Tapping into the Past: First Approach to a Diachronic Material Characterization of Mayapán Pottery

Abstract

The great city of Mayapan has experienced a technological change in pottery making, and our results confirm a shift in the raw materials and possibly the potters’ knowledge about them. The dynamics of change during the Postclassic period in the Maya area are reflected in the material changes used to make pottery. A comprehensive analysis was conducted on a collection of 248 pottery items from the archaeological site of Mayapán in Yucatán, Mexico, dating from the Middle Preclassic to Postclassic periods (700 BC–1500 CE). Non-invasive methods were used for the entire pottery set, including X-ray fluorescence (XRF) and fiber-optic reflectance spectroscopy (FORS). Additionally, for a representative subset, minimally invasive techniques such as inductively coupled plasma optical emission spectrometry (ICP-OES) and laser-induced breakdown spectroscopy (LIBS) were employed. The resulting data enabled the identification of materials used in the pottery’s manufacture. The elemental composition of the objects was determined, revealing correlations between elements such as Si with Al that yield a $R^2$ factor of 0.94. The results indicate the presence of smectite clays, carbonates, and iron oxides. The results show that a higher proportion of carbonates was found in the pieces from the Postclassic period compared to those from the Preclassic period, which may be associated with a change in the manufacturing process. Likewise, the Postclassic pieces are distinguished by a greater contribution of the Mg-OH signal, unlike the Preclassic and Classic, which show a greater contribution of the Al-OH group. The implications for the technological knowledge of the potters suggest the use of different technologies across various periods and material changes driven by shifts in political and economic relations in the city and the northern plains.

1. Introduction

Archaeological pottery in different regions of México has been widely investigated across various scientific disciplines, including archaeology, history, anthropology, and materials science. The study of the Mayapán pottery is significant as it is regarded as one of the last capitals of the Maya civilization, and their study has revealed information on different aspects of the region [1,2,3]. For example, it has shown possible exchange between this city and other sites in Central Mexico and Guatemala [2]. The region is characterized by having material features that distinguish it from those found in different zones of Mexico, determined by its geological history [4,5,6]. Recent work has focused on the analysis of these materials, which could be related to pottery manufacturing. There are remarkable studies on the analysis of processes involving the transformation of these materials using fire [7]. Only a few previous works in the same region have been carried out involving the material characterization of ceramic objects. For example, Pérez et al. [8] conducted and analyzed censer fragments using various spectroscopic and imaging techniques.

In previous similar studies, from different regions of the world, a similar combination of spectroscopic analytical techniques has been effectively used to characterize pottery objects from archaeological contexts [9,10,11,12]. In particular, XRF has been valuable for non-invasive analysis, enabling the identification of major, minor, and trace elements in these objects [13]. LIBS has also been employed in recent studies, allowing for the elemental characterization of pottery samples [14]. However, its application in pieces from Mexico, especially those from the Yucatán Peninsula, remains limited. Lastly, ICP-OES is less commonly used in methodologies for cultural heritage due to its invasive nature. Still, it has the potential to yield accurate quantitative results, as shown in some works [15,16].

The present work aims to systematically characterize the elemental and mineral composition of the pastes of pottery objects from the Mayapán region, focusing on the changes that this composition presents during different periods, from the Middle Preclassic to the Postclassic (700 BC–1500 CE). This is relevant because the chemical composition may reveal manufacturing-related information, such as the provenance of raw materials and processing, as well as the techniques used for object shaping. Then, a diachronic record of material features can provide insights into changes related to social, cultural, and technological backgrounds. Regarding the materials used in pottery objects from the Yucatán Peninsula in México, it is known that they have characteristics closely linked to their manufacturing technology and intended purposes [2,17]. For instance, the region is rich in resources traditionally used for pottery production, even today [4,18], and contemporary pottery work showcases the utilization of local materials in creating a diverse range of objects. Previous research has identified differences and relationships between these contemporary objects and the region’s raw materials [7,19].

2. Materials and Methods

2.1. Materials

The complete collection of analyzed objects included 248 pottery items (191 sherds and 57 objects) from the region of Mayapán. Pieces of the ensemble were previously classified under the type-variety system (See

Table A1. List of pottery items analyzed by XRF and FORS.

