional group evidence for phosphates is strongly suggestive of
Chlorapatite. A carbon peak in these four spectra generally indicates the presence of organic materials so
these specimens where categorized as candidates for additional studies using instrumentation and
techniques appropriate to assisting. such identification, are documented later in this report (see Table 3,
etc).
Initially, the EDS information gathered for the CG 1929 green was obtained from a tape collect sample. The
data was somewhat straightforward and suggested a halide chemistry while its morphology appeared
mineralogical. Efforts to study any possible organics present was hindered by the adhesive on the tape. A
decision was made to forward the shell containing
the green for further studies. X-ray Powder Diffraction (XRD) was selected. Its suitability is well established
and there exists an extensive spectral inventory well standardized by the Joint Committee for Powder
Diffraction Standards (JCPDS).

Sample preparation proved difficult. The green (see Fig. 6)
deposit occurred only on an area of approximately 4 sq mm (see Figure 7). When examined with binocular
light microscope, a white, flowery deposit suggesting an evaporite covered the co-mingled wi th much of the
green. The presence of an efflorescence would be consistent with the elemental spectrum

X-Ray Powder Diffraction (XRD)

Mineralogical identification of this sample was established by XRD, a non-destructive, non-invasive
method of spectroscopy. A recent model SINTAG/USA was used located at Coors Porcelin, Golden,
Colorado.

The instrument's Cu-tube bathes the specimen with an X¬ ray beam that moves over a fixed arc while the
specimen remains stationary. The reflection of the incident beam is measured as two theta (20) and the
data gathered for storage in a computer. Interpretation of the spectra comply with the rules of the Joint
Committee of Powder Diffraction (JCPDS) and calibration of the alignment is to ground quartz traceable to
the National Bureau of Standards. The bar graphs seen in Figure 9 are JCPDS references.

This technique's ability to establish most components in a given sample is of particular value in
archaeological studies where suspected artisan's mixtures are being analyzed.

Figure 8/9. XRD fingerprint spectrum, 10 to 47 degree, 2 theta along with the JCPDS
identification cards for the constituents of the green material from the shell (CG 1929)
(see table 2 . All peaks correspond to the four bar standards collectively with the exception of
15.6 degrees (see text).

Table 2. XRD results
.

Green material on CG 1929 green. Specimen 5. In order of relative abundance from top.
Atacamite:                  Cu2CI(OH)3           Copper Chloride Hydroxide
                                                          naturally occurring mineral

Paratacamite:             Cu2(OH).3CI           Copper Chloride Hydroxide
                                                          naturally occurring mineral

Gypsum:                     CaSO4 .2H20          Calcium Sulfate Hydrate
                                                          naturally occurring mineral

Aragonite:                   CaCO3                   Calcium Carbonate, polymorph of calcite (CaC03)
                                                          naturally occurring mineral

Atacamite is a mineral Dana (1944) and Hurlbut and Klein
(1974) occurring naturally as a secondary formation in association with malachite and azurite
and is found, among other places, in northern Mexico and southern Arizona (Roberts et al.
1992). Dana (1944) suggests that gypsum can be in association in situ, but Kay (Phagan and
Kay 1993) suggests that gypsum/anhydrite was deliberately added by craftsmen in
the preparation of certain colored artisan's mixtures. Another possibility, by by no means final,
is contamination during burial or curation. Aragonite/calcite are normal constituents of shell to
which the green substance adhered when sampled (see Figure 10). A photomicrograph,
Figure 11, shows the fragmented green atacamite. The sharp angular fracture surfaces may
demonstrate mechanical alteration (such as grinding) as could variability in particle size.

