By Natalya BREDIS, head of the Department of Optico-Physical Research, National Restoration Research Center, Ukraine (Kiev)
In times of old, one of the most coveted gems in many parts of the ancient world, especially the Arab East, was known by the name of "lou-lou". Its name in Old Russian chronicles and legends was "zhenjchug" or "zhemchug"-a derivation from Chinese "chjenchzu". It was used in a whole range of ornaments. Strings of pearl beads adorned the garments of princes and gentry and also of the common folk. In that day and age embroidery with pearl beads was regarded in Russia as a purely feminine craft and whole workshops were set up in cities and towns in which teams of females of different ages worked long hours in well-lit "svetlitsy" chambers. Our museums possess a wealth of objects decorated with pearl beads. One of the questions facing art experts now, however, is this: Are all of these different pearls really what we think they are? This question is now on the agenda of what we call scientific attribution.
WHAT IS PEARL?
Speaking in strictly scientific terms, natural pearl is a calcareous concretion of globular or irregular form. A pearl is formed around a foreign body either against the inner side of a shell or within the mollusc, sealed off as a cyst. It consists of concentric films of nacre, consisting of aragonite, which also forms the smooth lustrous lining (mother-of-pearl) in the shells of pearl-bearing molluscs. This radial or concentric structure is the main distinction of a genuine pearl.
Its main components include aragonite, organic conchiolin, water and micro admixtures of aluminum, barium, iron, silicon, manganese, copper, and molybdenum the levels of which can vary both in quantities and quality. Aragonite present in the pearl shells and beads belongs to the class ofcarbonites Ca (?03) with its hardness factor, by the Mohs hardness scale, being 3.5 to 4.0. Its density is from 2.65 to 2.75 g/cm 3 . Its structure fits that of the shell with ferrous cells of the mantle generating microcrys-tals of aragonite in the form of fine scales of an oval or polygonal shape. Glued together by conchiolin, they are superimposed one upon the other, like brick cladding, forming layers. In the process of pearl growth these layers continue to increase, showing on microphotos what we call "tracer" surface. Thanks to this structure natural pearl possesses viscosity, resilience, elasticity and resistance to mechanical destruction. Genuine pearl, as different from artificial one, bounces off a hard surface when it hits one.
Since ancient times the quality of pearl was measured by its shape, size, color and shine with preference being given to round or pear-shaped beads and the colors, or shades, of choice including pink, lemon, golden, bluish or black.
The color of a grain of pearl cultivated by a mollusc depends on the translucency of its outer shell and the nature of the substrate (calcium carbonate in the form of calcite or aragonite- colorless or white, conchiolin in a fine cut-yellowish, and in thicker layers-brown and sometimes black). Some of the weaker shades of different colors depend on the presence of small amounts of micro admixtures in the absorbed water.
Native pearls are mostly of irregular shape. The Muscovites of old nicknamed them as "uglies" or "hornies" and high-quality beads were respectfully called "rollies". By modern classification beads with
natural deformities are referred to what we call the baroque type.
By their size pearl beads can be fine-up to 2.5 mm, medium-from 2.5 to 6 mm and large-of more than 6 mm in diameter and even bigger stones, of 7 to 8 mm, are quite rare.
The qualities of natural pearl depend on the environment of the pearl-bearing molluscs. If it could move freely in different directions its "products" possess the best spherical shapes. When its movement was somehow restricted to one direction, the layers turn out to be oval or drop-like in shape. And the key factors also include the depth of habitation, stable temperature, bottom relief, water currents and its levels of oxygen.
Linked directly with shape is another property-shine or glitter, which depends on the orientation of the fine aragonite scales and their levels, or depth, of translucency.
Since pearl consists of carbonite substances of biogenic origin, its physical condition can be negatively affected by rising temperature, prolonged exposure to weak solutions of acids, alkali and even soapy water which triggers decarbonization. On the other hand, one can "brighten up" a pearl bead by washing it in a weak solution of hydrochloric or acetic acid which removes the eroded surface layer and the products of erosion.