Item ID	Count	Temporality	Type-Variety (Figure 11)	XRF Analyzed Objects	FORS Analyzed Objects	Analyzed Pigments
1–76	76	Middle Preclassic (700–450 BC)	Chunhinta–1 Kin–2 Loche bicromo–3	15	74	Orange Yellow Black White Beige Paste
79–118	40	Late Preclassic (450 BC–250 CE)	Grupo Sierra–1	6	39	Orange Yellow Paste
119–169	51	Early Classic (250–600 CE)	NA	25	31	Orange Yellow White Paste
170–227	58	Late Classic (600–750 CE)	Chablekal–1 Chuburna–6 Kinich–1 Teabo–2 Ticul–1 Yokat–1	36	49	Orange Yellow Ocre Green Black White Paste
228–237	10	Terminal Classic (750–1100 CE)	NA	7	10	Orange Yellow Ocre Black Paste
238–248	11	Postclassic (1050/1100–1543 CE)	Tecoh–1	8	11	Orange Yellow Ocre Green Black White Beige
Total	248		20	97	214	Paste

for representative pieces) based on the concept of Wheat et al. [20], the work of Smith et al. [21,22], and the research of Peraza et al. [23]. The objects include those with slip surfaces, some of which are decorated with features such as incisions, spots, bichromy, polychromy, and streaks (see the work of Nagaya et al. [24] for some details on these features), and span through the different representative periods of Middle Preclassic, Late Preclassic, Early Classic, Late Terminal Classic, and Postclassic (700 BC–1500 CE).

Because of the invasive characteristics of the ICP-OES and LIBS analysis methods, a small subset of 11 sherds was chosen (Figure 1); the selection of the pieces was made by considering representative characteristics from each period.

2.2. Methods

The pottery items were characterized using a combination of spectroscopic analytical techniques. The methods included LIBS, ICP-OES, and XRF, which were employed to determine the elemental composition of the sherds and objects [25,26,27]. Additionally, FORS was used to provide a basic mineral identification, based on specific absorptions of the functional groups in the chemical structure of the compounds.

2.2.1. Laser Induced Breakdown Spectroscopy (LIBS)

Laser-induced breakdown Spectroscopy (LIBS) measurements were conducted using a laboratory system that employs a neodymium-doped yttrium aluminum garnet (Nd/YAG) laser (model Brilliant EaZy, from Quantel, Les Ulis, France). This laser emits a fundamental wavelength of 1064 nm and was operated at a frequency of 1 Hz. The laser beam was focused on the sample analysis spot using a 10 cm focal length lens. The energy output was set to 50 mJ, corresponding to a fluence of 375 J/cm². The light emitted from the sample was collected using a collimating lens connected to a 400 µm optical fiber, which directed the light to an Echelle Spectrograph (Aryelle 200, LTB Lasertechnik, Berlin Germany). This spectrograph has a spectral resolving power of 9000 and operates within the 210 nm to 850 nm wavelength range. The resulting spectra were captured by an intensified charge-coupled device (ICCD) camera (Istar 334-18F-03, Andor, Belfast, United Kingdom), with a delay gate of 1450 ns and a gate width of 50 ns.

Lines in LIBS spectra are generated by the atomic emission of elements in plasma created through laser ablation. Various factors, including the concentration of the element, the system’s sensitivity, and the sample’s density, influence the intensity of these lines. Following these effects, emission intensity is directly proportional to elemental concentration, enabling semi-quantitative elemental analysis. When selecting lines for the spectra, care was taken to exclude interferences from other lines, saturation, and auto-absorptions. Signal-to-noise ratio values were estimated assuming a normal distribution for the signal and noise. To prevent saturation of elemental peaks, relative intensities were determined for the lines associated with the same element throughout the spectrum. For details on this procedure, system specifications, and line selection, refer to the work of Sobral et al. [28].

Measurements with LIBS were only performed on the subset shown in Figure 1. For each item, the following zones were measured: front, back, and exposed paste (on a broken side of the pottery). In each area, five spots were taken, separated by 1 mm in line, and in each spot, 100 consecutive shots were fired. For data processing, the spectra were normalized and then averaged across the five spots to obtain 100 spectra representative of different depths. Spectra from 1 to 100 made in the same spot were used to perform an elemental characterization at different depths corresponding to each shot.

2.2.2. Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)

Inductively coupled plasma analysis was performed using the commercial instrument Optima 4300 DV (Perkin Elmer, Waltham, MA, USA). This instrument features an optical emission spectrometry (OES) system that provides two acquisition modes: axial and radial. For each sample, 100 mg was extracted and finely ground. The digestion process involved mixing each sample with 8 mL of nitric acid (HNO₃) and 2 mL of hydrofluoric acid (HF). This digestion was carried out in a Milestone Microwave mega MLS 1200 system with six Teflon reactors (Gemini, New York, NY, USA) at 210 °C for 30 min. After digestion, the samples were diluted to get a final volume of 100 mL with distilled water. Concentration calibration was performed using instrument calibration standards with concentrations ranging from 0.5 ppm to 200 ppm.

