Museo del Vetro

Glass Museum

IN-DEPTH

Types of glass and glassmaking techniques in the 19th 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 repeatedly, 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. In the 1800s aventurine paste was stretched into little canes  and melted again for blowing: “The very capable Cav. P. Bigaglia, in the making of the beautiful aventurine paste, was the first to stretch it into little canes in order to entwine it with others of different colours in working filigree blown glass, however the above paste was used initially with the best of success in Salviati’s laboratories like any other glass by re-melting and blowing. In fact, cups are made with this splendid enamel, which maintains itself unaltered in its beauty, vases, tumblers, plates, bowls, and other varied objects, which all present a magnificence and richness no longer to be seen, all the more appreciable inasmuch they are more easily marketable due to their lower cost compared to similar objects made with the same paste by a wheel, needing a very long working time and more distinguished artistic skills in execution, that are very expensive” (La Voce di Murano, 11 July 1868, N°. 27, page 116).

 

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 acts in matting), cobalt oxide, potassium dichromate, etc., that are dispersed by stirring the fusion. The streaking, caused by the oxides, is 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 on 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. Lorenzo Radi brought this glassmaking composition, which had seen great success in the 15th and 18th century, to life once more by presenting it at the Istituto Veneto in 1856, together with other vitreous pastes.

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. In this manner, a cylinder with the rods close together is obtained 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, with a straight inner thread. It is obtained by preparing rods characterised by a white or coloured straight inner thread. The glassmakers 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 by 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 a further blowing is executed, attaching a small bridge at the opposite end and detaching the blow-pipe, then passing onto 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 it on its opposite side to the bridge and detaching it from the blow-pipe. The glassmaker blows another identical half filigree, but twisted in the opposite sense, into this first artefact. 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, twisted into a spiral, and arranges 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. This decorative device is also named zanfirico.

Although with variations, 19th century filigrees derived from those of the 18th century. In fact, in both types of manufacturing one may see netted filigrees, twisted ones, skeins (a variation of the twisted), half filigrees, mono and double bicolour twisting (filigree with one or two twisted ribbons made of two sides in differing colours). In the 1800s Murano glass art, the taste for 18th century origin filigrees continued probably until the first period of Salviati & C. Limited, 1866 – 1868 c.

 

Granzioli. Small sized glass shards, generally coloured, used for spotted colouring. The glass being processed was rolled on the bronzin (slab of bronze, marble, nowadays in cast-iron) where the shards lay and which gave the blown artefact a wrinkled effect, by sticking onto its surface.

 

Incalmo. Murano technique, dating back to the 16th-17th centuries, consisting in welding together two open blown pieces when hot, generally of a different colour. In this manner, the two blown pieces heated at the mouth of the oven and having the same diameter, are immediately welded while hot, that is “incalmati”. After this first phase the object thus obtained is freed from the bridge (a solid iron on which the piece is fixed at the end opposite to the blow-pipe) and after levelling the surface with a marbling operation, the shaping of the object one wishes to create follows.

Cameo glass. Technique conceived in the second half of the 1800s in England to reproduce Roman age glass cameos. It was subsequently widespread in the whole of Europe, in particular in Bohemia and in France. This process featured two distinct stages: the first consisted in creating a thin encased blown glass, “a first specimen of very dark (blue, black, etc.) coloured glass is submerged in a crucible of milk white glass after being lightly blown, so as to be covered with it uniformly“. At this point, the specimen is completely developed by blowing and subsequently transferred into the re-baking muffle oven. The second phase consists in engraving the obtained artefact when cold with acid and the lathe. “The decoration is outlined on the outer milk white glass surface and covered with Judaea bitumen or paraffin. With its mouth adequately sealed, the vase is submerged in a continually shaken solution of hydrofluoric acid. The surface of the vase not protected by the Judaea bitumen is progressively removed by the corrosive action and by the hydrofluoric acid solvent, until the inner layer of blue or black glass is revealed. The vase is then taken out of the acid and abundantly rinsed. Once the bitumen protective layer is removed, the decoration outline in milk white glass appears in relief on the surface of the vase and one then proceeds to the lathe-engraving, to finish off and perfect the contours and thickness of the glass decoration in relief. In the end, the background areas and decoration requiring it are polished” (S. Hreglich, L’arte del vetro – Silice e fuoco, Vetri del XIX e XX secolo, 1992).

