*This dissertation is not a peer-reviewed article
*The author is head of the Mosaic Conservation Section in the Conservation Department of the Israel Antiquities Authority and was personally involved in the conservation of the Lod Mosaic.
The Lod Mosaic
The famous Lod Mosaic (Fig. 1) 1Comprehensive information about the mosaic can be found at www.lodmosaic.org was uncovered in 1996, during a salvage excavation conducted by the late Miriam Avissar on behalf of the Israel Antiquities Authority (IAA). 2Photographs in this article are taken from the www. lodmosaic. org website; some of them were photographed by the author, courtesy of the IAA Over the past years, complementary excavations were conducted at the site in preparation for the construction of the Lod Mosaic visitor center. These excavations were directed by Amir Gorzalczany and Hagit Torgë of the IAA. The involvement of the Mosaic Conservation Section of the IAA at the mosaic site began as early as 1990, when two relatively small polychrome mosaics were exposed during an excavation directed by Alon Shavit and Arieh Rosenberger. Today it is clear that there many more mosaics, installed from the Early Roman to the Byzantine periods, are yet to be unearthed in the environs of the Lod Mosaic site.
How craftsmen created mosaics in the Roman period
During the construction of a building, its owner would ask a craftsman to create a mosaic floor for it. There were several stylistic schools of mosaic artists throughout the Roman world. Craftsmen were differed in their level of skill and talent—both of which would determine the cost of the work and probably also the quality of the final product. First, the craftsman would have to examine the space’s dimensions and choose the floor and stone types. He would then consider the nature of the floor that was required, depending on the function of the building: Did the structure call for a design of geometric patterns or animal motifs? Would a cultural-artistic or religious nature be fitting? The mosaic was then designed based on the answers to these questions. Beyond the artistic aspects, it was important to plan in advance where the floor would slope to drain the mosaic of water. In earlier days, the statumen, the substrate layer, was effectively used as the floor of the house. When mosaics came into vogue, this layer became the basic infrastructure. Sometimes two statumen layers were laid: Large stones were placed first, and above them, a layer of river pebbles or construction debris was spread, sometimes mixed with potsherds. The nature of the preparatory layer(s) depended on the location of the mosaic’s construction. Where the soil was heavy, a very stable infrastructure was required, while when the floor was laid on a rocky outcrop, it was sufficient to fill cavities and depressions, creating an even surface. Once the substrate was ready, two layers were laid on it. The first was the rudus—a sloping layer consisting of poor-quality lime, stone fragments, potsherds, and marl soil. Above its was the nucleus layer, which served as the foundation for the mosaic stones, the tesserae (pl.; sing. tessera, from the Greek word meaning cube). On the nucleus the sinopia was drawn—a polychrome preparatory sketch that the artisans followed when creating the mosaic’s outlines with tesserae. This layer needed to be spongy so that it could withstand pressure and blows during the work process. It was therefore made of higher quality lime, clay, and sand. The sinopia had to be kept moist for several days to ensure that the mosaic adhered to it well. If this task was not carried out skillfully and cautiously, the mosaic would detach from the base layer relatively easily. The materials for preparing the substrate were chosen according to the economic and natural resources available to the customer and in the vicinity of the site. Very wealthy customers were less likely to compromise on their desires, and would order raw materials even if they had to be imported from overseas. When the nucleus layer was ready, the final details were negotiated with the owner regarding the mosaic—for example, would it abut the walls or not? The artisan would usually arrive with stencils of patterns from which the owner would choose one or, at most, a combination of several. At the owner’s request, inscriptions were sometimes added.
We know of various methods for constructing mosaics. Most often they would be set in situ, directly into the floor. Only delicate creations requiring great precision, such as surfaces made of small tesserae with sides measuring 2–3 mm or especially intricate works, were prepared in the workshop and brought to the construction site. Laying geometric mosaics began with the creation of a black outline. The craftsmen would lay down wooden rulers according to which they determined the pattern and the slopes of the floor. The tesserae were placed on a ca. 5 mm-thick layer of lime or, in the case of Roman mosaics, sometimes on concrete. At times, the chosen designs were of natural motifs, including animals, plants and landscapes. The first step in creating a mosaic was usually building a frame that delineated its area. The center of the “carpet” was subsequently set. Next, diagonal strips would be laid opposite one of the frame’s corners so that the first strip was triangular; then trapezoidal strips were laid parallel to each other. Once all the tesserae had been set, the surface would be smoothed with a circular motion, using a wooden board or a stone. This would force the substrate material out into the gaps between the tesserae. Excess material would be wiped away with a wet cloth until an even surface was achieved. After the floor was completed, it was sometimes planed with a stone to smooth it. In the past, experts in finding colorful tesserae would know how to locate deposits of rock that could be cut smoothly in straight lines, thus obviating the arduous task of leveling the surface. About three-quarters of the tesserae were white. Sometimes they would be prepared on site and only special tesserae would be brought from afar. Glass tesserae (smalti) integrated into high-quality floors were usually made from waste glass. Smalti were produced mainly locally, except in the case of unique quality mosaics. In antiquity, tesserae, almost without exception, were made of natural rock. They were also never colored; at most, the stones from which they were cut were burned to change their hue. First, a block of rock would be cut by a guillotine placed high above it. Next, finer cuts were made using a smaller and more delicate guillotine, which produced “pencils”—long square-sectioned rods with sides measuring 8–10 mm. Finally, the rods were cut into tesserae according to the required size.