These standards enabled the search and quantification of 24 elements in the subset of 11 representative pieces, which are indicated in Figure 1. The method was validated with NIST standard reference materials 2709a San Joaquin soil, 2706 New Jersey soil, 2710a Montana soil, and 1d limestone, which have comparable matrix composition. Reference materials were digested and analyzed simultaneously with the micro-samples from the sherds to estimate the recovery rates. The measuring system is optimized to enhance the signal-to-noise ratio by utilizing a 257.61 nm Mn emission line, which allows a precise alignment of the torch with the optics, ensuring optimal argon flux and sample gas mixture flux.

2.2.3. X-Ray Fluorescence Spectroscopy (XRF)

A home-built portable X-ray system (SANDRA) was employed to describe the elemental composition of the studied pottery items. This system, along with some initial applications, is described in more detail in the reference [29]. In brief, the system is based on a Mo X-ray source collimated to a 1 mm diameter spot, along with an SDD detector (Amptek, Bedford, MA, USA), forming a geometry of 45° irradiation and a detection angle of 45°. The system’s energy is calibrated using a NIST multicomponent glass standard reference material (SRM 1412). Certified reference materials from NIST (SRM 1d limestone, SRM 88b dolomitic limestone, SRM 2709a San Joaquin soil, and SRM 2706 New Jersey soil) were used to generate a sensitivity curve, allowing us to utilize this information as a semi-quantitative method (empirical calibration as described in the reference [19]). Acquisition parameters were fixed during the entire experiment as 45 kV, 0.200 mA, and a recording time of 90 s, with no filters. Spectra analysis, deconvolution, and fitting were achieved by using the ESRF-BLISS PyMca analysis toolkit [30]. XRF measurements were performed on a subset of 98 samples, 47 sherds, and 51 objects. In each item, four analysis points were selected in representative regions. When possible, the analysis was carried out in areas of the exposed paste. The list of these objects is indicated in Appendix A, Table A1.

2.2.4. Fiber Optic Reflectance Spectroscopy (FORS) and Colorimetry

A standard FieldSpect-4 ASD acquisition system was employed to generate FORS spectral measurements, which provided spectra in the ultraviolet (UV), visible (Vis), near-infrared (NIR), and short-wave infrared (SWIR) wavelength ranges—delivering two different spectral resolutions: 3 nm for the 300–1000 nm range (Si-CCD) and 10 nm for the 1000–2500 nm range (InGaAs). The analysis area can be adjusted between 0.1 cm² and 1 cm², with the probe in contact with the surface of the studied object during data acquisition.

The system uses a calibrated halogen illuminant (A type), and the acquisition time for each spectrum can be adjusted, depending on the analysis area, typically between 0.3 s and 10 s intervals—an average of 25 repeated measurements in each analysis zone was employed to produce reduced noise spectra. Normalization was performed using a certified reflectance standard, Labsphere, with 99% reflectance from ASD Inc., Malvern, United Kingdom (CSTM-SRM-990-362). Additionally, Labsphere reflectance standards were used for wavelength calibration verification (WCS-DO-010, WCS-EO-010, and WCS-HO-010).

The spectra were preprocessed by performing a baseline convex hull correction, and no smoothing was performed. A principal component analysis was performed using the covariance matrix to calculate the statistics and the transformation matrix. The eigenvectors associated with the specific absorptions in the original spectra were included.

The FieldSpect-4 ASD system enables colorimetry measurements by processing the spectra with the ViewSpec Pro v6.20 analysis software, with the results expressed in the CIE XYZ color space. For the color calculations, the CIE D65 was used as the standard illuminant, and the CIE 1931 2° as the standard observer. Subsequently, a conversion of the color values was carried out to the CIE Lab* space for a more direct and efficient interpretation of the results. In this system, the axes represent opposite colors: on the horizontal axis, red and green, while on the vertical axis, yellow and blue. In the diagram, each point corresponds to a calculated color value for each FORS spectrum. Color calibration verification was performed using a set of Labsphere color standards SCS-BL-020, SCS-RD-020, SCS-YW-020, and SCS-GN-020.

FORS analysis was performed on a larger subset comprising 192 pottery items, 155 sherds, and 37 objects; the average number of measurements per piece was 3 in representative areas of the surface, including exposed areas of the paste when possible.