 

Corinth glass. So called because it imitated the corrosion on excavated ceramic. It is a matt paste, speckled in gold or silver and green on a dark background, with which small vases with shapes inspired by Greek ceramic were made. This corinti glass, probably conceived by Lorenzo Radi junior, is also found amongst the works created by Giuseppe Barovier and amongst the drawings in certain albums at the Salviati dott. Antonio company. The Francesco Ferro & Figlio company made vases whose outlines repeated the shapes of Egyptian, Etruscan and Graeco-Roman vases with “gold or silver dust ornamenting the surface“.

Enamel glass. It is the most opaque among opaline glass, and also becomes matt like girasol glass due to the abundant precipitation of lead hydrogen arsenate, which can even reach 30% of its matter. Enamel is mostly used in processing beads and filigree. Mosaic enamels are the canes with all the ranges of colours necessary to make tesserae. Enamel for tesserae is opaque coloured glass, brilliant and rich in lead. Such enamels were generally prepared apart by adding opaque (body) and coloured (soul) vitreous material to the colourless meld with a procedure over various steps. Non transparent, normally white glass is called enamel at the furnace. It can be defined as “fixed” (it is the best) or “claret” (it is the poorest).

Feathered or Festooned glass. Technique introduced in the Murano glass factories at the end of the 16th century or more probably in the 17th century, obtained by applying vitreous threads around a blown object of another colour when hot, 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.

 

Flame glass. In the 1800s, the term a fiamma was used by abbot Vincenzo Zanetti to describe a new vitreous texture presented by Salviati & C. in 1868 at the Industrial Exhibition in the Doges’ Palace in Venice: “a new graffito that may be recognised by the name a fiamma. This graffito is offered by combining within the same object various tonalities of glass, not excluding the opaline, and various designs interleaving the waves in the various colours with aventurine threads, something of difficult success, nor executed so far by anyone else. In fact, all the older works, not excluding the most recent ones carried out in this genre, present naught but an extremely simple graffito, that is, showing a single tint and a single design” (La Voce di Murano, 11 July 1868, N°. 27, page 116).

Gold-graffito background glass. Technique resumed at the end of the 1800s from late Roman and palaeo-Christian glass (3rd – 4th century A.D.), consisting in applying a very thin gold-leaf with glue when cold on the bottom of a glass or a cup, which was then engraved with an ivory or bone awl to obtain the desired designs, whether scenes of sacred topics, characters’ portraits, mythological allegories or written dedications. With further work when hot, the gold-leaf was covered and protected by another thin layer of transparent glass, very often coloured in green, turquoise or red.

Granite glass. This typology was probably born from an attempt in imitation of natural marbles. While spotted effects are traditionally obtained by gathering fragments (shards) of glass in one or more colours on a cylinder, in the case of granite glass the mixture is achieved by blending multicoloured glass shingle without a basic background.

 

Iridescent-Metallic glass. Iridescence is an effect created when a very thin film of metallic origin is deposited on the surface of a finished object, by evaporation of metallic oxide (tin or other metals), on which light then reflects in an irregular way. This technique saw its maximum development in the second half of the 1800s, when, with the purpose of techniques imitating archaeological finds, they thought of reproducing surface deterioration undergone by glass over time. It was Antonio Salviati, around 1880, who introduced this iridation procedure in Murano, already applied for some time to foreign crystals. Glass thus produced was called metalliforme.

 

Maculated glass (macie). Spotted glass is obtained by gathering granules or shards of coloured vitreous scraps scattered on the bronzin (slab of bronze, marble, today in cast-iron) around a small quantity of crystal at the paste stage, attached to the pipe. An apparently homogeneous spotted colouring is obtained once they are included into the crystal mass and dilated by blowing.