After the mosaic was installed, it was cleaned and grouted with hydrated lime to fill the grooves between the tesserae. Finally, the floor was waxed. It was customary to clean the mosaic daily with a damp cloth.
Conserving ancient mosaics on site
Conservators accompanied Miriam Avissar and her team during the actual excavation of the Lod Mosaic, and its documentation began on site. The conservation team replaced missing tesserae with lime-based material temporarily, to prevent the rest of the mosaic from collapsing, and instructed the archaeologists on how to protect it during the excavation. The mosaic was covered until the entire floor was exposed, and then the conservation work began. The team first surveyed the mosaic to determine its conservation status, and intervention was planned accordingly, taking into account the mosaic’s condition, its building materials and construction methods, and historical processes that it had undergone. In addition, the mosaic was thoroughly documented by photography, color-copying (down to the level of the individual tesserae where there were intricate artwork; Fig. 2). A written description of the floor’s technical condition was
also produced.
Copying the mosaic pattern is significant for the reconstruction process of the floor. For example, the mosaic floor at the Tiberias Hot Springs was vandalized, and it was reconstructed on the basis of its initial documentation, which included counting the tesserae and examining the type of stone, the thickness of the substrate layers, and the composition of the substrate materials. There are three methods for cleaning mosaics: dry cleaning using brushes; wet cleaning with water, which can be performed only on a highquality substrate; and mechanical cleaning, designed to remove biofilm surface coverage or cementing materials that are stuck to the floor. In special cases, silicate materials are injected to stabilize stones, or even surfaces. So, for example, conservation intervention is almost always required for smalti tesserae, which are very delicate.
A more thorough treatment of the mosaic includes filling in lacunae and securing the mosaic’s borders. If there is a bulge in the floor, it is emptied out, the substrate is treated, and the floor is restored to its original level plane. In the case of a depression, the substrate layer is injected with lime mixed with grog and stone powder to stabilize it and stop the floor’s sinking. Sometimes there is no choice but to remove a section of the mosaic to determine the cause of the depression. If the cause is a pit, it is filled and sealed before returning the surface to its place. In many cases, the depression can be stabilized without lifting the surface.
Our goal is always to preserve the mosaic in situ, either by completing its conservation and creating proper conditions for displaying it or by covering and protecting it without displaying it.
Transferring mosaic floors from the site
After documenting and studying the structure of the mosaic, an informed plan was prepared for dividing it into strips in a way that would allow future restoration without leaving any cut marks (Fig. 3).
After the mosaic is cleaned, it is standard practice to fasten sheets of raw cotton fabric to it, using chemical-based adhesives or bone glue (usually used for woodworking). Plastic glue can also be used. An antimicrobial agent was added to all glues to prevent the development of bacteria, especially if the mosaic is expected to be stored away for many years before it is treated further. The manner of gluing and the viscosity of the glue are selected based on the mosaic’s size. The fabric adheres well to the mosaic tiles after applying a few layers of glue. When the glue is dry, the most accessible and weakest point of the floor is chosen, and the conservators begin to detach the floor from the lime-bed layer in which the tesserae are embedded, using chisels. If this layer cannot be penetrated, they must go a layer deeper, and remove the substrate as well. The mosaic must be completely loosened by using long chisels. Small mosaics (with an area of up to ca. 20 sq m) can be detached in their entirety. A wooden board is placed on the floor surface and fastened to it using clasps. The surface is then flipped over onto a second board for transport. If the mosaic is large, a large wooden board of the size of the mosaic is prepared and a similar procedure follows.
