Glass / Ceramics (Smalti, Roti)
For over 2000 years, glass has been used in the making of tesserae. The basic elements of glass are silica (usually sand) mixed with a flux (soda or potash) and compounds called stabilisers. To this mixture or batch are added colouring agents such as copper, chrome and selenium. The mixture is then heated in a process called fusion, in which the gases, in the form of bubbles, are eliminated and the solids are dissolved. This heating process may last for days. The result is a viscous substance, a molten glass, which is poured onto a flat surface in the form of a disc (”pizza”) or oblong slab, and cooled in a carefully controlled process called annealing.
The resultant glass plate is then cut into tesserae by hand, using either a hammer and chisel or a semi-automated cutter. the variants of colour, opacity and brilliance have intrigued glassmakers throughout the long history of these amazing materials, and many secrets or recipes, known by the few, are carefully guarded.
In its’ earliest use, probably in the first century AD, the tesserae were generally of glass paste, not always opaque but more often semi-translucent. these can be seen, for example, in the Roman mosaics of Jordan. During Byzantine times, smalti in the red and yellow shades were produced with a higher lead content, which gave a more brilliant optical effect. During the fifteenth century, on the glassmaking island of Murano in the lagoon close to Venice, the glassmakers began to produce the material we know and love as (Venetian) smalti - an opaque material of intense colouring with a wonderfully brilliant reflective surface. This brilliance was obtained by greatly increasing the quantities of lead oxide. Another advantage of this addition was the greater facility when cutting the material, since it is less brittle and therefore less prone to splintering.
Venetian Smalti is normally bought by the pound or kilo, in a colour of one’s choice. In Venice they can offer about 5000 colours and they will make up any other colours required. Sample colours are made available by each company. Colour can be ordered to an exact shade but the cost would be greatly increased. Venetian smalti “pizza” can be bought whole, but usually the glass is bought already cut in a size approximating just over and just under ½” or 15 x 10 mm. Any irregularities of size and colour add interest to this exquisite mosaic material, and pre-determine a lively surface whenever used.
Smalti filati are threads or glass rods of Venetian smalti, used almost exclusively in the making of miniature- or micro- mosaic. They were first used in the eighteenth century by the Studio of Mosaic at the Vatican to suit the taste of the time, when mosaics were created to imitate oil paintings.
Gold smalti (metal-leaf glass) is gold which has been used in mosaic work since the fourth century. It is sumptuous material which was used at its’ height in late Byzantine times when its’ qualities were understood in practical and spiritual terms.
Gold Tesserae are made from a glass support base - these days transparent and coloured yellow, green or aquamarine, and ranging from around ¼” to ½” (3 to 7 mm) in thickness - and an upper protective layer of thin glass, the cartellina, which is generally clear but may be coloured for producing coloured golds. Interspersed between the two layers of glass is the metal leaf, a 24-carat gold, silver copper or alloy of gold and other metals. Sometimes the base is granulated to give a rippled finish, though generally the surface is smooth. Colours range from deepest gold to white or may include blue, green, or pink - the colour being made respectively by the cartellina, the metal or the backing glass. A sample card of the golds produced today is obtainable, but they are subject to availability. At present there is only one company in the world which produces this most exquisite, brilliant, and elusive of mosaic materials.
VITREOUS GLASS
This is the most commonly available mosaic glass and has the advantage of being cheaper than Venetian smalti. It is standardised mosaic glass, used industrially, and is generally bought by the sheet (12″ x 12″, 30cm x 30cm). Being a manufactured glass, vitreous glass is regular in format - normally about ¾” x ¾”, 2cm x 2cm - with a smooth upper edge, sloping shallow sides and a generally roughened bottom edge. The smooth side is used whenever a completed even surface is required. However, the roughened surface can be used for textured effect. Among the wide variety of colours and finishes that characterise vitreous glass are the brilliant imperial primary hues, the milky opaque colours, and finishes which are grainy or have a coppery irridescence.
CERAMIC MOSAIC TILES
Clay has been used in mosaic since the ancient times in the form of terracotta tiles, glazed Samian ware, and bricks. Today clay tesserae can be bought glazed or unglazed. In the unglazed tile, the colour is uniform throughout, while in the glazed tile, the glaze is superficial. They are readily available and inexpensive to purchase.
Characteristically, their colours are earth colours, and they can be used whenever a Greco-Roman type of palette is needed and when marble is hard to acquire.