Research and conservation of pietre dure tabletops

Rocks and minerals used in pietre dure as based on a tabletop with fruits, flowers and birds

The stonecutters of the Grand Duke's Workshop skillfully used natural variations of the colour and structure of minerals to show light and shadow, to emphasize the variety of colours of fruits, flowers and plants, as well as the colours of the animal world. At the end of the 17th century, the number of minerals described by Agricola in the middle of the 16th century was limited to 80-100 mineral species (to date, 5593 have been discovered). The availability of diverse minerals was limited and not many species were ornamental. However, the mosaic gives the impression of a rich variety of materials. Despite the range of colour presented in the tabletop, very limited quantities of minerals were used to encrust it.

One of the key minerals of the mosaic is calcite. It is calcium carbonate (CaCO3) — the main mineral of limestone and marble — sedimentary and metamorphic rocks, as well as "building material" of shells of mollusks. In monocrystalline form, calcite was not used in mosaics because of its softness (3 on the Mohs scale) and perfect rhombohedral cleavage — the ability of the crystal to split in three directions. However, both limestone and marble have a higher hardness (up to 4), isotropy (homogeneity) of stone-cutting properties and are widespread in Italy, particularly in Tuscany. Classic black marble (Noir Belge) mined in several Belgian regions was used for the background. This material has been known since Ancient Rome; it was widely used for the production of pietre dure mosaics in the Medici Workshop because of its velvety black uniform colour and sheen. It can be seen in the inlays of the Taj Mahal, in large decorative objects — vases, fireplaces, sculptures, floor lining — in various buildings in France (Palais Versailles), Belgium and the United States. The overwhelming number of green and olive leaves, as well as stalks, trees, shrubs and flowers are carved out of marble of the local variety Verde dell’Arno. Siena orange-yellow dolomite limestone (Siena Gialo) is used in the trimmings of ornamental elements — hearts and rhombuses on the edges of the tabletop, as well as in thin ribbons that separate the inside from the outside. One of the varieties of marble — marble onyx — has a striped texture and decorative qualities: it is visible in the image of the shell of the snail, and the bodies of some insects. Often marble and marbleized limestone contain remains of fossil flora and fauna, similar elements with fossilized coral can be seen in the mosaic (moth tendrils). Fully organogenic calcium carbonate — internal fragments of sea mollusks-gastropods — is used in many elements of the mosaic: feathers of parrots and other birds, petals and buds of flowers of pomegranate, tulips and carnations. It is clear from the characteristic striated appearance, moire colours and fluorescence under UV light.

The second most used material of this tabletop is quartz. In addition to the coarse-crystalline varieties — amethyst and colourless quartz (rock crystal)and cryptocrystalline varieties — chalcedony and agate, this is the main mineral of jasper: microcrystalline rocks consisting of quartz and a mixture of clay minerals, giving them colour and thus a variety of textures. Flint — the natural mineral binding of silica in sedimentary rocks (limestone) — also consists of cryptocrystalline quartz with an admixture of opal — amorphous SiO2; in addition to strength, flint can demonstrate smooth colour transitions arising as a result of impregnation by oxides of various metals and, as a consequence, the diffused texture, called "Liesegang rings ".

The colour scheme of jasper сconsists of white-beige-red, bluish burgundy, and green and brown shades. The main advantage of jasper is a fantastic variety of textures, which, being properly selected, create the impression of painting made by nature. On this tabletop you can see brecciated jasper in the white and red petals of damewort, green, white and brown leaves of Arabian jasmine, brown and red branches of fruit trees, as if "growing" from the corners of the tabletop to the center, in the back of the wood snake, orange and green dried orange, purple and burgundy figs. The Sicilian orange-red jasper Diaspro di Sicilia was used for the pomegranate fruit, flowers and sepals. Patterned jasper in orange-brown tones was used for morning-glory flowers. To the naked eye, this jasper is almost indistinguishable from the marble leaves of the plum tree, the natural pattern of the stone is so skillfully chosen. Striped abdomens and chests of the insects — some flies, wasps, moths, as well as locust wings and caterpillar bodies — are composed of striped jasper and agates. Finely striped weightless wings of moths, flies and butterflies are made of greyish-white agate with occasional red flakes. Bright red, spotted fruits of dogwood and orange azarol seem to have been carved from Sabina agate. White with yellowish, pinkish and grayish shades petals of jasmine, cherry, hibiscus, dogwood, narcissus, tulip, rosehip, orange, cloves, hyacinth, honeysuckle, as well as yellow and orange fruits of cherries, plums, apple trees, pears, lemons, quinces and peaches are made mainly from Calcedon di Volterra, an area famous for its white alabaster production.

Transparent, yellow and orange variety of chalcedony — carnelian — is used for the juicy cherries, a Martagon Lily flower, and the wings of a hoverfly.

