The ceramic vessel analyzed in this study was discovered during archaeological excavations conducted by Abel Viana in 1931, at the site then referred to as Cova da Moura (Viana, 1932a, 1932b, 1932c, 1932d, 1932e, 1932f, 1932g, 1932h, 1932i, 1943, 1955). The site is in the parish of Carreço, within the municipality and district of Viana do Castelo, Northwest of Portugal. It lies on a platform at an average altitude of 186 meters a.s.l., halfway up to the western slope of the Santa Luzia Mountain overlooking the Atlantic coastal platform (Oliveira, 2024: 854-913) (Fig. 1).

The geological substrate of the site consists of fine-grained porphyroid granite, which is abundantly exposed, particularly to the north-northeast and south-southwest area of the monument. Less than 1 kilometer to the northeast, there is an occurrence of Andalusian schist (Teixeira & Medeiros, 1972) (Fig. 1).

The monument consists of a large artificial mound measuring approximately 45 meters in length (north-south), 28 meters in width (east-west), and up to 11 meters in maximum height. It was constructed by depositing sediments over various granite outcrops that feature natural shelters and tafoni (small natural cavities), and was covered with a lithic armor, which remains particularly prominent on the western side (Viana, 1943; 1955: 487-488; Bettencourt, 2013: 165; Oliveira, 2024: 861-867).

There were two walls inside the monument, at least in its northern half. The western wall, constructed from unworked granite blocks, was double-layered and designed to contain the sediment on that side. However, it left exposed the entrance to a granite cavity, which Viana (1955: 486) describes as being “(...) very irregular, decreasing in height from the entrance to the bottom and from the North side to the south, but with enough space to fit a dozen crouching people” (Fig. 2).

On the opposite side, there was a wall measuring over 4 meters in length, which was heavily damaged during the excavation. This wall was constructed from granite blocks approximately 30 to 40 centimeters in length, arranged without fitting and without the use of mortar. Its original height could not be determined.

These walls appear to have been constructed in at least two distinct phases (Viana, 1955: 487-488), due to the differing construction techniques employed, although the phases may not have been far apart in time. Notably, the walls do not extend to the top of the monument, raising several questions. Could these walls have delimited a corridor that may have been covered, or might the space have remained open during a certain period of use, serving solely as a circulation passage for depositing materials in the side cavity or central ground-level cavities? Were the sediments visible on the walls contemporary with the walls’ construction, or were they deposited later, during the monument’s closure?

According to Viana, the shelters and small cavities within the structure may have been used for cremation deposits, sometimes accompanied by lithic, ceramic, or metallic artifacts, as well as eight mammalian bones and teeth that were also recovered.

He noted that “remains of ashes” were deposited in the central part of the monument, within the shelters, and beneath other excavated layers. He further elaborated: “The fragments of ceramic vessels that were  discovered suggest that part of the ashes were enclosed in funerary urns, or more likely in simple pieces of pottery (...) It is to be believed that these containers were placed under the slabs, where they were, without any arrangement that would favor their preservation, as a result of which everything was crushed under the formidable weight of the roof” (Viana, 1955: 489).

The pot under analysis was found near the center of the monument, close to the entrance of a granite cavity. In reference to this context, Viana noted: “Only in the layer discovered in the center, in which the copper bead and snare were found, was there a larger number of shards, especially the one that corresponds to the bottom of a vessel; but equally small and scarce are the fragments that can be attributed to the bowl, and the rim only a very limited portion” (Viana, 1955: 489) (original in Portuguese).

Although Bettencourt suggested that this monument was constructed during the Late Bronze Age, based on the discovery of a Rocannes-type sickle (Bettencourt, 2013: 165), a recent review of archaeological materials stored at the Geological Museum of Portugal in Lisbon (Oliveira, 2024: 873-904), alongside with radiocarbon dating obtained from organic matter found in the ceramic container studied in this paper (see below), indicates an initial construction phase at the end of the Middle Bronze Age. Nevertheless, later reconstructions cannot be ruled out, as the site was used over an extended diachronic period, including the Late Bronze Age and the Early and Late Iron Ages, spanning the 1st millennium BCE (Oliveira, 2024: 905).

The ceramic was handled with nitrile gloves and stored in aluminum foil to minimize the risk of contamination. Powdered samples were collected from the inside bottom of the vessel, where organic residues had adhered, and analyzed using gas chromatography-mass spectrometry (GC-MS). The analysis, conducted at the HERCULES Laboratory of the University of Évora, successfully identified the organic residues present. Special care was taken on the detection of contaminants, being the chromatograms examined for common organic contaminants, such as squalene, a compound from skin oils indicating ungloved handling of the ceramic material, personal cosmetic products typically linked to ungloved handling, and plasticizers.

