Museo del Vetro

Glass Museum

IN-DEPTH

Types of glass and glassmaking techniques in the 18th century

Aventurine Vitreous brownish-red coloured paste with metallic sheens due to the presence of shiny flakes of copper crystals. The first recipe for creating aventurine is found in the manuscript by Giovanni Darduin (1644) but already in a letter dating to 1614 there is mention of “a sort of stone with golden stars inside”. The technique for making aventurine glass consists of adding lead and/or tin lime, red copper oxide, iron oxide, and strongly reducing substances, in small doses and various stages, directly into the molten glass in order to reduce the copper oxide into metallic copper to the maximum extent, which precipitates it within the glass in crystalline form. Having reached a certain stage, the oven is extinguished and left to cool naturally over a few days, during which a slow but almost complete separation of the metallic copper from the glass base takes place. This compost is then extracted in blocks from the oven, cut and polished cold as with a hardstone or worked under heat with special skills.

 

Chalcedony Variegated opaque glass, red in transparency, with multicoloured veins, in imitation of semiprecious stones like streaky agate, natural chalcedony, onyx, malachite, etc. It is obtained by mixing scraps of white opaline glass, coloured opaline glass and crystal. Once the fusion is ended a mixture of colouring compounds is added repeatedly, such as nitrate of silver (which also creates matting), cobalt oxide, potassium dichromate, etc., that are dispersed by stirring the fusion. The streaks, caused by the oxides, are then further enhanced by the curving motion conferred by final blowing. The most ancient known specimens of this variegated opaque glass date back to the Roman age and were produced in Alexandria, while the first known mention of chalcedony dated to 1460. This type of glass was greatly successful between the end of the 15th and the beginning of the 16th century. In blown chalcedony glass, cups, saucers, bottles, etc., between the 17th and 18th centuries, small spots or veins of aventurine may be noticed at times amongst the streaks: it was fragmented into chips, then collected and englobed into the glass at its paste stage.

 

Bohemian style Crystal At the beginning of the 18th century crystal from Bohemia eclipsed the crystal from Murano in brilliance. During this period a certain Sola, in contact with producers of Bohemian glass, got the exclusive for importing Bohemian scrap glass to Murano to add to the vitreous mixture, in order to enrich its contents. In 1737 Giuseppe Briati received an exclusive from the Serene Republic for the production of a crystal in Bohemian style which he had conceived, also due to his probable collaboration with Bohemian glassmakers. It was a glass with consistent percentages of lead oxide in addition to potassium oxide, which could be worked while hot according to the Murano tradition. Even chairs, small tables, wardrobes, mirrors, lamps, etc. made of finest crystal in azure blue colour were numbered amongst Briati’s famous works in the second half of the 18th century. Attention is brought within this category of products to a mirror with a gilded wooden frame and a Rococo chair in carved and gilded wood, with inserts of aquamarine glass, on display at the Museum in Murano. It was also possible to create blown glassware without exceedingly thin sides with this type of crystal, or objects in filigree, mostly with multicoloured braided ribbons, rounded off with pinched filaments and vitreous flowers in coloured glass.

Crystal of Bohemia: glass with a high content of potassium and calcium, conceived in Bohemia in the first half of the 17th century using depurated potash and limestone. It is a particularly transparent and colourless glass, and is usually devoid of lead oxide, being particularly suitable for decoration by etching and mill-grinding.

Lead Crystal (flint glass): typology conceived in England by George Ravenscroft in the second half of the 17th century. Its formulation featured the use of quartz pebbles, as well as nitrate potassium, tartar, borax and red lead with a concentration in lead oxide between 24 and 30%. Due to its brilliancy this glass is well suited for the production of objects with a very faceted surface and may be worked with any etching technique.

 

Deseri or Trionfi. The trionfo is an elaborate centrepiece made of several pieces laid together at the centre of a laid table to compose a grand decoration. In the 18th century, table centrepieces, called deseri in Venice, were produced in a great variety of shapes and materials: glass, ceramic or porcelain, silver and also cast wax. In grand banquets the deseri were sumptuous and of great dimensions and the various elements were made so as to create complex figurative compositions. They made up truly scenic sets with a historical or mythological flavour, otherwise they might offer a glimpse of Venetian life on holiday by representing games, gardens, cavalcades, etc., as in the case of the one set up for the St. Mark festivities in 1767. The most important were made in Venice by Giuseppe Briati, and subsequently by Giacomo Giandolin, and sometimes represented an Italian garden with statues, fountains, columns, arches, balustrades, flower pots, often complete with bowls, saucers, salt cellars and oil cruets, all in monochrome or coloured glass.