Murrino Glass or Mosaic Glass or Mosaic Fusion. Extremely ancient technique developed in Alexandrian and Roman times (1st century b.C. – 1st century A.D.), rediscovered by the Venetians at the end of the 15th century (when the custom of enclosing sections of canna rosetta in blown artefacts took over in Murano glass factories, characterised by concentric layers of different colour, at times forming typical star or flower motifs) and resumed by them in the second half of the 1800s (around 1870). This technique consists in laying 1-2 cm high sections of variegated canes, characterised inside by a design visible on cutting, or segments of rods or even prefabricated elements of various shapes and colour prepared beforehand, on a metallic or refractory slab when cold, following a certain design, all forming a multicoloured mosaic. The empty spaces between murrina and murrina are filled in with pieces of cylindrical rods. The whole is then gradually heated at the mouth of the oven so as to cause a softening of the mosaic elements; the adhesion of the different elements is facilitated by pressing the edges towards the centre with wooden palettes. This heating and clamping operation is repeated several times to eliminate all the voids present between the canes. Once the operation is over the artefact thus obtained is laid still hot in a previously heated refractory material mould and then put back into the interior of the oven where, under controlled softening, it acquires the final desired shape. Once cooled, the object thus obtained is adjusted, mill-ground and polished. The technique of blown murrini was also resumed in Murano in the 19th century, consisting in gathering up sections of murrina cane or millefleurs (cut from canes prepared beforehand) from a marble or bronze (bronzin) slab with a bolo gob of glass paste attached to the blow-pipe, which were then included within the glass mass by marbling (rotation of the vitreous gob on the bronzin). Afterwards, after further heating, processing continued as with any blown product.

Netted glass. Glass introduced for the first time by Salviati & C. at the Industrial Exhibition at the Doges’ Palace in Venice in 1868. On that occasion abbot Vincenzo Zanetti wrote “we must not forget to recall a very astonishing product. This is a vase, a tumbler, any object made with various bizarrely uneven spots of varied enamel in imitation of marbles, imprisoned within the finest net of little white enamel rods with bubbles. This work of rare beauty, and as far as we know unique so far, truly amazes, for it seems that the hand of the most gentle and skilful embroiderer has applied that finest netting to the object made of glass, enrobing it all like a delicate lace” (La Voce di Murano, 18 July 1868, N°. 28, pages 117-119).

Opal glass (Girasol or Sunflower). Opalescent glass with an orange sheen, more matt than opaline, introduced into Murano technology since 1693. This glass is slightly opaque due to the presence of lead hydrogen arsenate crystals in the glassmaking mixture, that due to their size confer an original colour to the glass, which appears bluish when observed under reflected light and light brown, or pink, when observed under direct light. In the 1800s, this type of glass was presented by Salviati & C. at the Industrial Exhibition at the Doges’ Palace in Venice. On that occasion abbot Vincenzo Zanetti wrote “Opaline glass, which after repeated and very expensive experiments was finally obtained in Salviati’s laboratories due to work on glassblowing in all security and a very beautiful quality, was one of the most reluctant to mingle with other colours. In our opinion it must have been so also in the past, as it is true that the ancients knew very well how to produce the lovely opaline paste and wrought objects of diverse forms, but such objects of past ages, which are not found profusely and that are therefore paid fantastic prices, are not pure, meaning without other colours. Well, Salviati was the first to show us this lovely glass merged and decorated with any other paste and even with ruby and with aventurine itself. …” (La Voce di Murano, 18 July 1868, N°. 28, pages 117-119).

Overlay glass. The overlaying technique consists in superimposing one or more succeeding layers of glass in various colours on the pèa gob, in the first phase of work. Enclosed glass has thin depth generally characterised by a double vitreous layer. It is an ancient technique, largely employed even today, obtained by submerging the blown artefact being processed in a crucible with glass of different colour. In the end one will get a thin blown artefact of one or two layers in different colours (for instance an opaque blue may be obtained with milky lattimo covered with transparent blue, or coloured glass can be covered in turn by a light vitreous layer of crystal). The first news relative to this technique regards Benedetto Barbaria, who made blown glass with two enamel layers in 1815.