Slow cooling process starting at around 500°C in a special oven. The newly moulded glass (or glass slabs) are cooled to prevent stress due to the different cooling times (and therefore different contractions in volume) between the surface and the mass of the object. At one time the annealing oven was situated in the upper section of the melting furnace and made use of the combustion fumes to re-heat the objects. Nowadays the annealing oven (also called the lehr or tempering oven) is separate from the melting furnace and may be an independent refractory chamber re-heated to 500°C (closed when production is complete and left to cool down naturally) or part of a continuous tunnel structure.
The operation carried out to obtain the tesserae from the glass slabs or smaller tesserae from larger ones. The tools used since ancient times are the hammer and chisel. The glass slab or disc is placed on the blade of the chisel and by delivering a sharp blow with the hammer a clean-cut break is made with no splintering.
To facilitate cutting, the surface may be scored beforehand with a diamond tool. Since it is more brilliant, the cut edge is placed facing outwards (except in the case of metal-leaf tesserae) in the mosaic. Today automatic or semi-automatic machines are used for cutting.
Cartellina (pl. cartelline)
A thin layer of blown glass (usually as thin as 0.2 to 1 millimetre, but occasionally as thick as 10 millimetres) covering the metal leaf in gold and silver tesserae. The cartellina is applied by firing and protects the metal leaf from oxidation. If the cartellina comes loose, a common phenomenon in ancient mosaics, it leads to the loss of the metal leaf and the tessera is discoloured.
The transformation of the vitrifiable mixture (batch) or frit in the pots (crucibles) into glass. The process takes place at high temperatures (around 1000-1200°C in ancient times and over 1400°C in modern furnaces). During this process the solid compounds dissolve completely (homogenisation) and the gases present as bubbles are eliminated (fining). Given the limited technological means in ancient glassmaking, fusion often lasted several days until a workable glass (i.e. sufficiently homogeneous and refined) could be obtained.
Inorganic material obtained by the fusion of a vitrifiable mixture to form a viscous plastic fused substance which solidifies on cooling without crystallising (an amorphous solid). Glass may be transparent, translucent, opalescent or opaque and colourless or coloured. It can be shaped when hot using various techniques such as blowing, moulding, pressing or casting. Glass objects may be decorated with enamel, painting, engravings, abrasion, appliques with metal leafs, etc.
In mosaic this term is used for tesserae produced with traditional| almost lead-free glass (less than five per cent lead oxide). Ingait glassmaking terminology, on the other hand, glass paste or pâte de verre is made by grinding glass into a powder (with the possible additions of a flux and colorants), which is then softened by heating in a mould prepared according to the cire perdue technique.
The furnace consisted of a bench, the shelf for the various pots (crucibles) over the fireplace, and a cubicle where the firewood was stored. The air for combustion entered through the same opening used for loading the wood, while the waste gases were exhausted through a central hole in the bench thus allowing their heat to reach the pots, and a hole in the furnace vault. In a level above the bench, on another shelf, the so-called ara, glass artefacts were annealed. Through one or several arched openings (mouths) closed by small doors made of refractory clay, the glassmaker carried out the various operations (placing the batch, gathering the fused glass, etc.). In modern methane -or oil- fired furnaces the flame moves round the pots and the temperatures are higher (around 1400°C); thus two-stage fusion involving the frit is no longer necessary.
A thin sheet obtained by beating gold (20 grams yields around i cubic millimetre or 6 square metres). In tesserae the colour shade is determined by the purity of the metal, the thickness of the leaf, the colour, if any, of the cartellina and of the support. Gold-leaf tesserae are made up of a glass support layer (usually transparent, at times opaque red or coloured) less than one centimetre thick. The metal leaf is then sandwiched between the support and a thin protective glass layer (the cartellina).
Mosaic made of tiny tesserae (at times less than one millimetre long) obtained from thin rods of uniformly cut polychrome glass paste.
The decoration of a surface with small juxtaposed fragments (tesserae) of glass paste, enamel, pebble, or terracotta arranged according to a design (sinopia) traced on a specially prepared plaster base. The earliest glass-paste wall mosaics date back to around the first century AD.
The container or crucible in which the frit and cullet (broken glass) mixed with colorants or decolorisers are heated to fusion point to prepare glass. They were usually made of silica and refractory clay, although at times other locally available refractory materials were used, such as magnesium silicate. Modern pots vary in shape and size. They contain anything from a few kilos up to several tonnes of fused glass. Before the industrial age, they only ever contained a few dozen kilos at most.
Clay-based material mixed with silica able to resist heat and the contact with fused glass without being deformed or corroded to any great extent. These materials are used to make the melting furnace and pots (crucibles).
The most common form of silica used to make glass. Sand is not usually pure silica, since it contains other minerals in various quantities depending on the location of the sand deposit. Before being used it was usually washed and exssicated and then sieved to eliminate the larger grains, which are more difficult to fuse. It can also be prepared from ground quartz pebbles.
The thin mosaic strip or disc (pizza) is obtained by pressing the fused glass on a flat surface or by drawing through two cylinders. After annealing, the tesserae are obtained by cutting with diamond-pointed instruments or hammering with hard steel points or blades.
Smalto (pl. smalti)
In the world of mosaic smalto stands for particularly brilliant, completely opaque, usually prepared by adding crystalline material (corpo) and coloured material (anima) to the colourless or coloured fused glass. These smalti provide a vast range of shades (several thousands) with obvious advantages over the few dozen hues of glass pastes. The term smalto is also incorrectly used to describe metal-leaf mosaic tesserae. In general glassmaking it stands for enamel, that is an intensely coloured low-melting (i.e. it softens before the support material) usually opaque (but also translucent or transparent) glass used in decoration or to clad gold silver or copper objects as well as blown glass and ceramic.
Small, usually square pieces of glass or other material used to make a mosaic. Their size generally ranges from a few millimetres to two centimetres long and five to ten millimetres thick. The term derives from the Greek word meaning "four-sided". They are obtained from a glass slab, initially incised by a diamond-edge tool (or at least a tool made with material harder than glass and thus able to dig into the surface). The glass slab is then placed on a hard-steel sheet and struck with a hard-steel chisel (martellina) to break it up into small pieces. The same technique may be used by mosaicists to break up the tesserae further, as
required. The origin of glass mosaic tesserae is uncertain, but they probably date back to the first century AD and may be made of glass paste, smalto or metal leaf.