Girasolor or Sunflower. Opalescent glass with an orange sheen, more matt than opaline, present in Murano as an achievement in glass technology since 1693. This glass appears slightly opaque due to the presence of lead hydrogen arsenate crystals in the vitreous mixture, as due to their size they confer an original colour to the glass, which appears bluish when observed under reflected light and light brown, or pink, when observed without light striking the object directly.

 

Engraving, carving and mill-grinding The technique of engraving, already in use in the classical age, may be found occasionally on artefacts from the Middle Ages. The modern custom of engraving on the wheel or mill to decorate glass was developed in Prague and then in the whole of central Europe between the end of the 16th and the beginning of the 17th century, after the thin Venetian calcic-sodium glass, unsuitable for relief-work, was substituted by Bohemian calcic-potassium glass, much thicker and more resistant, and by English lead crystal. Engraving on glass with a small abrasive stone or metal wheel, derived from work on hardstones or rock crystal, was imported to Venice by German engravers at the end of the 17th century. The basic instrument is a small turning lathe, at one time driven by a pedal. In view of the type of engraving, copper wheels with differing sizes and profiles were used, applied alternately to the lathe axis during the various stages of work. The copper wheels engrave an opaque decoration by lightly scratching a design transferred onto the glass by means of a dusting of talcum fixed with Indian ink. Then, to make the whole operation more supple, an oily suspension of abrasive powder is scattered on the wheel rim surface. Erstwhile abrasives were quartz or granite powders mixed with paraffin and rapeseed oil. Nowadays powder of silicon carbide (carborundum) or corundum is used. Pumice or red iron oxide were used for final polishing, today the use of cerium oxide is widespread. The procedure for mill-grinding and carving includes outlining on a cast-iron wheel (more recently in carborundum or very hard abrasive material) fixed to the flattening part (when the object is worked on the surface) or on the lathe (for faceting and grooves), continuously fuelled by a jet of abrasive sand and water. The grinding phase takes place by means of a very fine-grained sandstone mill, also fuelled by water. The polishing phase takes place with a cork-bark wheel wet by a pulp of water and pumice, to make the milled surfaces transparent: lastly comes the stage of shining, obtained with a felt wheel wet by a pulp of water and iron oxide. Excess surface glass is removed through carving, while deeply etched geometrical designs, then polished, are obtained with grinding.

Chandeliers The use of glass for the production of chandeliers is attested starting from the 17th century, when the ancient shapes (already in use in the Middle Ages) in metal and wood are abandoned. The shape of a chandelier is generally characterised by a central column support with long arms, suitable for bearing candles or lamps. In the 18th century, Giuseppe Briati (1686-1772) created chandeliers (ciocche) made with a metallic structure covered in transparent, even coloured, glass, formed by tubular elements in blown crystal, with decorations of colourless or coloured flowers, leaves and fruit (an investment system). These chandeliers are divided into column ciocche, characterised by little blown and twisted columns, Chinese ciocche, shaped like a Chinese pagoda with Oriental inspiration elements, and modern ciocche, probably recognisable in the great chandeliers with rich multicoloured floral decoration, an example of which is housed at Ca’ Rezzonico.