3. Results

Results are presented grouped by the technique that produced them, and indicate the analyzed region in the pottery.

Table 1. Summary of results.

Studied Region	Technique	Result Presented in (Figure, Table)
Paste	ICP	Figures 2 and 5, Table A2
	LIBS	Figures 2 and 3
	XRF	Figures 5–9, 11 and 12
	FORS	Figures 13–15
Surface	LIBS	Figure 4
	XRF	Figure 8
	FORS	Figures 13 and 14
Pigments	LIBS	Figure 4
	XRF	Figures 8 and 9

summarizes the information presented in this section.

3.1. LIBS and ICP-OES

3.1.1. Analysis of Pastes

LIBS enabled semiquantitative and minimum-invasive elemental analysis of the subset mentioned above, as well as the comparison of elements present in the objects. Main advantage over XRF is the identification and comparison of light elements such as Li, Na, and Mg. Only a subset of pottery items was selected as described previously. Elements identified by LIBS include Li, C, Na, Mg, Al, Si, K, Ca, Ti, Cr, Mn, Fe, Ni, Rb, Sr, and Ba. Employing ICP-OES, Li, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Zn, Sr, and Ba were detected in the pastes. Low amounts of Cu, 3.05 ppm and 2.35 ppm, respectively, were detected in sample number 131 (Early Classic) and sample 236 (Terminal Classic). Likewise, the amount of 73.23 ppm of Pb was detected in sample number 186 (Late Classic). The table containing the values for each pottery item is included in the Appendix A(see

Table A2. Elemental concentrations determined by ICP-OES. The lines used for identification are indicated.

Sherd/Sample	18	54	90	97	112	131	180	186	231	236	238
Temporality	Middle Preclassic	Middle Preclassic	Late Preclassic	Late Preclassic	Late Preclassic	Early Classic	Late Classic	Late Classic	Terminal Classic	Terminal Classic	Postclassic
Al 396.153 nm (%)	1.63	1.61	1.57	0.76	0.79	0.11	1.97	0.94	1.56	1.71	0.26
Ba 233.527 nm (ppm)	52.83	45.91	69.02	17.52	28.77	19.59	56.64	35.54	67.32	63.91	6.48 *
Ca 317.933 nm (%)	10.63	10.45	7.396	16.37	15.28	20.55	4.461	12.08	1.67	1.92	11.54
Cr 267.716 nm (ppm)	19.38	11.25	14.55	20.19	40.45	5.32	23.63	31.54	14.25	14.6	<LOD
Cu 327.393 nm (ppm)	<LOD	<LOD	<LOD	<LOD	<LOD	3.05	<LOD	<LOD	<LOD	2.35	<LOD
Fe 238.204 nm (%)	0.48	0.40	0.50	0.45	0.34	0.07	0.72	0.62	0.66	0.56	0.06
K 766.490 nm (%)	1.38	1.40	0.75	0.90	0.80	0.25	0.93	0.45	0.95	0.95	0.44
Li 670.784 nm (ppm)	0.93	0.56 *	0.78 *	0.28 *	0.55 *	<LOD	0.90	1.03	1.15	1.21	<LOD
Mg 285.213 nm (%)	0.96	0.83	0.62	0.61	0.74	0.37	0.31	0.32	0.35	0.24	4.65
Mn 257.610 nm (ppm)	30.93	30.88	25.11	78.56	12.92	19.85	201.33	59.70	82.48	80.89	11.27
Ni 231.604 nm (ppm)	10.22	9.08	8.47	10.57	10.70	4.42 *	14.28	12.94	14.46	9.76	5.36
Pb 220.353 nm (ppm)	<LOD	<LOD	<LOD	<LOD	<LOD	<LOD	<LOD	73.23	<LOD	<LOD	<LOD
Si 251.611 nm (%)	19.54	19.93	18.12	11.52	11.98	4.43	20.18	10.39	22.60	21.07	3.95
Sr 407.771 nm (ppm)	23.46	17.59	92.82	11.30	33.75	45.64	16.03	20.53	17.73	14.50	25.21
Ti 334.940 nm (ppm)	443.43	470.50	400.79	346.30	380.49	98.20	568.59	588.77	535.94	438.74	83.05
V 290.880 nm (ppm)	70.00	80.00	40.00	70.00	70.00	10.00	80.00	70.00	70.00	60.00	<LOD
Zn 206.200 nm (ppm)	18.92	18.38	11.96	8.45	7.89	3.39	20.46	7.73	13.51	12.14	7.81

* Values over LOD and under LOQ.