Because none of the chemical/mineralogical constituents have a hardness above 3-3.5, the
resistance to collection and attemped fragmentation suggest that some form of processing
(such as roasting with an undetermined organic binder, or cementing action during burial)may
have taken place to alter this material physically. Nothing extraordinary was detected with XRD
as a possible explanation. All peaks are identified from 10 through 47 degrees with the
exception of a single peak at 15.5 degrees; this solitary signal is insufficient for identification
and probably results from trivial preferred orientation, and explicable phenomena in XRD.
Why the atacamite mixture adheres so well is unexplained by this research, beyond the
possibility of the grains of atacamite being small enough to attach physically into the shell's
structural porosity. Now that the mineralology has been conclusively established, procedures
should be undertaken to identify any organics that may be integral.
Several probable explanations can be posited for the presence of the copper chloride on this
shell and include, but are not limited to the following:

1) The shell may have been strung adjacent to a copper object. McLane (1993)
suggested that exposure to moist air would have encouraged oxidation of the copper. That
suggests that if the position of the tangent items, the shell and copper, were static for a
sufficient period of time, the shell may have stuck to the corroding metal while a chopper
chloride was being formed, especially in the presence of salty groundwater movement. The
re-dissolution seen in Figure 11 would seem to support this thesis. Subsequent separation
possibly left the shell with a well anchored deposit of the corrosive end product on the areas
of the manufactured perforation (see Figures 7 and 16).

2) The copper salt may have been the end product of one aspect of copper smelting certainly
must be given consideration. A modern process is outlined by McKelta (1980) and
Kirk-Othmer (1979) that given reductive modification to the contemporary conditions at
Precolumbian Casas Grandes, makes this seem feasible.

FT-IR Explained and Results

Fourier Transform Mid-Infrared (FT-IR) was selected for the initial efforts to detect organics.
FT-IR or mid-infrared spectroscopy provides information about vibrational dynamics of
molecular bonds of both inorganic and organic specimens. This nondestructive method
permits either direct observation and analysis of microscopic sized samples; examination of
prepared powder wafers, which alter only the morphology of the specimen; or solvent
extraction procedures that separate the organic from the inorganic fractions enabling various
multiple studies. In all cases the specimen fractions can be archived.

The resultant spectra convey information about organic functional groups that can be
interpreted by comparison to reference material when available. All organic molecules contain
at least one functional group structure. There are 21 functional groups from which most
naturally occurring and/or synthetic organic compounds are basically constructed.
Fortunately, most have distinguishing features that provide characteristics enabling
discrimination at the group level by FT-IR. However, mixtures such as those encountered in
this study may require highly skilled interpretation by the investigator, as various peak
locations may overlap (be superimposed).

Those functional groups of general interest in this study are alcohols, carboxylic acids, and
related esters, amines, and the usual hydrocarbons. Alcohols are widespread in nature and
are probably the most versatile in that they can be made from or be precursor to many different
organic compounds via various reduction and addition reactions. Some familiar alcohols are
ethanol in a beverage, peppermint oil as a flavoring or fragrance, and cholesterol. Alcohols are
organic derivatives of water (H2O or H-O-H). When one H is replaced by a hydrocarbon (CH) it
becomes an alcohol CHOH. Carboxylic acids, aldehydes, esters, and albenes can be
transformed to or from alcohols.

Fatty acids are found in lipids, animal fats and vegetable oils. They are insoluble in water and
this insolubility gives definition to the lipids by physical property. This fact is due to the large
hydrocarbon structure. Lard, beeswax, and palm oils are familiar examples. Some are solids
like lard or butter, while others are not solidified, such as corn or peanut oils. Technically, a fat
or oil is a triglyceride having three glycerols and three carboxylic acids. When a fat is cooked
with an alkalai such as wood ash, it will form a soap (amine) and glycerin. Yucca soap is a
common preparation used by natives of the Southwest. A carboxylic acid can react with an
alcohol (naturally occurring in tissues) to form a fatty acid ester. The flavor and aroma of fruits
is a common example. The latter process is reversible and thus new precursor materialsare
produced in a living organism allowing farther and reactions such as the sour taste of
unripened fruits, sour milk, etc. caused by a carboxylic acid combining organic salt. Formic
acid found in ants and responsible for the pain of a bee sting, acetic acid known as vinegar,
citric acid from fruits, and tataric acid found in wine or used in baking are some of the familiar
compounds resulting from the reactions of fatty acids and alcohols.