This sensitivity of pearls to acids prompted the ancients to use beads as a kind of "quality sensor" for drinks. A pearl was usually dropped into a glass of wine offered to a guest of honour and this was taken as a sign of special respect. If the wine was old, or poisoned, the pearl lost some of its gloss and shine.
FROM SEAS AND RIVERS
Valued most at different periods of history were pearls from the Orient- from the Persian Gulf and off the shores of the Arabian Peninsula, and also from the Bay of Manaar off the
northwestern coast of Ceylon. Pearls of that origin, called "oriental", stand out for their special shine and translucency.
And the bulk of the 10-12th-century pearls came from the rivers of Scotland, France and Ireland, with Britain being the main supplier of pearls to Rome. But these pearls were of rather poor quality, small and of irregular shapes, and were mainly crushed into powder and used in medicinal preparations.
In Old Russia (with the earliest chronicle mentions of the 15th century) pearls were obtained in quantities from the Northern Dvina and its tributaries, and in rivers near Novgorod the Great from where bigger beads came which were used for necklaces.
Fresh-water pearls differ from marine ones by the type of what we call deformities, which result from their rapid growth in rivers, and also by their chemical composition. Typical of them is the presence of manganese in admixtures the identification of which ranks among the major diagnostic clues.
One distinguishes native, artificial and imitation pearls. Although from the commercial angle artificial, or cultured pearls, came on the scene in the early 1920s, the Chinese were using for some 3 thousand years the shells of molluscs for coating some small objects with layers of mother-of-pearl.
To do that the shells of pearl-bearing molluscs were partway opened with a bamboo stick, and small figurines made of bone, wood or bronze placed inside. After an interval of several months, or after 2 or 3 years, the insert "clad" in the mother-of-pearl was taken out. This technique was used, for example, for making figurines of Buddha. From the 13th century local craftsmen used the technique of making cultured pearls by inserting tiny beads into molluscs.
For some considerable period of time experts in this field were trying to work out objectively reliable
methods of pearl identification. The main difference between natural and artificial, or cultivated beads lies in their inner structure: in the former all layers on a cut are located concentrically around the core, and in the latter case there is a solid core surrounded with an external concentric shell.
Used most often is what we call the classical and nondestructive identification method of X- raying which rules out any mistakes. As X-rays pass through a sample, a picture indicating its origin is formed. In the case of a genuine pearl it is a dotted hexahedron and in all other cases the image is a tetrahedron (or in the shape of the Maltese cross). The difference is the result of radiation scatter, or diffraction, in the crystalline structure of aragonite layers.
A more recent method, suggested in the 1980s by American scientists A. Alexander and N. Sherwood, is known as radiographic technique. In cases of natural pearls the picture produced by sample irradiation has in its center a sharper spot than along the periphery, with soft and contrastless transitions. In cases of artificial or cultured beads there is a dark borderline between the more and less clear areas of the image.
As for telling pearls of river origin from marine beads one has to resort to ultraviolet luminescence. A freshwater bead under irradiation emits greenish glow (due to the presence of manganese), while marine beads produce no such effect.
ACROSS TIME BARRIERS
Depending on the purpose of investigation, experts use different attribution techniques. But even with the present state of the art, so to speak, identification of ancient pearl, especially in
archeological finds, is a difficult task. This is because in such situations it is important to have an idea not only of beads formation and ageing, but also of the diagenetic(*) changes in their physical conditions at different stages or erosion and transformations of the carbonate material.
Relying on the available general information and also on the strength of analysis of chemical composition and structural matching, experts studied medieval beads from decorations on women's dresses of the time of the 10-13th-century Kievan Rus.
A treasure trove, dating back to the 12-13th centuries, was found in 1997 during excavations in the grounds of the St. Michael "Golden Dome" Cathedral in Kiev (excavations were conducted by the Architectural-Archeological Expedition of the Institute of Archeology of the National Academy of Sciences of the Ukraine). The find included gold and silver objects, rings, three-bead earrings, bracelets and 23 "grivnas"-old monetary units of the Kievan type. One of the bracelets was styled in the Iranian fashion and originally consisted of interconnected golden elements adorned with lacework and amethyst inserts. In grooves along the edges were pearls strung upon a fine silver wire. The segments of the bracelet and the silver wire were preserved but in bits and pieces while the pearls remained in a satisfactory state of preservation. All of them were of medium size of 2.5 mm and are of the same round shape and of dark color with metal glitter. This color, however, hardly matches the original. It was established that after 200 years, as a result of the decay ofconchiolin, pearls can assume a darker color-like blue steel.