). The presence of Al, Si, Mg, Fe, and Ca may be related to the presence of aluminosilicates [9,10]. These elements and K may also be associated with the possible presence of micas and K-feldspars, some of which have been reported in the region [4,31,32,33]. Silicon may also be present in quartz minerals, and the presence of this has been reported as detritus in clays in the region [33]. Fe and Ti could be present in the form of oxides, which have been detected in regional samples [4].

Some of the detected elements may be present because they are deposited within the structure of the clays; for example, this has been reported for Li and Ba. Sr may be present, replacing Ca in carbonates, and Rb may be related to K substitutions in feldspars, as they belong to the same atomic groups II A and I A, respectively. Calcium is primarily associated with the presence of carbonates and limestone [34], substances abundant in the region that can serve as both a degreaser and flux in this type of pottery.

These two elemental techniques were employed to characterize and quantify the elements present in the sherds shown in Figure 1. ICP and LIBS results are compared as shown in Figure 2, for Ca (line at 634.91 nm for LIBS and 317.93 nm for ICP-OES) and Si (line at 390.55 nm for LIBS and 251.61 nm for ICP-OES), elements in high concentrations in the subset pottery items. The error bars in the LIBS measurements represent the standard deviation of measurements taken from different spots within each sherd, averaged to calculate the overall mean. The error bars in ICP correspond to the uncertainty of the method used, considering the systems and the calibration process.

The comparison of results from ICP and LIBS for detected elements in the pottery items shows consistency between the two techniques. The results obtained by LIBS enabled correlations between some of the identified elements, which in turn could be used to indicate the presence of specific compounds in the pottery (Figure 3).

From the LIBS analysis of the pastes of the pottery subset, direct correlations were found between some of the identified elements. For example, Si with Al yields an $R^2$ factor of 0.94. This may be related to the presence of principal compounds, such as aluminosilicates. A correlation of Fe with Al was found with an $R^2$ factor of 0.94, and Ti with Al with an $R^2$ factor of 0.99. This may be associated with the presence of substitutions within the clay’s structure or the presence of Fe or Ti oxides in the clay sources used. Rb vs. K has an $R^2$ factor of 0.9, which supports the idea of elemental substitutions in feldspars or micas present in raw materials. Ba vs. Al with an $R^2$ of 0.89; Li vs. Al correlates to 0.8 in $R^2$, indicating the presence of minerals deposited within the structure of clays. In contrast, Ca is inversely correlated with Al, with a Pearson correlation coefficient of −0.92 and an $R^2$ of 0.82. This indicates that these elements are principally present in distinct compounds and are added as different ingredients. This does not reject the presence of Ca compounds mixed with clay, as reported in previous works with raw materials, but it suggests the use of a raw material with a high proportion of Ca.

3.1.2. LIBS Analysis of Pottery Surface

The use of LIBS allowed for an elemental depth profile analysis, measuring the element signal at a fixed point for 100 shots. As we estimated in a previous work [19], the depth of the analyzed region is about 100 µm, with an uncertainty of around 20%. This method enabled us to track changes in relative concentrations at different depths from the surface. Figure 4 shows two examples for sherd 238 (Postclassic period) and sherd 54. The results indicate the existence of a finishing layer where the increased concentration of some elements was detected. In sherd 238, a relatively higher concentration of Fe and Ti was detected on the surface, followed by a gradual decrease. In sherd 54, a higher concentration of Al, Fe, Ti, and Si was detected in the first shots, followed by a similar gradual decrease. In contrast, a lower signal was observed in Ca near the surface, followed by an increment.

3.2. XRF

3.2.1. Analysis of Pastes

XRF analysis allowed the noninvasive identification of elements; also, a larger number of objects were analyzed: 98 pottery items (47 sherds and 51 objects). The elements identified include Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Zn, Rb, Sr, and Zr. As mentioned earlier, this technique cannot detect light elements or elements with low concentrations, so information on the other elemental methods used is essential for validation and characterization.

We compared the average results obtained from XRF with those obtained from ICP-OES for the subset of pottery items in pastes (see Figure 5). The comparison shows a close correlation between the two techniques, as shown in the previous work of Perez et al. [19], where the consistency between the elementary techniques used was evaluated. While some differences are present, they primarily occur with elements in low concentrations, specifically those below 0.1%. These discrepancies can be attributed to the heterogeneity of the pastes used and the varying measurement regions.

The measurements corresponding to pastes allowed for an unbiased analysis of the pottery items, facilitating a comparison without interference from any potential additional surface materials.

