Considerations on the State of Conservation

Thanks to the varying conditions of the specific contexts that have been excavated, it is known that the state of preservation of these faunal remains depends both on their morphology and their interaction with the environment.

On one hand, the vertebrae always present greater stability, due to their compact shape, and a higher degree of calcification, regardless of the humidity conditions of the site. The same occurs with the caudal spines of rays, their dermal denticles, and their teeth, both rostral-whose chemical composition gives them greater resistance to the environment-as well as those from the lower and upper jaws.

On the other hand, segments composed of tessellated cartilage are much more unstable, and their preservation inside the matrix depends on the loss or conservation of the unmineralized cartilaginous nucleus, or its substitution by the surrounding area´s sediments, as well as the stability of the specific conditions of the ritual deposit.

The elements found in dry contexts filled with sediment are in good preservation conditions and are resistant as a unit. The anatomical sections found in constant conditions of waterlogging or very high relative humidity without filling are generally the most degraded, as is the case with those found in deposits with fluctuating conditions, since the pressure caused by the elements placed over the cartilaginous tissue as well as the presence and flow of water favor the hydrolysis of the protein and the dragging of the mineralized sections. Finally, the sections in conditions of high humidity, but with filling, are extremely fragile and show considerable deformations and fractures (Figure 2).

Source: (Photograph: Mirsa Islas, 2016; courtesy: Proyecto Templo Mayor [PTM], Mexico).

FIGURE 2 Offering 171 

There are also cases in which, thanks to the stable conditions of the context, it is possible to recover sections of the skin of these marine animals. Although it is difficult to notice their presence in situ, a thin, shiny layer above the tessellated cartilage can be identified by a trained eye. These remains are recognizable under the microscope by finding dermal denticles which, although easily shed under fluctuating conditions, can be recovered when analyzing the deposit’s sediments (^{Barajas et al., 2015, p. 39-50}; ^{Barajas et al., 2016; p. 63-67, 159-162 and annex 2}; ^{Barajas & Sanromán, 2017, p. 172-178, 188-194, 204-205 and 211-213}; ^{Barajas & Sanromán, 2018, p. 173-178, 200-201 and 221-235}; ^{Barajas & Sanromán, 2019, p. 222-240}; ^{Robles et al., 2018, p. 24}; ^{Bolaño et al., 2022}) (Figure 3).

Source: (Photograph: Leonardo López Luján, 2008; courtesy: Proyecto Templo Mayor [PTM], Mexico).

FIGURE 3 Offering126 

Preventive Measures in the Context

The participation of a field conservator is essential upon the discovery of this type of remains within the ritual deposits since decisions must be taken immediately to ensure the permanence and stability of the cartilaginous skeletons during the excavation and recording process.

First and foremost, it is crucial to always maintain the context’s conditions, recording and studying the humidity and temperature data with digital thermohydrometers, using water containers, plastic covers, or humidifiers, if necessary, in order to maintain stable parameters at the levels at which the remains were found. In addition, it is important to have a cover over the excavation area, to minimize alterations due to insolation; depending on the characteristics of the context, it can be temporary, using tarpaulins and sheets, or permanent, when closed spaces are built to protect the research area.

Excavation and Registry

The excavation of offerings is carried out following arbitrary levels in which the superficial layer of visible objects is defined and freed in a very meticulous and controlled manner, facilitating the detailed registry process and the subsequent analysis of each object. The work around these faunistic remains is performed with soft delicate tools, including bamboo swabs, wooden or bone spatulas and styli, and brushes, to avoid any possible marks on the surface of the cartilaginous tissue, which could cause confusion during taphonomic analysis.

