Here is a short guide to get you up to speed on mosaic foundations – the hidden layers beneath ancient tessellated pavements.
Mosaic foundations: five layers of making.
Layer 1: The SURFACE
I was standing in the Louvre, hunched over the mosaic of the Judgement of Paris, when a young American came up to join me. He must have been in his early 20s and I suspect that the only reason that he was drawn to stop and look at the 2nd century mosaic from Antioch was the intensity with which I was examining it. I wasn’t particularly delighted to have my reverie interrupted and scarcely looked up as he began commenting. He wondered how long it would take to make, how many pieces were used in its composition and began to notice the details of the work.
At this point I began to warm to him and his enthusiasm for what he was looking at and together we picked out the birds from the border design, discovered a cricket nestling in the acanthus and noticed the infinitely delicate placement of the tesserae to delineate the folds of cloth and facial expressions.
The 1.8 square metre work was once the centre piece in a Roman dining room and depicts an encounter between the messenger of the Gods, Hermes, and Paris, the son of Priam, the king of Troy. The young prince is alone on Mount Ida in Crete tending his sheep, when Hermes approaches him and asks him to judge which of the three goddesses, Aphrodite, Hera or Athena, is the most beautiful. Paris chooses Aphrodite in return for a promise that Helen of Sparta will be his bride. The subsequent abduction of Helen, who was married to King Menelaus, leads to the Trojan war.
The Judgement of Paris is a work of exceptional sensitivity and skill and is thought to have been based on a much earlier Hellenistic painting from Pompeii. The artist brilliantly creates a sense of the tension between the participants and the viewer in this all-important scene. A bronzed Hermes leans causally forward with deliberate nonchalance while Paris looks up at him, away from the three goddesses, naively oblivious to the consequences of his decision. Meanwhile, the women are placed slightly apart in a row; Aphrodite, the natural winner, looks almost bored as she twirls her right hand around a gauzy veil as if knowing that even a partial view of her face is all that’s needed to establish her superiority.
According to the plaque nearby, the mosaic was acquired after a joint mission by the Louvre and the University of Princeton led to its discovery in 1932. Other parts of the same floor were distributed among the museums of Baltimore, Princeton and Worcester. Date, story, provenance: these all important facts are mostly what attracts us when we look at ancient mosaics. Yet, for all their accomplishments, there is a lot more involved in their making than can be told purely by looking at the surface.
LAYER 2: THE SETTING BED. This is the layer of mortar immediately under the tesserae.
It is important to remember that the side of the tessera that is visible is only one side of a six sided cuboid shape, the rest of which is pressed into the mortar of the setting bed. The setting bed had tremendous gripping power, in many cases holding tesserae in place for two thousand years or more. In cases where mosaics have been damaged by earthquakes you can see that the foundations were made so well that they have remained reasonably intact, buckling with the mosaic rather than breaking apart.
However, the mosaic foundations were not infallible. In some cases, especially where glass was used, the grip wore away over time and the tesserae worked themselves loose.
Depending on the degree of detail required in the finished piece, the mosaic design would be etched or painted onto the setting bed while the mortar was still wet by the painter, or pictor imaginarious. This job was more prestigious than that of the ordinary workers who cut the stone and were responsible for the majority of the hard labour of setting the tesserae into the mortar. There are many examples where the painted guidelines, or sinopia*, have been revealed during excavations.
LAYER 3: THE NUCLEUS.
Most of the information we have about how mosaic foundations were prepared by the ancients comes from the Roman civil engineer, Marcus Vitruvius Pollio, who wrote an architecture book in the 1st century BC in which he describes how to lay foundations for pavements. The layer immediately underneath the setting bed is called the nucleus and according to Vitruvius it is made up of three parts crushed tile or potsherds to one of lime and ideally it should be about 15 cms thick.
LAYER 4: THE RUDUS
As you work your way down through the layers of mosaic foundtions the composition of the materials used gets rougher and coarser and the layers get thicker. The rudus, which is more than 20 cms thick, consists of broken bricks mixed with lime which were compacted down. The first two layers, the setting bed and the nucleus, are part of the mosaic itself and are lifted when a mosaic is moved. The rudus, however, was constructed in order to provide a solid, flat surface for the mosaicists to work on and can be considered as separate from the the mosaic itself.
LAYER 5: THE STATUMEN
This layer is more or less just rubble made of broken bricks and roofing tiles. Not all ancient mosaics contain all the layers exactly as set out in Vitruvius’ instruction manual – the quantity and quality of the foundations of ancient mosaics varied enormously but some foundations also included a base layer of impacted stones laid on their sides. The result would have looked similar to this cross view of a pebble mosaic from Rhodes.
Mosaics of the Greek and Roman World, Katherine M. D. Dunbabin. P. 281.
Ancient Mosaics, Roger Ling. P. 11