Ternary diagrams featuring the Si, Ca, and Fe elements were created to provide insights into the pastes’ diachrony (Figure 6). These diagrams were derived from the average values of XRF measurements of the exposed paste areas in each object. In this way, each point represents an object of the analyzed set, color-grouped by its corresponding period. A scaling was performed by normalizing the standard deviation per element to 1 of the complete datasets; later, the ternary diagram was elaborated with the elements Si, Ca, and Fe. They were separated by period and presented in two graphs for greater clarity. The scales are equivalent, and the data are comparable. This analysis was conducted using these elements, which are primarily linked to compounds such as aluminosilicates, calcium carbonate, and iron oxides. We have found that items labeled with the numbers 243 from the Postclassic period, and 156 and 222 from the Classic period, show significantly different concentrations compared to the other pieces. It can be observed that artifacts associated with the Preclassic and Postclassic periods tend to form independent clusters, but are still contained within data corresponding with the Classic periods. A possible trend is observed, where Preclassic objects are grouped in an area with a lower proportion of Ca, while Postclassic objects tend to be found in a zone with a higher Ca proportion. Considering the correlation found with LIBS between Si and Al for the subset, their relative quotients in the pastes to Ca were compared.

The ratios between Si and Al, corresponding to 1.2 for the Preclassic period, 1.8 for the Classic, and 1.25 for the Postclassic, indicate a change in the proportions of these elements, suggesting a variation in the type of aluminosilicates employed (see Figure 7).

3.2.2. Analysis of Color Regions from Classic Epoch Objects

Elemental analysis of the Classic epoch was applied to red and orange color regions and compared with the corresponding pastes to enhance elemental differences (see Figure 8). Measurements from 119 pottery items from the Classic period were analyzed, as these represent the most numerous artifacts. The analysis focused on comparing the paste and the red and orange areas on the surface.

The comparison between the red regions and the pastes reveals some essential differences. Principally, the paste areas exhibit a higher concentration of Ca. Additionally, the red and orange areas indicate a higher concentration of iron (Fe) compared to pastes, which could be associated with iron oxides that function as red chromophores. Al, Si, and the K distribution are similar in the paste, the red, and the orange, indicating that they are not primarily related to these color compounds.

The overall measurements in the black areas indicate almost no increase in Mn or Fe relative to pastes, suggesting that an inorganic black chromophore is not being used. Only a few specific points in these black areas show a significantly higher concentration of Mn (see Figure 9). This analysis helped identify outlier objects with concentrations that differ considerably from the rest of the analyzed samples. The concentration of Mn is similar in the zones; however, three pieces have a higher concentration of Mn: pieces 199, 174, and 225 from the Classic period. This could be related to the presence of Mn oxides in zones recognized as parts of the decorations (see Figure 10).

One notable finding from this analysis was the identification of specific objects containing unique elements. In particular, the object 200 exhibited a signal associated with Hg in a red region of the object, related to the presence of cinnabar.

3.2.3. Analysis of Archaeological Paste Varieties

The set of objects included some that were associated with different varieties. For the pieces where the exposed paste areas were measured, the concentrations of trace elements such as Zr, Sr, and Rb were normalized to Ca to characterize them. These relationships may be linked to the origin of the raw materials (see Figure 11).

Rb and Sr may be related to substitutions for K and Ca, respectively. This relationship can arise due to their similar valence shells, as they belong to the same groups in the periodic table. The proportions of these trace elements can serve as indicators of common sources. For instance, a correlation is observed within the Chuburna-type objects. The Rb/Ca ratio is similar across most pieces, except for one from the Chablekal type (archaeologically classified as an import), which exhibits a relatively higher value. Overall, this analysis allowed us to identify consistency among certain sets, correlated groups, and individual pieces.

3.2.4. Characterization of Radiographic Classification Types of Pastes

A complementary approach for characterizing the various types of pastes of the sherds was previously achieved by using digital radiography, as reported in the work of Nagaya et al. [24]. Here we present the relation of those results with the average composition and distribution of paste types for each period analyzed in this work, as presented in Figure 12a.

Violin diagrams illustrate the distribution of radiographic analysis results for paste types across the pottery items studied, showing that the Classic periods have a greater variety of pastes. In comparison, the Postclassic pieces exhibit only two kinds. Notably, the paste type with the highest number of samples is type 1, which has similar elemental proportions of Al, Si, and Ca. In contrast, paste type 6 shows a smaller number of samples, representing the Early Preclassic, Early Classic, Late Classic, and Terminal Classic periods. This type is distinguished by a higher proportion of Ca compared to the other pastes (Figure 12b).