 

Elements of a chandelier:
Crown: terminal (high) part of the chandelier
Chain: ornament, festoon applied to the arms of the chandeliers
Charm: pendant linked by a copper wire to the arms of the chandelier, to leaves, etc.
Crest: decoration high up, in the terminal part of the chandelier, made of flowers and leaves
Final: lower terminal part of the chandelier, made of the actual final and by the fiocco bow, which is a slightly larger pendant
Flowers: decorative elements of the chandelier
Fiocco: bow (see final)
Fogie leaves: decorative elements of the chandelier. They are high, low, squarae, a pinzi tirai, with sberlotto, etc.
Fondin: bowl in glass covering the fondin bottom (disc) of iron or wood, equipped with holes into which the glass elements of the chandelier (arms, leaves, flowers) are inserted; the passasorze gasket, the finale and the fiocco are found in this order at the bottom.
Goto: drinking glass
Massocca: element of the chandelier shaped like a small mallet
Papaor: small glass cylinder, attached to the arm of the chandelier, in order to house the candle
Passasorze: cylinder in glass shaped like a spool, serving as a gasket between the fondino and the finale of the chandelier
Perolo: pendant shaped like a pear (see charm)

 

Lattimo Also called laterolo and porcellano (in imitation of Chinese porcelain). Opaque white glass obtained by mixing the crystal amalgam (impure vitreous mass, obtained from the initial fusion of raw materials, to make the final fusion quicker), a bleach, manganese dioxide, and a matting agent, tin dioxide (to be found in recipes introduced under form of tin oxide and/or lead and tin oxide) or lead and arsenic or calcified bone ashes, used specially for achieving a semi-opaque or opalescent white. Precedents on the use of a white opaque glass already existed with the Romans. From the 14th century onwards it was used for enamels to be applied on gold and silver. From the second half of the 15th century, with the purpose of imitating the first Chinese porcelain then reaching Europe, lattimo was used for blown (suppiadi) artefacts mostly destined to enamel decoration and gold. Venetian lattimi in the 18th century differed from those of the previous centuries due to the matting agent (lead arsenic) introduced into the mixture, while the oldest pieces were matted through lead and tin oxide. Decorations on this glass were multicoloured, or monochrome enamels and gold, showing genre scenes, chinoiseries, mythological objects, and Rococo motifs. Lattimo glass production between the 17th and the 18th century spread to Germany, in Bohemia, in France, in England and in Spain. Nowadays the matting agents employed are made up of minute crystals of calcium fluoride and sodium fluoride which separate rapidly and in noticeable quantity from the molten glass during the cooling stage. The homogeneity with which the crystals precipitate is favoured by the presence of zinc oxide in high concentration.

Rui These rollers or circular sheets of modest size are obtained by centrifugal force subjecting a globe of glass, appropriately pierced, to a rotation movement until it is completely flattened. The panes for making windows were frequently obtained from such discs. Smaller discs, listed in charts since 1405 as rotuli or rui, were left whole and linked into series with lead. The larger sized ones were also cut so as to form windows in stained glass, with figures, linked by lead.

 

Enamel Glass melting at low temperatures (700-900°C) destined for decoration of gold, silver and copper in gold-smithies, and also in blown glass or ceramic. It was poured into plates on which the factory seal figured, and sold to the goldsmiths. Mattness was given by tin dioxide. Already in 1317, we may find opus smaldorum mentioned in Venetian charts.

Mirrors The first glass mirrors were produced in the 16th century and had a great circulation in the 18th century. In Murano, the Dal Gallo brothers were granted the production of mirrors of glass “according to a secret procedure” in 1570. A little later glass-sheet mirrors were widely produced by the Murano mirror-makers (spegeri) who set up into a corporation in 1569. Mirror sheets in Murano were prepared in the furnaces belonging to the colonel of the glass Art of the quari and sheets. The masters who made mirror sheets were called quari masters. Mirror sheets were obtained with large blown cylinders (vessighe), uncovered and opened when hot by cutting. Then, keeping the cylinder hot, it was opened to shape a sheet which was laid upon a layer of ash (Lorraine method). The raw quari were then delivered to the mirror-makers for flattening, polishing and the application of tin leaf. In 16th century Venice the mirror sheets were silvered at the back with an amalgam of tin and mercury, but after 1840 the system of silver or platinum deposit was adopted. Another method for making mirror sheets was the so-called “crown” one, in which the glass sheets were obtained by blowing a sphere that was transferred from the blow-pipe onto a metallic rod and then cut to allow its opening. Then the glass was rotated rapidly until it reached the shape of a large flat disc, tempered and finally cut into rectangular pieces. Another method was the French one, with which the glass sheets were made through pouring molten glass into custom rectangular moulds towards the end of the 16th century. Today flat glass production is carried out with completely automated new technologies.