Parallel to the excavation, graphic and photographic records are carried out, using a methodology designed and implemented in 2010 by the team of the Templo Mayor Project (TMP) in which, using scaled zenith photographs of deposits, printed in a proportion of 1:1, the elements that comprise a layer are traced in a plastic sheet placed over the print. The tracings serve as a guide for the digital AutoCAD^® drawing; osteoarcheological field analysis are also carried out (^{Chávez et al., 2011}) and the conservation registry, which consists of the elaboration of materials that support what was done by the archaeological team, to avoid the loss of information and ensure a good intervention a posteriori. The latter includes a written record of the conditions upon discovery and preventive treatments used, drawings and detailed photographs of the elements, and even Mylar^® or acetate tracing, which will facilitate the intervention processes in the field laboratory.

Likewise, a written record is kept, using a database developed in FileMaker^®, in which the team of archaeologists fills in the records for artifacts and organic materials, and the restorers fill in the conservation records.

In situ Intervention

It is sometimes necessary to carry out direct intervention processes simultaneously with preventive conservation and registry actions when the faunal remains are still in situ. The type of procedures carried out depends on the characteristics of the burial matrix, the state of conservation of the cartilaginous material, and the excavation plan.

The superficial cleaning of these remains is carried out using fine tools in a generalized manner over them to delimit the forms and make an adequate record; soft brushes and water applied with a dropper are used for humid contexts, while soft brushes are used in dry ones; occasionally specific actions are performed with swabs slightly moistened with water. The sediment recovered from these processes is registered as a sample for laboratory analysis, where it is possible to recover dermal denticles for future identification.

Another common process is veiling the surface with Japanese paper or non-woven materials to help maintain the humidity of the anatomical segment, or to ensure its unity and prevent its disintegration. The first are secured to the surface of the materials using potable water, using a sprayer when considerable desiccation is observed. For the second ones, various adhesives have been used throughout the years, such as EMA copolymers (ethylene methacrylate copolymers, Paraloid B72^® in different concentrations, with polar and nonpolar solvents) and cellulose derivatives (hydroxypropyl cellulose, hydroxypropyl methylcellulose, sodium carboxymethylcellulose), each presenting advantages and disadvantages for the conservation process and a posteriori analysis. This veil is usually planned as part of the extraction that has evolved and varied over time.

Consolidation -for remains composed of tessellated cartilage mosaic- is another process that has been carried out in situ in almost a generalized manner. This is mainly carried out in those cases where the element´s state of conservation is poor, and presents fragility, fissures, fractures, deformations, and general loss of cohesion. Different EMA and _PVB resins (polyvinyl butyral PVB, Mowital B60H^®) have been used in different concentrations and in solution or emulsion (EMA, Primal AC 33^®) whose application has been by dripping, taking care not to contaminate the rest of the context. (^{Hasbach, 2000, p. 135-136}; ^{Sanromán, 2009, pp. 8-12}; ^{Escalante & Soto, 2012, p. 113-114}; ^{Barajas et al., 2015, p. 132-143}; ^{Barajas et al., 2016, p. 63-67, 159-162 and Annex 2}; ^{Barajas & Sanromán, 2017, p. 172-178, 188-194, 204-205 and 211-213}; ^{Barajas & Sanromán, 2018, p. 173-178, 200-201 and 221-235}; ^{Barajas & Sanromán, 2019, pp. 222-240}).

Once the veils and consolidates are dry, the team starts to recover the ichthyological remains from the context.

Extraction

This process varies, depending on the anatomical section found in the deposit and its state of preservation, as well as each offering´s specific characteristics.

The sections corresponding to vertebrae, teeth and caudal spines are recovered manually or with fine tools, such as tweezers, since they are better preserved due to their morphological characteristics and calcification or mineralization. Dermal denticles are recovered in the same way but under the stereoscopic microscope due to their dimensions.

On the other hand, the anatomical sections composed of tessellated cartilage present a greater complexity, depending on their preservation conditions. The most common way of lifting these elements is through the creation of blocks and using various types of supports to ensure that the faunal remains maintain their coherence and do not deteriorate further.

A wide range of materials have been used to build these blocks, depending on their availability, temporality, and previous experiences with bone remains and other archeological materials.

