History of Murano Glass

by
Michele Zampedri

English translation by Juli Van Zyverden

Art of the glass (page. 1/4)

The specific characteristics of glass is the way in which it solidifies, passing from liquid to solid by increasing the viscosity and passing from the rigid to the to the solid state which is obtained at a temperature of about 500 degrees C. (centigrade). In this interval of time, the so called "workable thermal interval", the Glass Master can give shape to objects, the finished products of which will retain the rigidity of a solid body while maintaining the transparency of liquid. Glass is composed of about 70% sand and silica which is transformed into a liquid state at a temperature of 1700 degrees C. In order to melt the silica at a lower temperature a "fondente" or "flux" which is used as a melting agent is added. This composition is incisive in glass technology not only because of the economic savings but also because it becomes a protagonist in the characteristics of Murano glass for which it is famous through out the world. The primary melting agent is soda, which has the property to lengthen the solidification time thus allowing optimum conditions in which the Glass Master may work the glass. The higher the percentage of soda the slower the glass solidifies ("slow" glass), in any case the presence of a melting agent must not be excessive, in fact there is an equilibrium that must be respected. If this equilibrium is not respected, over a period of time the glass will bring the flux to the surface and the object will become opaque (in "Muranese" terms it is said that the glass " sputa" spits out the soda). In order to limit this tendency a stabilizing agent is used: limestone or calcium carbonate. Other components which are added to the composition are nitrate and arsenic which have a refining action, facilitating the expulsion of air bubbles and making the fusion more homogeneous. If colours or opaque agents are added to the primary ingredients indicated the famous coloured or opal glass is created. Today the pureness of the soda is guaranteed by the Solvay process which gets its name from its inventor, while in ancient times melting agents came from the Orient. In fact an analysis of ancient glass indicates that plant ash containing a high quantity of potassium oxide and magnesium was used as flux. In the Syriac language these substances are known as "allume di catino" and "cenere di soria". It may be suspected that the decision to use this particular potassium based ash which was sanctioned by a Major Council edict of 1306 which prohibited the use of potassium ash made from processed ferns had a political basis. In fact such an edict ensured that the "Galee" (Venetian ships) of the Venetian patriarch would return from the Orient with their holds filled. The plant ash under went a purification process in order to obtain the "sale di cristallo" or the "sale di vetro" or glass salts, which when used together with pure silica and magnesium from Piemonte was the most precious decolorant used by Angelo Barovier in the XV Century to obtain that most precious of Murano glass: crystal. As far as silica is concerned, from 1300 to the XVIII century stones from the Ticino river were used. The so called "cogoli del Tesin were very pure while the "cogoli de Verona were less precious because as is written an anonymous manuscript from the XVIII C., it makes the glass "zaleto" (yellow). Later excavated silica sand was used and is still used today. The most famous silica is that which extracted in Istria and along the Dalmatian coast which is quoted in documents as sand from Pola and Lisa. The pureness of the glass today is guaranteed, not only because of the quality of the raw materials but also for the manner and ease with which fusion takes place thanks to the use of methane gas as fuel which quickly reaches high temperatures.The most widely used furnace is the "crogioli or slow-baking furnace with a medium capacity of 500 kilograms a day. These slow-baking furnaces are known by the Glass Masters, in order of size, as "palea", "ninfa" and "curisiol". The composition is loaded into an empty slow-baking furnace in two or three stages.

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