A chemical analysis of the pearls revealed some carbonate admixture and no micro impurities. Micro structural investigations of the inner structure at a section with the help of an electron microscope revealed a concentric (annular) pattern consisting of densely packed aragonite laminae. Under 2,000 fold magnification one
* Diagenetic-relative to transmutation. -Auth.
could also see, apart from the fme "tracer" structure of the surface, formed in the process of growth of the bead, also some radially oriented microcrystals of aragonite of which every individual ring of the structure is made up.
All of these details attest to the natural origin of these pearls making it possible to refer them to the precious "oriental" category.
And it was interesting to compare them with beads from a diadem of the 12-13th century Kievan Rus whose pieces are preserved in the Museum of Historical Rarities of Ukraine in Kiev The embroidered diadem belongs to royal feminine decorations from a treasure found in Kiev in 1939 during archeological studies of the basement of the historic Desyatinnaya Church. The old jewellery was found in a cache in the western part of the central nave and must have been hidden there after the church was sacked by the invading Tatars in December 1240.
According to archeologists, it was near the bones of a woman, who must have perished under the ruins of the old church, that the hoard of valuables was found. This included the diadem consisting of 9 fine golden leaves sewn upon a textile base of cotton threads. Each of the leaves, or blades, had in the center a relief image of a lily flower against the background of pearl beads.
Yet, as was proved by subsequent studies, the precious decor was... but an imitation in which a whole metal leaf was replaced with a dense layer of fine-grain cement with some glue substance of dark-brown color in between. Under it there was some fine foil from an alloy on the basis of gold with an admixture of silver and copper.
On the surface the article was enveloped in a transparent organic film.
At first sight, the pearls did not look like genuine or natural ones. Of small size (1.5-2 mm), with the exception of the carbonate substance, they contained micro traces of strontium, copper, lead, zink, and iron. The strongly corroded surface of the beads consisted of fine and radially oriented scales. The brittle beads disintegrated when touched, revealing a hollow inside the spheres.
This kind of structural peculiarities does not belong to the hallmarks of natural pearls. But in this particular case one must bear in mind that the diadem was exposed to a fire of the 13th century and during the next 700 years it was under a negative influence of some soil solutions. The corroded condition of the surface can be explained by the processes of decarbonization, typical of decaying pearls. Specialists know that this process is promoted by drying up and disintegration of the organic substance, mechanical lesions, erosion or melting of the surface layer, a transition of the aragonite into calcite (in consequence of a prolonged exposure to high temperatures), cracking due to sharp changes of the environment. Taking into account all of the above factors, one can draw the conclusion that the diadem was adorned with some inexpensive small natural pearls which had gone through all the stages of "ageing" and "dyeing out" and were preserved to this day only in the form of "decarbonized" remains. It is nonetheless of considerable historical value as proof of the ancient ornaments-making techniques brought to Kievan Rus from other countries.
And one more bead, 1.5 mm in size from a pendant dating back to the 12th-century Kievan Rus (originally believed to be "pearl") was in for expert analysis from the National Museum of History of Ukraine. But under slight pressure the "pearl" broke into tiny fragments, being hollow inside. At fracture edges of the sphere we saw a conchoidal structure, typical of glass surface, and on the inner side the shell was covered with a fine silvery layer. Additional analysis of the chemical composition by the method of X-ray spectral fluorescence revealed the presence of lead, iron and copper in the absence of any signs of some carbonate material. Thus all this evidence confirms the fact that the "pearls" trimming the pendant were nothing but imitation made of glass beads.
Summing up, there is no denying the obvious fact that diagnostic analysis and assessment of pearl, and especially from archeological finds, requires some very "individual" approach and due caution. Ancient pearls, like ancient history, are full of mystery and trying to penetrate this veil of time is quite a challenge for modem research.
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