The veil is an essential component in the creation of said blocks, which serves as the first insulating layer between the element to be extracted and the filling material of the block. Isolating both the element to be extracted and the rest of the context is vital for the correct extraction of the block, and to avoid contaminating the offering. Materials used in addition to the veil include layers of plastic (usually PE polyethylene), self-adhesive plastics, aluminum foil, and layers of cloth with various adhesives, mainly acrylic resins. The materials that support the block are placed on top of these layers.

Depending on the specific characteristics of each case, plaster bandages, wood or cardboard is used to make the blocks, which are made to contain the filling materials, including: plaster, plaster with polystyrene beads, synthetic adhesives with polystyrene beads, expansive polyurethane resin (PU) and low-density epoxy resins (EP) (Araldit^® or Ren Paste^®). In almost all cases, these blocks have served as auxiliary support for laboratory work. Some of these form part of the permanent exhibition at the Museo del Templo Mayor (MTM) (Figure 4).

Source: (Photograph: Néstor Santiago, 2015, courtesy: Proyecto Templo Mayor [PTM], Mexico)

FIGURE 4 Offering 165, extraction with plaster bandages 

It is important to highlight that the production of this type of block has been recently avoided, with a preference for instead using several layers of veils, providing greater visual control of the element being extracted, along with the actions carried out on the other objects deposited during the extraction.

A great range of auxiliary supports are used to lift the blocks, they are inserted gently between the element and the lower layers of the area, making sure to provide enough support for the cartilaginous remnant and to also completely separate it from the sediment and other materials in the offering. The use of wooden plaques with metallic edges, metallic sheets, and polyethylene or other rigid or flexible plastic sheets is recorded (Figure 5).

Source: (Photograph: Mirsa Islas, 2017, courtesy: Proyecto Templo Mayor [PTM], Mexico)

FIGURE 5 Offering 174, placement of facing  

To ensure a homogeneous and simultaneous action on all the elements, lifting the block requires at least two or three people.

It should be noted that once the block has been lifted, in most cases it is turned over to cover, isolate and fill the underside ensuring the objects are not damaged during subsequent transportation.

More recently, after the extraction of cartilaginous elements is completed, they are then placed on a pillow of polystyrene balls, isolated with self-adhesive plastic and Tyvek^®, which serves as a support that can be modified as needed during the intervention procedures (Figure 6).

Source: (Photograph: Mirsa Islas, 2016, courtesy: Proyecto Templo Mayor [PTM], Mexico)

FIGURE 6 Offering 171, Polystyrene balls support 

Several considerations must be considered during the extraction of a block, including its final size and weight, how the block will be transported to the field or formal laboratory, as well as the amount of time that will elapse before the direct intervention begins.

In our personal experience, the blocks were moved a very short distance because the field lab was a few meters away from the excavation site; however, ensuring that the whole team is careful and aware when moving these fragile materials, helps avoid unnecessary accidents. Also, intervention should start almost immediately and any issues solved promptly, for example, by placing additional supports or coverings.

Intervention in the Laboratory

The processes that took place in the field are continued in the laboratory, in a controlled environment.

For caudal spines, teeth, and vertebrae, the direct intervention begins by finishing the surface cleaning; using paintbrushes and tap water, though in certain cases a rolled swab, dampened with ethanol or water, is used to control the humidity applied to the material.

In all cases, following the aqueous processes, controlled drying is performed, to avoid any abrupt changes that may cause new alterations.

Tessellated cartilage mosaic and certain vertebrae -mainly from young individuals- need a consolidation process, where low-concentration dispersions of acrylic resins (usually between 2 and 5% in ethanol; greater concentrations in less polar solvents were used previously) are applied locally, or in the case of the vertebrae, by immersion. As this is an irreversible process, systematic sampling helps subsequent analysis.

Sampling is performed jointly with archaeozoologists; it is done by selecting and removing segments, usually those already detached, and others with specific characteristics, and separating the teeth and vertebrae, which will be left untreated. This procedure is the same for specimens that are treated in situ for which, segments that are less impregnated or that have certain diagnostic elements are selected.