Fatty acids or carbonyls group substances give characteristic spectra by FT-IR in the carboxyl
and or ester ranges of 1670 to 1780 cm-l, and therefore are very diagnostic. The amines
(soaps) occur distinctively in three different regions while alcohols are seen in two yet
differing locations. Many of these substances are preserved in archaeological remnants that
can be seen by several spectometric devices.
While most fats and oils are straight chain hydrocarbons, some of their relatives are formed in
a ring structure giving the molecule more stability.

Such is the case with the indigo which is an aromatic (six-sided ring) amine (five-sided ring)
(Figure 12). The blue (pantone 301U, Table 3) material on the tiny Nassarius from prehistoric
Paquime, has been identified as natural indigotin (indigo) by FT-IR (see Figures 12a, b, c &d ).
This culturally prepared natural dye stuff occurs mixed with at least one fatty acid ([ester] at 17.
35 ~-1), which
probably acts as a binder and dispersive agent that also helps to provide adhesion to the shell
body. A very brief immersion in chloroform (CHCI3) allowed an appropriate extraction which
was evaporated on a NaCI crystal and examined in the transmittance mode. The resulting
spectrum, Figure 12a, clearly establishes natural indigo. Several reference spectra are
provided for comparison; Aldrich 394B and Wong's authentic indigo, and Paracas T220
(Figures 12a, 12b and 12d). It is important to remember that, while a fatty acid ester can be a
contaminant derived from human body oils as a result of wearing or handling, the blue colorant
on CG 1929 possessed a remarkable quality; it could be moved on the surface by a ridged tool,
but not removed. The substance defied collection, the efforts resulting only in the removal of
some shell matrix, carbonates. This smearable, "dry film", greasy coating even more
remarkably did not accumulate particules or fibrils. Therefore, perhaps, indigo dye stuff
contains some fats, oils, or waxes that gives rise to this unusual feature, or such was added by
the artisan to effect clinging to the shell. In any event, this substance may be a carbonyl
responsible for the peaks in the 1700-1735 cm-l range; its absence is notable in the synthetic
spectrum as well as the authentic. The

Scanning Electron Microscope equipped with Energy Dispersive Spectra (SEM/EDS).

The instrument is a JOEL model JSM-840 equipped with KEVEX DELTA FIVE accessories. A Poloroid-like
film holder attchment permits the production of photomicrographs. All procedures are non-destructive and
non¬ invasive. The specimens of whole shells were recovered for numerous additional observations.

The sample is placed in a vacuum chamber and where it is bombarded by an electron beam that excites
the specimen causing the emission of x-rays. These x-rays are detected and the data processed by the
KEVEX resulting in a video display of a spectrum (see Figures 1-5) and qualitative information on the
chemical elements. The detection of all elements above boron (B) is possible if their occurrence is 1% or
more by weight. Only the identified peaks on the x axis are absolute data. The height of the individual
peaks and their relative heights are not comparably valid data.

Secondary electrons, which are also produced by this electron bombardment, are processed by a different
set of devices that create an image both on a video monitor and into a film holder, thus enabling the
production of a photo record of chosen morphologies etc. Optimum results can be obtained (see figures 10
and 11) when the specimen is well grounded electrically. The laboratory coating needed for this process
deposits a film of gold (Au) and palladium (Pd) onto the subject and for various reasons was avoided during
the initial observations. A more comprehensive discussion of this method and results can be found in Kay
and Phagan (1993).

Figure 1. SEM/EDS spectrum of the red coating on the
surface of a Nassarius shell from a late 1980s Seri
necklace, Kino Bay, Sonora, Mexico (Specimen 1).

Figure 2. SEM/EDS spectrum of the blue coating on the
surface of a Nassarius iodes shell from a late 1980s Seri
necklace, Kino Bay, Sonora, Mexico (Specimen 2).

Figure 3. SEM/EDS spectrum of the green material on the surface of a Nassarius shell
from prehistoric Paquime, Casas Grandes, Chihuahua, Mexico (CG 1929) (Specimen 5).

Figure 4. SEM/EDS spectrum of the blue coating on the surface of a Nassarius shell
from prehistoric Paquime, Casas Grandes, Chihuahua, Mexico (CG1929) (Specimen 4) .