The pastes can be categorized into two groups based on the Zr/Sr ratio. Pastes 1, 2, and 4 have a Zr/Sr ratio greater than 1, while pastes 3, 5, and 6 have a ratio below 1 (Figure 12c). This distinction is particularly significant for sets with similar overall compositions, like pastes 1 and 2, which share relatively comparable proportions of Al, Ca, Ti, and Fe. However, their trace elements differ considerably, potentially indicating the different sources of materials used.

The PCA (see Figure 12d), with a cumulative explained variance of 80.1%, indicates that paste types 1 and 2 have similar compositions, lying between calcareous and non-calcareous compounds. Set 5 has a composition comparable to types 1 and 2 but with a higher proportion of Sr and a lower proportion of P. Pastes 3 and 6 exhibit a higher proportion of Ca. In contrast, paste 4 has the lowest proportion of Ca. This suggests that while some pastes have distinct compositions, there are others with similar profiles, where the primary differences can be attributed to their microstructure, as described in previous works. It is essential to note that this analysis is still limited by the current dataset available.

3.3. FORS

3.3.1. Surfaces Reflectance Analysis

The FORS spectra were analyzed separately in the visible and infrared regions. The main characteristics identified in the visible region correspond to points of maximum reflectance, maximum absorbance, and inflection points. As described in previous studies, these features allow us to identify specific compounds. Reflectance spectra from the red regions show characteristics associated with iron oxide-based materials (Figure 13a). The main features of this spectrum include inflection points around 580 nm and absorption bands near 870 nm, which may indicate the presence of hematite. In the orange regions (Figure 13b), the spectra exhibit inflection points at 440 and 550 nm, a maximum reflectance at approximately 820 nm, and an absorption peak at 890 nm. These characteristics are primarily associated with the presence of goethite, and they are discussed in different references [35,36]. Iron oxides subjected to heating undergo a dehydration process, which produces a shift in the absorption bands in the visible region [37,38]. Considering this, the current red and orange hues could have been the result of changes due to the burning process during the manufacture of the parts.

Colorimetric analysis shows variations in the samples’ colors, emphasizing the predominant use of materials in orange and yellow tones (see Appendix B Figure A1). Previous studies indicate that the primary chromophores in the region’s raw materials are based on iron oxides, particularly those associated with goethite and hematite compounds [35,39,40]. The colors of these minerals relate to the color distribution observed in most of the analyzed objects. Furthermore, the areas with orange hues show a wider range of saturation levels than those with yellow hues. This could be due to mixtures of materials present in raw components or added during the manufacturing process. Notable areas with green and blue hues may be related to the presence of pigments. Changes in saturation might also be linked to the presence of white materials in the analyzed region.

3.3.2. SWIR Spectral Region Reflectance Analysis of Pastes

The SWIR region of the spectrum (1000 nm–2500 nm) enabled the identification of compounds by their characteristic absorption bands (see Figure 14). This spectral interval was used to characterize diverse mineral groups. Absorption bands at approximately 1420 nm, 1925 nm, and 2200 nm are associated with smectite minerals [38,41,42,43]. These features are primarily observed in Preclassic and Classic period spectra. In contrast, representative spectra from the Postclassic period exhibit additional absorption features around 2200 nm, 2260 nm, and 2300 nm, which are related to Al-OH, Fe-OH, and Mg-OH containing groups.

Principal component analysis was employed to classify the spectra, highlighting principal features within a limited spectral interval to minimize interference from regions with redundant information and noise (see Figure 15). The cumulative explained variance of the PCA analysis for PC1 and PC2 was 74.5%. Instead of the large number of loadings, the eigenvectors associated with the specific absorptions in the original spectra were included. Such a transformation considers each of the discrete wavelengths of the spectrum as the original variables (vectors), and then the new principal components are obtained as a linear combination of these. The calculation used a correlation matrix, treating the wavelength values as individual variables. The wavelength range selected was from 2130 nm to 2400 nm, as it includes features associated with the Al-OH (2200 nm), Fe-OH (2260 nm), and Mg-OH (2300 nm) groups [44]. Each point in the diagram corresponds to the average paste spectrum measured in each sample. This approach helped us identify differences in the sets of pieces linked to the Postclassic period, highlighting their unique characteristics compared to other periods.

