Venice, a city in northeastern Italy, is world-famous for its picturesque location, architecture and of course, artistic glass objects, especially made on the island of Murano. The glassware produced on this island stands above others in terms of the elegance of products, the craftsmanship of glassmakers and the wide variety it offers. As far as the quality of Murano glass is concerned, you can notice exceptional difference in transparency, homogeneity, decolourisation, and range of colours.
Since the beginning of the glassware production in Venice in the ninth century and its subsequent shift to Murano in the thirteenth century, the Venetian glassmaking industry has seen both prosperity and adversity in equal measures. While the industry was at its peak from fifteenth to seventeenth century, there was an era of sharp decline and the following rise in fortunes in centuries thereafter. In the twentieth century as well as in recent times, the art of Italian Murano glass has witnessed a sort of renaissance.
Before delving into the raw materials used in the ancient Murano glassware production, let’s take a look the characteristics of glass that set it apart from other materials, particularly metals. The specialty of glass lies in the manner of its solidification. It gets converted from the liquid state at temperatures as high as 1400 degree Celsius to the solid state very slowly. Due to this long gap during which the glass remains fairly malleable (paste-like), the glass masters are able to create stunning shapes of their choice. In the end, the object will have the rigidity of the solid and the transparency of the liquid at the same time.
Murano glass is generally obtained from the melting batch or mixture that consists of raw materials, primarily silica sand (around 70% by weight), soda ash and calcium carbonate. Other raw materials that a glassmaker might add to the mixture include sodium to create an opaque glass surface, nitrate and arsenic to get rid of bubbles, and colouring substances. For your information, the materials that facilitate the lowering of temperature are known as ‘fondenti’ (flux or melting agents). The mixture is then introduced in a crucible of refractory material contained in a furnace where a melting cycle is carried out up to a temperature of about 1400 degree Celsius.
Until the fourteenth century, sand was the primary source of silica. However, this silica source was marred by problems like complicated and variable composition. So, in a quest for better-quality silica source, quartz pebbles took the place of sand from the middle of fourteenth century. These pebbles were particularly sourced from the river Ticino. From the eighteenth century, sand quarries of Istria and Dalmatia (sands of Pula and Lissa) were also used.
When it comes to the flux or melting agent, the foremost choice had always been Venice soda or sodium carbonate, contained in the ash obtained from the combustion of coastal plants. The ash can be according to the type of plant from which it comes, sodium or potassium. In particular, the ashes used in Venice came from the family of coastal halophytic plants (Salsola kali, Salicornia, etc.). However, to preserve the quality of Murano glass, a decree of the Council in 1306 prohibited the use of the ashes of continental plants such as ferns, owing to their many drawbacks like variations in compositions and greater quantities of colouring agents. Hence, the Venetian glassmaking industry started importing soda ash from Syria and Egypt. The Egyptian import being less pure was used for low-quality glass only. From the seventeenth century, the soda ash was imported from Spain and southern France. In the nineteenth century, the industrial production of artificial soda was begun.
Until the end of the eighteenth century, glassmakers had no idea of the importance of stabilizers such as calcium carbonate. As a matter of fact, they unintentionally introduced this compound in the batch with the flux. In the ash like the ‘”alum Syria” or “alum bowl”, calcium carbonate was present in percentages almost equivalent to the sodium carbonate.
Cullet is actually the glass that is smashed and prepared to be melted again. It is a part of glass recycling process. Even in ancient times, and especially in the Middle Ages, the cullet was widely used by Venetian glassmakers to facilitate the merger. It was largely imported from the East and often recast in the Venetian furnaces.
Other Raw Materials
• Lead “brusado” was employed to produce the leaded glass. The lead oxide (PbO) was obtained by heating the metallic lead.
• The “grepola” (tartar barrels) consisting of potassium tartrate was used in small quantities as such or calcined in colored glass or as a flux.
• Manganese was used as a bleaching agent and in greater quantities as a colorant; the most valuable was the manganese of Piedmont.
Some Raw Materials Used as Colourants
• Crocco (iron oxide) was used to obtain yellow and green/blue colour in the glass.
• Ramina red and black (copper oxide) was used for blue, green and red.
• Manganese was used for violet.
• Sulfur was used for yellow-amber.
• Zaffera (mixture of sand and Cobalt oxide) was used for blue.
• Silver was used for yellow.
• Gold was used for ruby red.
Some Raw Materials Used as Opacifiers
• Mixture of lead and tin: Opacifying is obtained by roasting a mixture of lead and tin metal.
• Animal bones calcined: Opacifying is obtained by calcium phosphate used in the sixteenth century.
The aforesaid raw materials were an integral part of Italian Murano glass production for the last eight centuries. Today, the Venetian glassmaking has gone through several revolutionary changes which will be explained in another post. Till then, you may browse through an amazing selection of genuine Murano glass table lamps, chandeliers, sculptures, vases, plates and jewellery at Originalmuranoglass.com and order some masterpieces at competitive prices from there.