Figure 5. SEM/EDS spectrum of the red/black/bronze coating on the surface of a
Nassarius shell from prehistoric Paquime, Casas Grandes, Chihuahua, Mexico (CG1929)
(Specimen 3).

3) Another, but hardly exhaustive explanation, is that thematerial was gathered as a
discriminate substance to be used intentionally as a colorant. It was probably readily available
in the areas where raw copper was mined (DiPeso et al, vol.7, 1974).

Figure 10. SEM photo of Nassarius shell surface showing the
natural calcite/aragonite fiber construction (CG1929) X 1,100

Figure 11. SEM photo of emerald green material found on
surface of prehisitoric shell (CG1929) from Paquime. These
angular fragments show evidence of mechanical processing not
the result of collection procedures. ( X 4,100 )

Fig. 6

Fig. 7

information in the spectra for these two specimens was far less conclusive than for the blue shell from
Paquime, CG 1929.

As with CG 1929 black/bronze/red, the chloroform extraction revealed signals for fatty acid(s)/ester(s)
and nothing interprible regarding the coloring agents. The carbonyls are probably due to body oils, a
result of handling by various examiners, and were on the surface- thus easily and readily extracted.
Figures 14 and 15 are nearly identical in form, suggest nothing of color, and, other than the fats, give
only a weak indication for amine soaps. The dye colors were only weakly apparent in the solvent.
Additional studies are underway using other solvents in hope of retrieving more data. The inorganic
FT-IR spectral information is consistent with the SEM/EDS data.

Figure 12. FT-IR spectrum of chloroform extraction of blue CG 1929
colorant which establishes indigo (see Aldrich 19-¬ Figure 12a and Wong
19-- Figure 12b) along with fatty acid(s) and fatty acid ester(s) at 1735
cm-1. The broad region with sharp peak at approximately 3275 cm-1 is
related to the amide (NH) fraction along with those in the 1625 cm-1
region. The strong peaks in the 2850 to 2900 cm-1 region are typical
hydrocarbons (C-H) seen in many organics.

Figure 13. FT-IR spectrum of chloroform (CHCL3) extraction of black/red/bronze, prehistoric
shell CG 1929 which appears to yield no clue as to the origin of the color. Fatty acid (s) and
fatty acid ester (s) are seen in the region of 1735 cm-1 along with inorganic carbonates and
bicarbonates. Silicates are indicated in the region 1450 to 1000 cm-1. A possible mineral
coating is on the shell surface. The strong peaks in the 2850 to 2900 cm-1 region are typical
hydrocarbons seen in many organics-such as the fatty acids here.

Figure 14. FT-IR spectrum of chloroform extraction of modern Seri shell with
red coating.

Figure 15. FT-IR spectrum of chloroform extraction of modern Seri shell
with blue coating. Both spectrum show only fatty acid(s) and fatty acid
ester(s) along with inorganic carbonates and silicates. Color is
indistinguishable. In both cases the organic material may be oils from
human handling.

Figure 16. FT-IR microscopic examination of particles scraped from blue surface of
prehistoric specimen CG 1929. The spectrum shows basic carbonates which are the shell
matrix; calcite/aragonite (CaC03) in the areas of 1800, 1430, 840, and 750 cm-1, while
the single sharp peak at 1080 is discreetly aragonite. A slight indication of phosphorus is
seen at 2500 cm-1. The two peaks in the 2900 cm-1 general area are hydrocarbon stretch.

Perforations

The prehistoric shells have holes that appear punched with a round, sharply pointed tool.
The two Seri shells however, appear to be pierced with an oblong tool, forming a
rectangular perforation. Figures 20a and 20b show the aperture in Specimen l-Seri Red.
A close examination of the right side wall and the tangent of the basal surface clearly exhibit
a linear tool mark(s). These slice marks are evident in several places along the basal
surface. The shape of this perforation and the narrowness of the tool marks suggest
perhaps the use of a pointed exacto blade, a razor blade, or possibly a very thin knife point
or stone flake. Comparison with Figure 19, the prehistoric green shell from Paquime (Casas
Grandes) exhibits the differences in the shape of the perforation.