Once the consolidation process is finished, according to the anatomical section being worked on, the fragments are adhered using polyvinyl butyral (PVB) based resins, applied with a brush, and allowed to air dry. Subsequently, if needed, gaps can be filled with ceramic paste (kaolin and PVB) or Modostuc^® (as commercially known) reinforcing the union areas. When missing portions compromise the anatomical section’s structure and stability, replacements are made using a hard modeling paste³ made with kaolin, PVB, and fiberglass, avoiding distortion of the archeological data; these procedures are common on rostral cartilage from sawfish (^{Hasbach, 2000, p. 135-136}; ^{Escalante & Soto, 2012, p. 113-114}; ^{Malváez, 2011, pp. 20-39}; ^{Soto, 2013, p. 110-114}; ^{Barajas et al., 2015, p. 132-133}; ^{Barajas et al., 2016, p. 159-162}; ^{Barajas & Sanromán, 2017, p. 172-178, 188-194, 204-205 y 211-213}; ^{Barajas & Sanromán, 2018, p. 173-178, 200-201 y 221-235}; ^{Barajas & Sanromán, 2019, p. 222-240}) (Figure 7).

Source: (Photograph: ^{María Barajas, Néstor Santiago and Adriana Sanromán, 2016}, 2010 & 2017; courtesy: Proyecto Templo Mayor [PTM], Mexico).

FIGURE 7 Processes in the field laboratory 

Finally, color retouching with varnish paints or watercolor is applied to give visual unity between the cartilage, gap fillers and replacements.

Storage, Exhibition, Diffusion and Dissemination

To ensure the preservation of the collection in optimal conditions, once the conservation intervention has finished, and in parallel with the analysis, the permanent packaging is made. For this purpose, custom-made black polypropylene (PP) boxes lined with slabs of polyethylene foam (PE, Ethafoam^®) are used. To avoid alterations through movement or vibrations, the foam is carved in the shape of the most susceptible remains, such as sawfish rostral cartilage or sections of tessellated mosaic. These carved areas are then covered with non-woven textile made of high-density polyethylene (HDPE, Tyvek^®) fibers to avoid surface abrasion of the faunal remains. The vertebrae, caudal spines and teeth are packed in individual hermetic polyethylene bags that are placed in specific sections within the Ethafoam^® slabs. It must be stressed that to avoid confusion or loss of information, each element is always identified with its corresponding excavation data (^{Sanromán & Barajas, 2022}) (Figure 9).

Source: (Photographs: Adriana Sanromán; courtesy; Proyecto Templo Mayor [PTM], Mexico).

FIGURE 9 Packages of different anatomical sections  

Once the objects recovered by the PTM are transferred to the Safekeeping Storage for Archeological Collections at MTM, the storage conditions are stable, and the environmental parameters are constantly monitored. This helps to keep objects that have not been intervened, as well as restored ones, from suffering alterations due to sudden changes in relative humidity or temperature.

Tailored supports are designed and created to exhibit these cartilaginous remains, as well as for other materials, according to the curatorial discourse. However, because of their poor state of conservation, some anatomical sections that correspond to sawfish rostral cartilage were mounted in blocks to keep their unity. To integrate them into the museum’s permanent exhibition, the blocks were sought to resemble that of the soil matrix.

During the last two PTM field seasons, various studies have been made surrounding the identification, symbolism, and conservation of these type of skeletal remains, resulting in articles for the diffusion of scientific knowledge that have deepened the understanding of these animals and their relevance for the Mexica. Among them, a diffusion article (^{Robles et al., 2018, pp. 20-27}), and three scientific articles that delve into their biological aspects (^{Mendoza & Bolaño, 2022, pp. 423-438}; ^{Bolaño et al., 2022, pp. 439-457}), and on their conservation treatments (^{Sanromán & Barajas, 2022, pp. 537-551}) stand out.