The origins of mural painting in Ancient Peru: archaeometric preliminary study of the Ventarrón mural paintings, Valle de Lambayeque
Phase 2
Most of the samples taken from Phase 2 had just one paint layer generally relatively
thin (35–50 µm thick approximately, except the white M2 with a paint layer thickness
ranging between 50 and 300 µm) (Fig. 5). The grain is very fine, about 5 µm and homogenous. It was possible to notice that
only one layer of paint was applied in this constructive phase. Only two samples have
two pictorial layers, separated by a 200Â ?m thick plaster layer (Phase 2/M5/Gray,
Phase 2/M2/Yellow) as a result of being repainted.
Fig. 5. Microscopic cross-sections of white, red, yellow M5 samples and black M2 sample from Phase 2
The whole analyses achieved allowed to acquire information about each color composition
(Fig. 4): the white is a mixture of calcite (CaCO
3
) with a greenish clay, illite ((K,H
3
O)(Al,Mg,Fe)
2
(Si,Al)
4
O
10
[(OH)
2
,(H
2
O)]) (Fig. 6) 2]. The red and yellow chromogenic mineral belongs to an iron oxide (Fig. 7)—probably hematite (Fe
2
O
3
) for red and goethite (FeO(OH)) for yellow according to the FTIR results—mixed with
clay like illite. Should be noted the important concentration of calcium in both samples,
registered with XRF analysis. The characterization of the black pigment mixture was
more problematic. Thanks to the XRF and SEM–EDS data we suggest that the chromogen
element is iron mixed with clay of illite type (Figs. 4, 8). It was not possible to determine the structural composition of the iron oxide,
but we propose as a hypothesis the use of magnetite (Fe
3
O
4
). In addition, the SEM–EDS experimentations showed the presence of manganese in low
concentration. Structural complementary analysis could enable to determine if this
element constituted another chromogen component or belonged to the clay mineral environment.
The compositions of the black and the gray layers are similar, so therefore it seems
that they were made with the same materials.
Fig. 6. FTIR spectra performed on the white samples M1 (Phase 2) and M4 (Phase 3). The absorption bands reveal the mix of illite (I) and calcite (C) responsible
of the white tone
Fig. 7. X-ray fluorescence spectra of the red sample M2 and the yellow one M2 (PUCP Laboratory). Iron compound is identified as the chromogen element and characterization
of trace elements
Fig. 8. SEM–EDS analysis of the black paint layer M2. BSE, ×800
Fig. 9. SEM–EDS analysis of the green paint layer M7. BSE, ×350
Finally the XRF analyses have enabled to determine the presence of many trace elements
as minor compounds which belong to metallic inclusions of nickel (Ni), copper (Cu),
zinc (Zn) and arsenic (As) and rare earth element of yttrium (Y) in each sample (Fig. 7).
Phase 3
The results from the three samples taken from this phase, allowed us to demonstrate
that the green and white paint represent just one very thin pictorial layer (from
10 to 30Â ?m thickness), with thin homogenous grain (5Â ?m on average) just as Phase
2 (Fig. 4).
The white pigment is a mixture of calcite and illite. Its elemental and structural
compositions are the same than the white elaborated in Phase 2 (Fig. 6). The elemental analysis revealed that the green mixture is composed of silicates
with an important concentration of sodium, magnesium, aluminum, potassium and iron
(Fig. 9). These results suggested that the green color is obtained with a green earth, a
natural green pigment with low opacity. This postulate was confirmed by the FTIR analysis,
showing presence of illite mixed with green earth, very probably glauconite (Fig. 10) which chemical composition is approximately (K,Na)(Fe
3+
,Al,Mg)
2
(Si,Al)
4
O
10
(OH)
2
(with crystals of calcite, pyrite (FeS
2
) and gypsum (CaSO
4
·2H
2
O) that can be found as impurities) very similar to celadonite (K[(Al,Fe
3+
),(Fe
2+
,Mg)](AlSi
3
,Si
4
)O
10
(OH)
2
) 3].
Fig. 10. FTIR spectrum performed on the green sample M7
Furthermore, similarly to Phase 2, it was possible to demonstrate by XRF analysis
the presence of many trace elements as minor elements which correspond to metallic
inclusions of nickel (Ni), copper (Cu), zinc (Zn) arsenic (As) and rare earth element
of yttrium (Y) (Fig. 4).