Accumulation of soot layers on the ceilings and walls of ancient Egyptian tombs and temples is a very common phenomenon. In the Roman period, numerous isolated tombs and temples, dating back to the Pharaonic period, were inhabited by Christian hermits. When they suffered from persecution and economic pressure by the Romans, these sites served as shelters for them and consequently some of these pagan sites were turned into churches (1, 2). As a result of that occupation, the wall paintings of these Pharaonic sites became begrimed with thick soot layers produced by the domestic activities of the new inhabitants (cooking, lighting, heating, etc…). Incomplete combustion of materials produces smoke which in return results in the formation of particles of carbon (soot). Soot is mainly composed of carbon particles in combination with an oily matrix. The particles of carbon are of ca. 2.5 µm size and they form the visible soot agglomerations when they are attracted to each other by the oily materials (3-5). In addition to the adsorption of various pollutants, these soot layers obscure the wall paintings and reduce the permeability of their surface (6, 7). Moreover, conservators should consider safety requirements during the removal of these deposits as they can be irritant to the respiratory system or the skin (3).
In our study, cleaning tests were performed on the ceiling of the Osiris sanctuary in the temple of Seti I at Abydos, Egypt. The ceiling is covered with thick soot layers composed of carbon (C) as identified by means of XRD (8). Seti I dedicated one of the seven sanctuaries in his temple to the god Osiris. Osiris is one of the most important and popular gods in ancient Egyptian history. This importance is derived mainly from two circumstances. The first one is that he suffered from a dramatic death: either by drowning or being murdered by his brother Seth. According to the myth, his corpse was dismembered, reconstituted and mummified by his wife Isis and his sister Nephthys. Isis, after becoming impregnated by the mummified Osiris, gave birth to a son Horus who was secretly raised by her to protect him from his uncle. Upon reaching maturity, Horus managed to triumph over Seth and was crowned king of Egypt. This myth is significant as it ensured Egyptians that they could survive after death. Secondly, Osiris was popular due to the fact that, after the victory of his son, he was installed as the ruler of the netherworld where every deceased Egyptian had to travel through for regeneration after death. The cult of Osiris was connected to Abydos (situated in Thinite province – upper Egypt) in addition to Busiris (situated in the east – central Delta) (9-12).
The sanctuary of Osiris is the most interesting one in the temple and is located in the middle of a series of seven sanctuaries devoted to different deities; see Fig. 1. All the sanctuaries contain false doors on their west side except for the Osiris sanctuary that has a real door. The latter leads to a complex of small rooms dedicated to the celebration of the Mysteries of Osiris.
The paintings of the temple of Seti I have been studied previously (8, 14-16). They were performed by means of different techniques. In some cases, the stone (limestone and sandstone) was cut into sunk/raised reliefs or a combination of both, covered with a whitewash and then painted using the secco technique. While in other cases, the Egyptian artist applied a thin preparatory layer (composed of calcite, gypsum, and quartz) to the uncarved or carved stones. The pigments used were Egyptian blue, Egyptian green, red ochre, and yellow ochre. Gum Arabic was used as the binding medium for these pigments.
Numerous reagents have been used for the cleaning of soot particulates from artwork surfaces. They include solvents, salts, and surfactants (17, 18). The most commonly used reagents are as follows: a) solvents such as ammonia, ethanol, white spirit, acetone, trichloroethylene, and ethyl acetate, b) surfactants such as orvus*, vulpex**, and triton X-100***, and c) salts such as sodium bicarbonate, ammonium carbonate, sodium carbonate, tri-ammonium citrate, and ethylenediaminetetraacetic acid (EDTA). Traditionally, they are used individually or in mixtures and applied by cotton swabs or poultices (3, 17-20). As an example, in a recent study, Zn nanoparticles mixed with vulpex has been successfully used to remove soot, patches of waxes and blood from the wall paintings in the temple of Isis in Luxor, Egypt (7). In another study concerning conservation-restoration of the mortuary temple of Ramses III in Luxor, enzymes have been used to remove soot deposits, blood of bats and a deteriorated varnish of animal glue applied as a protective coating in a previous conservation treatments (21). An attempt to evaluate soot removal from acrylic emulsion paintings has been carried out using dry cleaning methods (commercial erasers, sponges, and homemade bread) and non-contact cleaning methods (atomic oxygen and CO2 snow) in comparison with wet cleaning (5). Water, homemade bread, and the so called ‘Absorene sponge’ were able to remove the soot without altering the treated surface. Agar gel has been tested for the cleaning of stone sculpted surface “Fuga in Egitto” high-relief of the Duomo of Milan (22). The most effective results were obtained by using 3% agar loaded with 1% Tween 20% which was able to safely and easily remove the accumulated soluble salts and soot particles after one hour contact time.
