Actes du deuxième colloque international de l'association Verre et Histoire, Nancy, 26-28 mars 2009

Venetian innovations in glassmaking and their influence on the European glass history

Marco Verità
Laboratorio di Analisi dei Materiali Antichi LAMA
Università IUAV di Venezia, Venezia (Italy)

In the history of glassmaking, the invention of crystal glass by Muranese glassmakers around the middle of the 15th century is an event of major consequence. This glass had acquired such optical properties as to be compared to natural rock crystal. Murano crystal objects were traded throughout the world and its secret formula was one of the factors that allowed Venice to maintain its European glassmaking predominance for two centuries. Venice imported silica and fluxer at extremely high prices. The right choice and processing of the two raw materials ensured the production of a soda-lime-silica glass with a stable and durable composition.

Several reasons caused the decline of the Venetian glassmaking in the second half of the seventeenth century: some glassmasters escaped from Murano to set up glassworks all over Europe; the first printed book on glassmaking was published in Florence in 1612, strongly influenced by the Venetian tradition, and translated into English, German, French and Spanish. In Bohemia and in England new glasses were invented which were probably more clear and bright than the Venetian cristallo and certainly less expensive.

Venice reacted to this competition by specializing in the production of small size mirrors and beads that were produced in huge amounts.

Les innovations vénitiennes dans la verrerie
et leur influence sur l'histoire européenne du verre

Dans l'histoire de la fabrication verrière, l'invention du cristallo par les verriers de Murano au milieu du XVe siècle est un événement majeur. Ce verre possède des qualités optiques qui le rendent comparable au cristal de roche. Ces objets en cristallo de Murano ont été vendus dans le monde entier et le secret de leur composition a été l'un des facteurs de la prééminence verrière de Venise sur le reste de l'Europe pendant deux siècles. Venise importait pourtant la silice et le fondant à un prix très élevé. C'est le choix et le mélange de ces matières premières qui assuraient la réalisation d'objets à la composition silice-soude-chaux stable et durable.

Plusieurs raisons expliquent le déclin de la fabrication verrière à Murano dans la seconde moitié du XVIIe siècle. Des verriers s'échappent de Murano et partent établir des verreries dans toute l'Europe. Le premier traité de verrerie est publié à Florence en 1612, inspiré par la tradition vénitienne il est traduit en anglais, allemand, français et espagnol. Enfin, en Bohême et en Grande-Bretagne sont inventés de nouveaux verres, plus transparents et plus brillants que le cristallo vénitien et moins chers à produire. Venise réagira en se concentrant sur la fabrication de petits miroirs et des perles de verre en grande quantité.

∧  TopIntroduction

From the fifteenth to the end of the seventeenth century, Venice has been the world leader in glassmaking. Murano's primacy was due to the extraordinary quality of its glass (homogeneity, transparency, decoloration, palette of colours), the style of Venetian glassware, the skill of glassmakers and the wide range of products. This supremacy could be reached and maintained thanks to the fact that glassmaking in Venice has always been a dynamic craft. Since its beginnings (the first document attesting a production of glassware in Venice is of the ninth century) it underwent radical changes and incorporated many innovations along the centuries.

The aim of the present paper is to examine the technological changes that took place in Venetian glass manufacturing from the fifteenth and sixteenth centuries that allowed it to reach an incomparable quality, and how Venice reacted during the seventeenth and eighteenth centuries when foreign competition was most intense, through the invention of new high quality glasses.

∧  TopGlass batch and raw materials

The major source of information on the Venetian glass technology is the chronology drawn up by L. Zecchin, who investigated the documents of the State Archives of Venice (Zecchin L. 1987, vol. I; 1989, vol. II; 1990, vol. III). A significant number of analyses of Renaissance glass remains available today supplement this source. Along with the study of the Medieval and Renaissance treatises on glassmaking these analyses permit one to have a good understanding of the evolution of glass composition and technology from different glassmaking areas throughout Europe. Several recipe books of Venetian glassmakers dating from the fifteenth to the seventeenth centuries have been published up to the present time. They include the 2nd and 3rd books of recipes of the three booklets (Trattarelli) lying in the State Archive of Florence (second half of the fifteenth century, Milanesi 1864), the so-called Montpellier (dated 1536), partially translated and commented by Luigi Zecchin (Zecchin L. 1987, vol. I, 247-76), the Anonimo of the fifteenth century (Moretti and Toninato 2001), the Darduin (16th, early 18th centuries, Zecchin L. 1986) and the recipe book of the Venetian glassmaker Brunoro found in Danzica (Polen) dated 1645 (Moretti et al. 2004).

Each of these texts describes the use of two basic raw materials: sand or quartz pebbles as a silica source, and soda plant ash as a fluxer. Despite this apparent simplicity, the glassmakers had to face several problems arising from the complex and inconstant composition of the natural raw materials. In order to obtain high quality products, since the middle of the fourteenth century sand was replaced in Venice by quartz pebbles, which testifies to the efforts made to have a high purity silica source. The pebbles came from the rivers Ticino and Adige (those coming from the latter were inferior in quality). To reduce them to a finely ground powder suitable for melting, they were roasted, cast into water, and subsequently ground and sieved.