4.1. Sample preparation and analysis

The samples were collected using a clean scalpel with sterilized blades and finely ground with an agate mortar and pestle. To this, 7 mL of a chloroform: methanol mixture (2:1 v/v) was added. The mixtures were vortexed, sonicated for 20 minutes, and centrifuged at 2500 rpm for 15 minutes. The supernatants were carefully transferred using clean glass Pasteur pipettes, and the extraction process was repeated. The supernatants from each sample were combined and dried at 40° C under a gentle stream of nitrogen. The dried extracts were then suspended in n-hexane and derivatized with N,O-bis (trimethylsilyl) trifluoroacetamide containing 1% trimethylchlorosilane (BSTFA + 1% TMCS) in a microwave oven (700 W, 30 seconds). After excess derivatizing agent was removed under a gentle stream of nitrogen, the extracts were dissolved in n-hexane and analyzed by GC-MS.

The chromatographic analyses were performed with a Shimadzu GC2010 gas chromatograph coupled to a GCMS-QP2010 Plus Mass Spectrometer device equipped with a column Zebron ZB-5HT. The identification of compounds was based on the analysis of fragmentation patterns and the comparison of resulting spectra with spectra from the commercial libraries Wiley 8 and Nist17.

4.2. Results

The GC-MS analysis of the total lipids extract (chromatogram at Fig. 4) shows the presence of degraded lipids, clearly presenting traces of triacylglycerol degradation products (Evershed, 1993, 2008; Roffet-Salque et al., 2017; Romanus et al., 2007; Rosiak et al., 2020), as the intense peaks of monoacylglycerols (2 and 1-monostearin, 2 and 1-monopalmitin, the most intense detected peak), and diacylglycerols (1,2 and 1,3-dipalmitin, 1,2 and 1,3-distearin, and 1,2 and 1,3 C_18:0/C_16:0 diacylglycerols) (Fig. 5a).

The chromatogram reveals a homologous series of alkanoic acids ranging from C₁₄ to C₃₄, with intense peaks of palmitic (C_16:0) and stearic (C_18:0) acids (Fig. 5b). The palmitic-to-stearic acid ratio is a key tool in lipid analysis for determining the origin of organic residues, such as differentiating between dairy, adipose, or aquatic sources (Kimpe et al., 2004; Romanus et al., 2007). Ratios below 1 reflect a higher proportion of stearic acid, which is typically associated with animal fats. As the value obtained was approximately 0.73, it suggests a strong predominance of animal fats over vegetable oils (Romanus et al., 2007). This conclusion is further supported by the presence of cholesterol (Fig. 4), a sterol of animal origin.

The possible detection of several isomers of C_18:1 fatty acid resulting from biohydrogenation processes in the rumen is a distinctive feature of ruminant fats. Additionally, ruminant fats typically exhibit diverse isomers of odd-chain fatty acids such as C₁₅ and C₁₇ (Dudd et al., 1999; Regert, 2011). In contrast to ruminant fats, monogastric animal fats exhibit a simpler profile of fatty acids, as they generally lack the diversity of C_18:1 isomers and odd-chain fatty acids like C₁₅ and C₁₇, as non-ruminants do not undergo ruminal biohydrogenation. Instead, their fatty acid composition more directly reflects their diet, with minimal microbial alteration in the gut. The absence of positional and geometrical isomers of C_18:1 and the scarcity of odd-chain fatty acids in non-ruminant fats are key indicators distinguishing them from ruminant fats, highlighting metabolic and digestive differences between these groups (Regert, 2011). Figure 4 shows that a single isomer of C_18:1 acid was detected, along with both linear C_15:0 and C_17:0 carbon acids. The absence of positional isomers indicates that the fat is derived from non-ruminant animals (Evershed et al., 2002; Mukherjee et al., 2008; Regert, 2011).

As noted by Malainey and Eerkens (Eerkens, 2005; Malainey, 1997), the ratio (C_15:0 + C_17:0)/(C_12:0 + C_14:0 + C_16:0 + C_18:0) serves as a marker to differentiate fats from monogastric and ruminant animals. When this ratio exceeds 0.04, the organic residue may originate from ruminant fats. In this case, the measured value was 0.034, aligning with the presence of fats derived from monogastric animals.

Besides the strong animal contribution, the sample also shows traces of vegetable fats (Fig. 5c), as the presence of long-chain alkanes and alcohols with an even carbon number, from C_22:0 to C_30:0 (Evershed, 2008).

Amides are chemical compounds typically formed through the reaction of free fatty acids with ammonia or amino groups, a process that is significantly enhanced at elevated temperatures. In archaeological contexts, this formation may occur during cooking or other thermal treatments involving animal fats. The thermal decomposition of triglycerides, combined with the availability of nitrogenous compounds, facilitates the generation of these amides, making their detection a reliable marker for heat-induced transformations in lipid residues. The presence of amides, such as hexadecanamide, octadecenamide and octadecanamide (Fig. 4), indicates that the fats were exposed to high temperatures during processing or use (Lejay et al., 2016; Lejay et al., 2019). The small peak of retene (Fig. 4), a polycyclic aromatic hydrocarbon (PAH) produced by the thermal degradation of resin acids, indicates the use of conifer wood or resin as fuel (Carpy & Marchand-Geneste, 2003; Marchand-Geneste & Carpy, 2003).