In view of the previous literature survey, a few studies have been dedicated to study the removal of soot from painted surfaces in general and ancient Egyptian wall paintings in particular. In addition, traditional cleaning methods were employed in the soot removal procedures such as cotton swabbing and poulticing which have a number of limitations/disadvantages. In the case of thick soot layers, the use of cotton swabs embedded in the cleaning reagents is not recommended by conservators as it causes damage to the paint layers. This is because there is not enough contact time for the reagents to soften the soot and consequently the removal depends mainly on the mechanical action of swabbing. Moreover, the aggressive mechanical action embeds part of the soot particles further into the pores of the surface. In such cases, conservators usually use poultices composed of cellulose pulp, carboxymethyl cellulose (CMC), and the proper cleaning reagents mixed into water. These poultices permit long contact time for the reagents to soften the soot which can be removed later with cotton swabs. However, poultices cannot retain the liquids for relatively long time as they evaporate fast in elevated temperature. In addition, part of the liquids is absorbed by the porous surface which introduces more of the chemical reagents into the treated surface. Finally, in some cases the poultice may leave cellulose fiber and CMC residues on the treated surface (23, 24). Due to the limitations posed by the traditional cleaning methods in the removal of soot from ancient Egyptian wall paintings, gel cleaning was proposed as an alternative cleaning approach to be tested for this task.
Gels are deemed to be controllable and safe cleaning method (25, 26); thus, they were evaluated for removing thick soot layers from the ceiling of the sanctuary of Osiris (see Fig. 2a, b). In previous studies, we developed a polyvinyl alcohol-borax/agarose (PVA-B/AG) double network gel and studied its properties. It was tested to remove deteriorated consolidant layers from wall paintings of the same sanctuary and it was able to successfully remove the consolidant without damaging the paint layers (27). In addition, we investigated the different characteristics of the PVA-B/AG double network gel. According to these investigations, the gel is characterized by the following features: a) it is a flexible gel with the ability to adapt its shape to complex surfaces such as the case of ancient Egyptian painted reliefs, b) it features good shape stability when applied on painted surface for extended contact times, and c) it can be loaded with a wide range of reagents such as mixtures of polar and non-polar solvents, chelating agents, and surfactants, d) and it has a good liquid retention, a relevant property required in sites characterized by elevated temperature and very low levels of relative humidity (28).
Based on the above-mentioned features, the PVA-B/AG double network gel was deemed suitable for cleaning tasks in the sanctuary of Osiris. For this reason, the gel was loaded with various cleaning reagents, already documented in literature to have the ability to remove soot particles. The prepared gel composites were applied to the ceiling of the Osiris sanctuary during two cleaning tests and the results were assessed visually and microscopically. In view of the difficulty of reaching the cleaning locations (vaulted ceiling ca. 4 m above floor height), the option was taken not to use other/more sophisticated (spectroscopic) characterization equipment on site to assess the difference before and after treatment. The most effective gel composite was adopted to treat a larger area of the surface as a final demonstration of its efficiency.
* An anionic surfactant composes of sodium dodecyl sulphate.
** An anionic soap composed of potassium methylcyclohexyl oleate.
*** A nonionic surfactant composed of octyl phenol ethoxylate.