Museo del Vetro

Glass Museum

IN-DEPTH

Types of glass and glassmaking techniques from the 15th to the 17th centuries

Colouring of glass. The colouring agents are generally metallic oxides which, added to the glass mixture, develop their action during the fusion stage, determining the colour of the glass. For instance, cupric oxide produces an aquamarine green tonality, cuprous oxide makes red, cobalt oxide blue, the combination of iron oxide with chrome oxide green, manganese dioxide, depending on the quantities employed, violet and black, iron makes brown and antimony yellow. Then there is colouring such as the range going from yellow to red which is produced by colloidal suspensions of microscopic particles, that separate from molten glass during cooling. Furthermore, ruby colouring can be obtained both by using a small percentage of gold, and both by a limited amount of copper. Obviously, the tonalities will be different, because lead oxide is used in the first case while a substantial dose of tin is in the second case. Opaque coloured glass is obtained with the same means as the coloured transparent ones, by using white opaline glass as a base.

 

Crystal. Colourless glass of extreme purity, similar to rock crystal, obtained in Venetian glassmaking around the mid-15th century through mixing quartz powder (vitrifying element), derived from grinding pebbles from the Ticino river with the ashes of marine-swamp plants from the Mediterranean basin boiled, ground and sifted containing soda and potassium carbonate (fusion element). Manganese dioxide was then added to these raw materials, as a bleach. Actually, this latter substance had already been known as a bleach since 1290 in the Murano furnaces, but almost two centuries were needed, before techniques and deeper knowledge of the peculiarities and the potential of manganese dioxide were refined in order to achieve a clear glass, devoid of impurities and bubbles, as well as extremely thin. Before then, all metallic oxides present as impurities in the natural raw materials employed used to colour the vitreous mass naturally, with tonalities varying from blue-green to yellow-green according to the reducing or oxidising conditions within the fusion environment. Murano crystal is a calcic-sodium glass, which makes it more suitable for a long and complex manual working. Bohemian crystal is composed of a calcic-potassium component, usually without lead oxide. English crystal is a very brilliant potassium glass with a high concentration of lead oxide.

Gold-leaf decoration. Gold-leaf is placed against the hot glass paste during the initial phase of the work, then it is covered by a further layer of glass taken from the crucible. After the definitive blowing of the vitreous paste the metallic leaf within breaks into small fragments or into dust. This technique was introduced in Murano in the second half of the 15th century and resumed in the second half of the 19th. The gold may also be applied onto the surface of a glass object. The operation consists in placing a gold-leaf against the cold glass, applying glue, and subsequently etching it with an ivory or bone awl to obtain the desired pattern. Through further work with heat the gold-leaf can be covered and protected by another thin layer of glass. Another technique consists in using a paste made of finely pulverised gold precipitated with mercury and fusion elements, all amalgamated thanks to a greasy essence. This supple substance is applied with a brush onto the vitreous surface, which may be further modified through a scoring process, once the solvent has evaporated. The object thus decorated is then passed through a muffle oven.

 

Filigree. Decorative technique, conceived by Venetian glassmakers between the end of the 15th and the first half of the 16th century to decorate crystalline blown glass when hot. The technique features the use of small crystal rods (canne) inside which threads of opaque (lattimo or milky) or coloured glass are found, structured following an axial symmetry or a series of spiralling curves. From this, it may be clear that the preliminary stage of the technique in question is producing a glass rod made of an outer sheath in crystal or lightly coloured transparent glass and an inner core in white opaline or coloured glass. The rod is cut in pieces about 20 cm long, placed close to each other and aligned on a metallic slab covered with fire-clay. The whole is put inside the oven so that the cane pieces, slightly softening, begin to stick to one another. In the meantime, after extracting a certain quantity of transparent glass from the crucible with the blow-pipe, the mocaura is prepared, a blown glass paste open at the base, with which the rods placed on the sheet will be collected. A cylinder with the rods close together is obtained in this manner. After the marbling operation and further heating inside the oven, the “sealing” between rods is perfected by pinching the tips free of the cylinder with suitable instruments, such as callipers and special scissors. Once the filigree draft of the object is ready, work proceeds by hand as for any blown artefact.

Half filigree: decoration with parallel rods, having a straight inner thread. It is obtained by preparing rods characterised by a white or coloured straight inner thread. The glassmaker picks up the rod sections, previously placed on a slab heated at the mouth of the oven, with a lightly blown crystal gob, enclosing them with the marbling process, then welding them with the callipers and the cutter at the opposite end from the blow-pipe so as to close the cylinder. At this point, after twisting the vitreous bulb, if a diagonal or a spiral rod direction is desired, further blowing is carried out, attaching a small bridge at the opposite end and detaching the blow-pipe, then passing on to the opening and modelling of the blown artefact.

Netted filigree (Redexello): filigree characterised by its doubly interwoven disposition of the rods used for the decoration. It is obtained by a half filigree blown artefact which is opened into a cup shape after attaching its opposite side onto the bridge and detaching it from the blow-pipe. The glassmaker blows another identical half filigree into this first artefact, but twisted in the opposite sense. Heat-welding the two sides produces an inner decoration similar to a delicate net in white or coloured glass, while a minute air bubble remains caught in correspondence with each mesh.

Twisted filigree (Retortoli): filigree in which the rods are set in a twisted thread or spiral and enclosed into transparent and colourless glass.

Glass technique patented by Filippo Catani in 1527. The glassmaker previously prepares round-section crystal rods, with white or variously coloured glass threads present inside and twisted into a spiral, arranging equal sections of this rod on the bronzin or slab. The rods get picked up with a blown crystal cylinder, sticking all around it, he then welds them at the ends and by blowing and working with marbling, he obtains a blown artefact with a regular lace-like effect radiating out from the bottom. All this within a depth just a few millimetres thick.