To give colour to the pale transparent materials and enhance their internal reflection metal foil was placed underneath them. This tabletop is no exception: the foil covers the backside of the cherries made of chalcedony, and the plums of amethyst quartz. Often to increase the intensity of the colour the foil was tinted: where the paint has faded over time, it is visible that the purple plum has acquired the colouring of natural amethyst, in which colourless rock crystal alternate with purple amethyst. For the plums and grapes amethyst aggregate was used, fragments of the geodes, which is visible in the direction of growth and alignment of quartz crystals. Quartz — is one of the hardest materials used in this tabletop. It is impossible not to acknowledge the skill of the masters who have worked with such a difficult material: it is hard, crumbles easily, but, nevertheless, the edges of the fruit are finished perfectly and fit one into the other with minimal gaps. Quartz also partially fills the volume of green stone jasper, the so-called Diaspro di Barga, which is heterogeneous in both colour and structure. It is used for the leaves of the hibiscus blossoms and the bindweed in the central part of the tabletop. Probably, the term Barga includes both jasper itself and other hard rocks (pietre durе) of green colour. These rocks, which are heterogeneous in structure, contain fibrous and flaky aggregates of complex silicate minerals — amphiboles, serpentines, micas — in the form of specs, blocks and strips "impregnated" with quartz. In terms of mineral composition and primary structure inherited, this rock can be classified as listvenite.

The bright, emerald-green material was obviously rare. It was used for several fruits of unripe almonds and some cuts of stems — Spanish jasmine and pink carnation. Like the previous material, it has a spotty texture ranging from white to grassy green. It is a so-called smaragdite, an emerald green mineral called actinolite. The word "smaragdite" comes from the ancient Greek Σμάραγδος and means "green mineral". Such a dense green actinolite aggregate with white spots is classified as jade nephrite. Jade nephrite — is an extremely viscous mineral with exceptional strength, which is difficult to process. In addition, the natural appearance of smaragdite is extremely rare, it could have been an accidental discovery at an asbestos deposit. However, rare minerals were often imported from abroad. One of the brightest examples of such imported material for Europe is Persian bright blue lapis lazuli. Lapis lazuli mining has been known in Afghanistan (Badakhshan province, Sar-e-Sang deposit) since the 4th century BC. From the 3rd century BC lapis lazuli was transported along the Great Silk Road, the part of it called the "Lazuli Road" — from the foothills of the Pamirs to the Middle East and then to the west. This gem was exotic in Europe: nothing like it was found here. Stone mining took place at an altitude of 4-5 thousand meters above sea level, transportation took a long time, so it is not surprising that this rare gem, which was also the main compound for blue mineral paint was valued as high as gold. It is believed that the best quality lapis lazuli is of uniform dark blue colour, almost without inclusions of other minerals. Such fragments are visible at the edges of the tabletop - in geometric ornaments with hearts and rhombi. This sort of lapis lazuli may contain typical inclusions of tiny pyrite crystals, whose bright yellow metallic appearance is often confused with gold. On the main part of the tabletop the blue flowers (ipomea, hyacinth, cornflower) and part of the bird plumage (blue tit, blue-tailed lory, blue jay and cardinal) are inlaid with heterogeneous pale blue lapis lazuli. Upon detailed examination it was found that lapis lazuli is combined with white sodalite and calcite and also contains small inclusions of pyrite crystals. The use of such heterogeneous lapis lazuli emphasizes the natural voluminous forms of flowers and birds, creating the illusion of wind.

The blueish-gray, with random streaks, flowers of bindweed are interesting because of their unusual colouring and the fact that they are rarely found in objects similar to this one. Samples have not been taken since the restoration treatments were completed, and the available range of research methods can only narrow down the range of possible materials. The colour and structure of the stone suggest a non-crystalline and possibly artificial material such as glass or obsidian. However, the fact that it was rarely used by the Grand Duke's Workshop suggests that it is a unique and rare material, which cannot be said about glass. A similar material is also known in nature - flint from the Arno Valley - Selce dell'Arno; colour streaks are typical for diffusion impregnation of flints, while bluish-gray is known in other forms of minerals, but the shades of the colour palette do not quite correspond to the material of the bindweed. At last, the colour and structure indicate minerals of a group of alkaline amphiboles, "impregnated" with quartz, — riebeckite, arfvedsonite. It should be pointed out that near Florence, the province of Grosseto (Monte Argentario commune) is known for having riebeckite, which is called "blue marble". However, the quartz impregnation of this material allows determination in situ only silica, which corresponds to flint, chalcedony and glass.

The dominant feature of the composition is an opened pomegranate fruit with even dark red grains made out of garnet picking from under its skin. The group of garnet has more than 15 mineral types; here – the most abundant one of them, almandine Fe₃²+Al₂[SiO₄]₃, was used. It is widely spread in metamorphic rocks and forms large and transparent dark red to cherry-violet crystals. Almandine is described in Agricola's works and was known as "Alabandian carbuncle" under Pliny the Elder. Ten crystals of transparent almandine in pink, red-violet, crimson, orange-red and intermediate shades with a substrate of gold and silver (?) foil, create a sense of volume and inner glow of ripe pomegranate grains.