Figure 15 shows that the spectra from the Preclassic and Classic epochs are more strongly oriented toward the 2200 nm vector. In contrast, the spectra of the Postclassic pieces align along the 2300 nm vector. This suggests that Postclassic pieces have a higher contribution from the Mg-OH group. In contrast, Preclassic and Classic pieces are predominantly aligned with the contributions of the Al-OH and Fe-OH groups.

4. Discussion

Finally, the joint results of the techniques indicate the possible presence of smectites, which can be considered part of the primary raw material. The three elementary techniques agree with the detection of the main elements of these compounds (Si, Al, K, Ca, Mg, Fe). Additionally, through FORS, characteristic absorptions were detected, supporting the presence of smectites, such as montmorillonite, with diagnostic absorptions at 1420 nm, 1925 nm, and 2200 nm. Observable absorptions were found at 2260 nm and 2300 nm, which may be related to features in the materials or manufacturing processes. Montmorillonite is the mineral reported in nearby regions, which agrees with the results [45]. The presence of calcium carbonates characterizes the geology of the Yucatán Peninsula, which is rich in limestone ranging in age from the Cretaceous. The clay minerals from the region originate from pedogenic processes, detrital sedimentation, and direct crystallization [4]. Additionally, the results of the work of Schultz et al. are consistent with the presence of smectites and iron oxides in sources close to the region. Observed, also in the work of Meanwell et al. [3].

The detected Ba may be related to minerals deposited in the clays, which have also been identified in the region, and their deposition has been associated with water sources [46]. The detected Ca may be associated with calcite, which has been distinguished as a principal compound in materials and in conjunction with some clays in the region. Employing FORS, the characteristic absorption at 2340 nm corresponding to the CO₃ group supports the presence of Ca carbonates. The inverse correlation between Al and Ca indicates the use of a Ca-rich compound; for example, it is reported that the use of Ca-rich material, such as Sascab, in the manufacture of traditional pottery is an existing practice. The detected Ti and Fe may be related to oxides that function as chromophores, and in some cases as pigments in pottery. Employing FORS, absorptions of iron oxides were detected in the visible region, confirming the presence of hematite and goethite-type compounds. The presence of these materials in neighboring areas has been reported in previous works [5].

The main changes identified are related to the possible mixtures used in the different periods. Preclassic pieces present, on average, mixtures with a lower contribution of carbonates. Later, the Classic period presents a greater variety of mixtures, represented by their distribution in compositions, and finally, the Postclassic period presents pieces with a greater contribution of calcium carbonates. This may be related to an evolution in their manufacturing processes, since calcium carbonate influences the process, for example, by acting as a flux [38,47].

In addition, the FORS results indicate a possible change in the mixture used. The spectra of the pieces from the Preclassic and Classic epochs exhibit signals with a greater contribution from the Al-OH and Fe-OH groups. In contrast, the Postclassic pieces display signals with a greater contribution from the Mg-OH group. This may be related to a change in the sources of clay used for these objects, which may have different explanations.

In the work of Perez et al. [19], the elemental and mineral composition of contemporary ceramics from the Uayma region, produced over the course of a year using traditional methods, was analyzed with the same analytical procedure as in this study. It was found that the mixtures also contain a high proportion of calcium carbonates and smectite-type clays, similar to the archaeological pottery objects. Different material mixtures were identified in relation to the raw materials used.

5. Conclusions

This research provides a diachronic study of the materials present in pottery items from the archaeological zone of Mayapán, spanning a time interval from the Preclassic to the Postclassic periods (700 BC–1500 CE). The compounds are consistent with the reported raw materials available in the Yucatán Peninsula region of México.

The analysis of samples from the Postclassic period revealed distinct substance characteristics that differentiate them from samples from other periods, particularly regarding the proportions of principal identified elements Ca, Al, Si, and Mg. Molecular analysis provided by FORS complemented the elemental characterization by describing at a first glance the compounds present in the studied pottery. Furthermore, LIBS enabled a depth profile analysis, exhibiting elemental concentration changes on the surface of Postclassic period sherds.

The main change observed in the manufacturing techniques across different periods is that in the Preclassic period, the composition of the pieces is more specific. In the Classic period, there is greater variation, and it seems that in the Postclassic period, the composition is more specific again.

This study shows that the great city of Mayapan experienced a technological shift in pottery production. Our findings suggest a change in the use of raw materials and possibly an alteration in the potters’ understanding of them. The changes observed during the Postclassic period in the Maya region are reflected in the material modifications used to create pottery. The implications for potters’ technological knowledge suggest different technologies were used during various periods, with material changes driven by shifts in political and economic relations in the city and northern plains.