Figure 17. FT-IR microscopic examination of particles scraped from the
surface of Seri red Specimen 1 from Kino Bay, Sonora. The spectrum
shows peaks for carbonates, bicarbonates, silicates, and phosphates.
Phosphates are seen in the general area of 1050 cm-1 and 2500 cm -1,
while silicates are in the 1100 cm-1 area (weakly). The phosphate and
silicate signals are somewhat superimposed in the 950 to 1150 range.

Fig. 17

Fig. 18

Figure 18. Microscopic examination of particles scraped from the
surface of the Seri blue shell Specimen 2 from Kino Bay, Sonora. The
spectrum shows carbonates, silicates, bicarbonates, and phosphates.
The bicarbonate is seen at 1795 and the companion peak at 1750 is
one of the carbonates. As with Figure 17 the silicates, carbonates and
phosphates overlap, but a discreet broad band for phosphorus is seen
at 2500 cm-1. The scrapings from this and Figure 17 Specimens 1 and
2 are of a material laying on the surface of the shell instead of that
which has bonded into the shell matrix.

Figure 19. Photo of Paquime shell specimen (CG1929,
green) from binocular optical light microscope showing
tool marks along edge of hole. (lOx). Actual size of shell
average 1/4" long by 3/16" round.

Figure 20a. Photo from SEM showing rectangular
shaped perforation in the Seri red specimen. Note
tool marks. (40x).

Figure 20b. SEM photo of the same perforation
seen in Figure 17a. Tool marks are at the base.
Notice the sharp corner in the lower right (15x).

Acknowledgements

The archaeological shells used in this analysis were obtained from the collections at Casas
Grandes (Paquime) with the support and permission of officials from the Instituto Nacional
de Antropologia y Historia, Mexico D.F. The generous help of Dra. Beatriz Braniff, Jose Luis
Perea Gonzalez, Profra. Lorena Mirambell Silva, and Dr. R. Ben Brown is gratefully
acknowledged.

The author is grateful to John E. McLane of the Adolph Coors Company for the SEM/EDS
analysis along with the excellent photomicroscopy and to Doug Allen of Coors Ceramics for
the highly refined, small sample XRD procedures. Both gentlemen contributed significantly to
the interpretation of the data.

The FT-IR research was accomplished by Donna Lewandowska of Coors who's great
patience and rigor made possible excellent spectral results from limited sampling
materials. Her willingness to collaborate contributed in a major refinement of her science as
applied to archaeological objects.

All the preceding donated services were made possible by the generosity of
Swede (Marvin) Johnson, Vice-President of Corporate Affairs, The Adolph Coors Brewing
Co., Golden, Colorado.

My thanks also to David McJunkin of the Isotope Laboratory, Institute of Geophysics at
UCLA for the unpublished Master's thesis of Ms. de Wong's from MIT. It proved useful for
comparative data.

References Cited

Abbott, R. T.
1986    Seashells of North America. Golden Press.

Dana, E. S., and W. E. Ford
1949     A Textbook of Mineralogy, 4th edition. John Wiley and Son, New York.

deWong, O. N.
1977    The Identification of Natural Dyes in Pre-Columbian Andean Textiles by
Mass Spectroscopy. Unpublished Master's Thesis , Massachusetts
Institute of Technology, Cambridge.

DiPeso, C. C.
1974     Casas Grandes,   A Fallen Trading Center of the Gran Chichimeca, Vol. 1.
Northland Press, Flagstaff.

Felger, R. S., and M. B. Moser
1985     People of the Desert and Sea. University of Arizona Press, Tucson
.

actual size
about
1/4" long
X 3/16" round

FOUND AT: www.chriscooksey.demon.co.uk/indigo/index.html

SOME NOTES ON THE PLANT SOURCES
FOR INDIGO DYE IN MESO-AMERICA/USA:
AUTHENTIC INDIGOFERA
and
WOAD

Wikipedia has a good discussion
of
Indigofera tinctoria (true indigo)
[with a great color swatch]
and other species