The Lod Mosaic was removed by rolling its surface onto a cylinder over 120 cm in diameter (Fig. 4). For this purpose, the conservators had to very thoroughly clean the substrate that adhered to the tesserae from below. As rolling the mosaic may damage the surface, it is not often the preferred method and is applied only when a large mosaic such as the Lod Mosaic must be turned over onto a surface prepared in advance in order to transfer the mosaic to the laboratory.
Once the floor is removed, the surface on which it was placed was stabilized and the heavy load was transferred to the IAA’s laboratory at the Rockefeller Museum. After removing the mosaic, it is always vital to examine whether there are any remains beneath it. For example, under the Lod Mosaic we found footprints of the craftsmen who prepared the adhesive material for the tesserae (Fig. 5).
In exceptional cases, it is necessary to divide the mosaic into metopes by cutting the cotton fabric and the mosaic attached to it in places where the mosaic pattern is monochromatic and not complex. The tesserae should not be cut using a circular saw but separated from one another. This enables the metopes to be connected and to restore the mosaic in its entirety after it has been treated.
Preparing the Lod Mosaic for museum display
In the labs the mosaic was cleaned again to remove any remains of the lime bed that adhered to the back of the tesserae. This layer helped to keep the tesserae in place during the mosaic’s initial transport to the laboratory. When we started removing the substrate layer from the floor of the Lod Mosaic, we noticed the presence of pigment on the material attached to the floor. We decided to detach the material from the floor in an orderly manner, thus exposing the sinopia drawings and revealing the preparatory sketch that was made for the floor before the tesserae were embedded. A laboratory test revealed that the sketch was made using the same colors planned for the mosaic cubes. These plaster fragments were preserved and later prepared for a museum display next to the mosaic (Fig. 6).
Once the mosaic was clean, it was placed face down on a level floor. A temporary frame was prepared for it as a mold for material that the conservators poured into it, and a back for the mold was also prepared in advance. A lime-based separator (acting as a substitute for the lime bed) was applied to the mosaic, epoxy glue is poured onto the separating material, and the glass-cloth frame back is affixed to the glue. Several days later the mosaic was turned right side up, and the glue and fabric were removed using scalpels and dentistry tools. Chemicals were applied as needed. Once the mosaic was cleaned, the conservators treated it with protective materials and silicate-based stabilizers, and a coat of Paraloid (a thermoplastic resin) was applied to the surface to protect the floor and emphasize its colors. This material can be removed, if necessary, using acetone.
The dimensions of the Lod Mosaic were given to a packing company that prepared boxes for transporting the mosaic around the world. A unique box was custom-made for each strip. The mosaic was transported as three floors divided into seven crates housing five surfaces of the central mosaic carpet, presenting animals, the fish carpet, and the rhombus carpet. Sometimes the mosaics were displayed on the floor, and various surfaces were connected (Figs. 7, 8). In other cases, such as at Ben-Gurion Airport, mosaics are hung on the wall. For this purpose, a frame is constructed on the wall with the safety approval of an engineer. The mosaic surface may reach 1 ton in weight, and therefore careful planning is required for its display on a wall. The frame is firmly fixed to the wall and supports the mosaic the way a picture frame supports a work of art.
Figs. 7–8: Transporting a section of the mosaic after conservation for display at the Altes Museum, Berlin
The travels of the Lod Mosaic around the world
As of 2011, the Lod Mosaic has travelled between some of the most important museums in the world. It was first exhibited at the Metropolitan Museum of New York, then at the Fine Arts Museum of San Francisco, the Field Museum in Chicago, the Columbus Museum of Art in Ohio, and the University of Pennsylvania Museum of Archaeology and Anthropology. The mosaic was later displayed at the Louvre in Paris, the largest museum in the world, visited by over nine million people a year––the only exhibition of antiquities from The State of Israel to be displayed at the Louvre to date. The mosaic was later exhibited at the Altes Museum in Berlin, and in London. It was on display in Saint Petersburg, Russia, and in September 2015 it was delivered to Venice, Italy. A representative of the IAA’s Mosaic Conservation Section was sent to each museum where the mosaic displayed, to inspect it before it is dismantled and packed for shipment. The mosaic was transported by companies specializing in shipping logistics, which operated following the instructions and under the supervision of the representatives of the Mosaic Conservation Section. The reinstallation of the mosaic at each museum was also accompanied by a representative of the Mosaic Conservation Section. Despite the mosaic’s long journeys around the globe, its condition has not changed since it was cast into the frames.
Footnotes
- 1Comprehensive information about the mosaic can be found at www.lodmosaic.org
- 2Photographs in this article are taken from the www. lodmosaic. org website; some of them were photographed by the author, courtesy of the IAA