 

Diamond point engraving. Very superficial and light engraving with a diamond point or a flint, particularly suitable for thin glass. This technique, used for the first time by the Romans, was introduced in Murano in 1534-1535 by Vincenzo d’Angelo, who obtained a “privilege” in 1549 to decorate mirrors and blown objects with a diamond point. Following the imitation of Murano manufacturing methods, this technique spread all over Europe. The process consists in applying a design on the object to engrave by means of talcum dusting, the tracing being fixed by Indian ink, then engraving being carried out using the same etching technique applied to copper sheets.

Morise. Decoration obtained by nipping a vitreous filament with special pinching callipers (borselle da pissegar), taking it up with a fine iron (speo) and applying it when hot, sometimes in a contrasting colour, to the object being worked upon, generally on the base, the rims or the sides, with a wavy or indented sequence.

Enamel painting on glass. The Roman technique of decorating glass with enamel, already enhanced to the utmost level of perfection by the Syrians, was inherited by the Venetians, who indirectly passed it on to the Spaniards and the Germans. This decorative practice, attested in Murano between the end of the 13th and the first half of the 14th century, developed in an exceptional way after the mid 15th century and in the 18th century, and was later retrieved in the second half of the 19th century. Enamel decoration identifies painting carried out with a mixture of colouring metallic oxides and an impure vitreous mass obtained from the first fusion of raw materials (fry), or with powdered coloured glass mixed with an oily substance. Nowadays coloured mixtures obtained from finely ground powder of opaque and transparent low melting glasses are used, and applied by brush onto the surface of glass already created beforehand. After desiccation by evaporation of the solvent, they are fixed permanently by baking. Enamel decoration may also be combined with gilding, in gold-leaf or with mercury precipitated gold. In order to fix the coloured enamel to the glass, at one time the painted object was re-attached to the bridge and taken back to the fusion oven at a temperature of about 800-900°C. Nowadays the object is placed inside a small second-baking oven (muffola), subjecting it to a heating cycle not over 500°C, so that through softening the brush-painted glass may adhere to the supporting glass surface. Cold painting is carried out with normal colours which do not need a second passage of the object in the furnace.

 

Rui. Rollers or cylindrical sheets of average sizes, achieved by centrifugal force, subjecting a suitably perforated glass globe to a rotating motion until its complete flattening. Panes for making windows were often obtained from those 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 to form windows in stained glass, with figures, linked by lead.

 

 

Mould-blowing. The presence of forms or moulds in the Murano furnaces has been documented since the Middle Ages. These are used to stamp a decorative motif on the surface of the pèa, or vitreous gob blown in first instance “levada”. The principal moulds are: the balloton or embossed, the rigadin or grooved, the coste or ribbed. The material the mould is made of may be of wood, bronze, cast-iron, of aluminium and of steel. The mould can have a circular or polygonal section, it can present etchings or decorations in negative which then emerge in positive in the blown artefact. Mould work can be done in first instance by blowing the glass within an opening mould, made of two or more hinged parts closed by the assistant (sera forme), to make the incandescent bulb blown by the master adhere to the inner walls of the form itself. Or by blowing the gob into an open mould, resulting in prominent decorative motifs on the glass surface. If the horizontal section of the mould is circular, the pipe, and therefore the glass too, may rotate along an axis during the blowing; if, instead, the section is not circular, then the glass and the pipe as its consequence, cannot rotate (still-mould).

Balloton engraving: based on the use of a metal mould containing pyramidal studs having a square base inside it, which give a criss-cross embossed effect upon blowing.

Rigadin engraving: obtained by blowing the gob into an open bronze mould featuring grooves with a triangular section which give pointed ribbing. If the glass is also twisted, by impressing a torsion movement on the pipe during the moulding before exiting the mould completely, a twisted rigadin is obtained (stampà e menà: engraved and dragged). After re-heating it is also possible to put the already engraved gob into the same mould again, twisting it in the opposite direction, thus obtaining criss-cross diagonal stripes.

Half engraving (meza stampaura): is a technique already used in Murano glass factories since the 15th century. During blowing the work is covered with a further, thicker, cap-shaped vitreous layer in correspondence with its bottom, then, pressed inside an open ribbed mould, it takes on its form, thus achieving a decoration with ribs in relief only on its lower part.

Ice glass. Technique in use in Venice since the mid 16th century (the oldest mention known to us dates from 1570), then also spread in the rest of the Italian peninsula and in the European continent over the two following centuries. This type of work is obtained by briefly submerging the yet hot gob in cold water, so that the sudden temperature shift causes some cracking on its surface, accentuated by the subsequent passage to the oven before blowing it definitively. “Ice glass” produces an effect of seeming cracking on the surface of blown glass, imitating the faults in ice. A glass similar to this is obtained by rotating the still hot gob on the bronzin slab, covered with minute fragments of colourless or coloured glass, so that these particles, upon melting, stick to the surface of the glass.

 

Feathered glass. Technique used in pre-Roman Phoenician and Egyptian glass, introduced in the Murano glass factories at the end of the 16th century or more probably in the 17th century, and broadly used in the following centuries, obtained by applying vitreous threads under heat around a blown object of another colour, which, combed upwards with a metal instrument with various teeth, called comb, sgraffon or also manereta, assume a sequence like waves, feathers or like repeated festoons. The object thus decorated is further heated, marbled on the bronzin slab and blown again.