In the Mediterranean area as in Venice ash of coastal halophytic plants (Salsola Kali, Salicornia …) was used as a fluxer, while in northern-central Europe ash of inland plants (potash ash) was used. The Muranese glasshouses were expressly forbidden to make use of wood ash. The reasons for this prohibition lie in the differences in composition existing between coastal and inland plant ashes. The larger amount of colouring elements (iron, but also manganese, which is absent in soda ash), and the wide compositional variations existing in inland plants ash (according to plant species, provenance, plant parts, burning temperatures and methods, etc., Wedepohl 1997), were probably among the main drawbacks of wood ash. Recent investigations have identified less pronounced differences for coastal plant ash (Barkoundah and Henderson 2006, Tite et al. 2006).

In Venice, soda ash was imported from Syria and Egypt. The Egyptian product, less pure, was not used for quality glass. Since the seventeenth century, it was imported also from Spain and southern France. Its trade was protected by the Venetian Government, with a view to restricting its use. The price of these ashes must have been extremely high, considering the transport costs of the time. It is surprising that these ashes remained the main Venetian fluxer even after the seventeenth century, when less expensive, high quality raw materials were available in Europe. The astonishing consistency of composition over several centuries resulting from the analyses of Venetian glass manufacts is certainly an indication of the importance of this raw material (Verità and Zecchin S. 2009). The constancy in glass composition is a critical parameter. The viscosity of the glass melt, and consequently the possibility of working glass even into most complex shapes and with the precision required by luxury glass, are strictly dependent on the constancy of the glass composition.

The third raw material, manganese oxide, was the decolorizing agent used to neutralise the natural light green to yellow hue due to iron impurities of the batch components. The quality of the raw materials was not as high as today and colouring impurities (mainly iron minerals) were introduced into the glass. The iron concentration in the Venetian cristallo was high (Fe2O3 between 0.2 and 0.3 wt% of the oxide) as compared with modern glass (0.02% or less). The iron content and its oxidation state produce a more or less intense hue ranging between yellow, green and blue. To obtain a colourless glass, this hue was eliminated by adding manganese oxide, but this process leads to the production of a grey glass.

In Venetian glass furnaces the addition of manganese was made directly to the melt (not to the batch) in controlled amounts, until decolouration was achieved. In this way the use of manganese was limited to the lowest amount necessary. Instead, until the end of the sixteenth century, the decoloration of glass in northern-central Europe was problematic. In this sense, the presence of manganese in the inland plant ash was a handicap over coastal plant ash (practically free of it).

∧  TopGlass quality

The optical quality of the glass produced from silica and plant ash during the Middle Ages and the Renaissance period, was weakened by several defects. It was impossible to make a glass completely free of seeds, cords, a certain turbidity and a slight colour (or grey) hue. Long-lasting skill and technical knowledge helped the Venetian glassmakers to control these defects, yet not to eliminate them completely.

The considerable amounts of insoluble salts (sulphates and chlorides) of the plant ash are practically unreactive with silica and could not be incorporated into the glass. When present, these salts cause a dispersion of light leading to glass turbidity. Droplets of sulphates and chlorides form a suspension in the glass, producing light scattering (reduced transparency, turbidity). The casting of molten glass into water and remelting was a common practice in Venetian glassmaking. This procedure, repeated several times, together with the scumming of the liquid salts floating on the molten glass, helped to reduce unreacted salts, thus increasing the glass clarity.

Beside the presence of contaminants in the raw materials and the complexity of the fluxer discussed above, these defects are to be ascribed also to the technological limits of glassmaking at that time. The temperature reached in the melting furnace was too low to allow a complete refining (elimination of gas bubbles and seeds) and good glass homogeneity. Layers (cords) or knots of glass with a different index of refraction inducing light distortion can frequently be observed in ancient glass.

Continuous improvement was a characteristic of the long-lasting glassmaking tradition in Venice, which attained its highest point towards the middle of the fifteenth century. From this time Venetian documents classify clear glass into three types (instead of the traditional two): vetro comune (common glass, slightly coloured), vitrum blanchum (uncoloured glass) and cristallo (crystal glass). Cristallo, invented by the Muranese Angelo Barovier around the middle of the fifteenth century, had acquired such a clarity (perfect decoloration and high light transmittance) and homogeneity as to be compared to natural rock crystal (quartz). Soon it was traded throughout the world, and the secret formula was one of the main factors that allowed Venice to maintain its predominance over other European glassmaking sites for about two centuries. For the preparation of cristallo, Barovier added a preliminary step to the existing technique, consisting in the purification of the plant ash. The raw ash was ground, sieved, and dissolved in boiling water; the resulting solution was filtered, concentrated and dried. The salt obtained was mixed in the right proportion with silica to prepare the frit. The purification process would lead to the elimination of insoluble iron compounds (colouring impurities) as well as the calcium and magnesium compounds, which are essential to stabilize glass against weathering (Verità 1985). The glass obtained thereby (practically sodium silicate) would become coated with an opaque, weathered layer even shortly after being produced, with consequent loss of the brightness and transparency. None of the Venetian cristallo produced using only purified ash and silica would still be in existence today. The good state of preservation of most of the Renaissance Venetian luxury glass attests that Muranese glassmakers searched for and discovered a solution to this problem.