History of Murano Glass

by
Michele Zampedri

Three-Tiered Furnace : Venice 1540

Art of the glass (page. 2/4)

The first load is placed in the furnace at about 5 pm at a temperature between 1200 - 1300 degrees C. and the last load is placed in the furnace between 9 and 10 pm. The temperature then is raised to boiling point at about 1400 degrees C. in order to allow the air-bubbles to escape from the liquid and allow the amalgamation of the glass. Around 2 or 3 o'clock in the morning the temperature is lowered to around 1200 degrees C. so that at around 7 am when the days work begins again the glass has the necessary viscosity that is required by the Glass Masters. The fusion happens in a slow-baking furnace placed inside of a wood burning "fornace furnace previous to the preparation of the "fritta". This is a pre-fusion mixture of ground "cogoli" (pebbles) and ash which at a temperature of 700 degrees C. becomes a cohesive solid masse. This masse is placed in a slow-baking furnace where after a few days, or sometime after a few weeks the real fusion takes place and forms a workable glob. During the fusion the glob is removed from the slow-baking furnace many times and emersed in water and in modern times even the residue in the bottom of the slow-baking furnace which was removed and cooled in the open air was called "cotiso". In the Master book of 1348 it was written that a certain Bartolomeo Tataro gave a "cacia de fero" (iron instrument) to be repaired, an instrument that was used to pull the fused glass out of the pan and poured into water and then by adding complementary materials bring it back to fusion. This is the first testimony of a technical expedient that the Murano Glass Masters indicated as a practice to "traghetar (cavar) el vero in acqua" remove glass from water. At the beginning of the 1600's the Florentine, Antonio Neri used this method to free glass from the excess soda. In chapter IX of his book "The Art of Glass-blowing", in fact, he explains how to "fare il cristallo in tutta perfettione" make perfect crystal by suggesting that the first fusion be thrown in ceramic jars full of fresh water since the effect of the water removes a type of salt called Alkali salt which inhibits the crystallization process and instead fogs the crystal. The oven that was used the most in the history of Murano Glass-blowing was the "forno a tre piani" three-tiered furnace (the glassªblowers constitution of 1315 ordered that the work be done with a furnace that had three openings "qui habet tres bocas"): one level for a bed of wood, the second level for the slow-baking furnace and the third level was used as a "muffola" or cooling oven which was necessary in order to lower the temperature of the glass very slowly to ambient temperature thus avoiding thermal stress which would make the finished objects more fragile and more likely to break. The furnace maintained this structure up until the mid-1800's when a grill was placed in the furnace to sustain the wood fire. This change made fusion possible in one day. Another important modification took place in 1900 with the separation of the reªbaking furnace from that of the melting furnace. Today, in fact, the "muffola" or cooling furnace with a temperature around 500 degrees C., is detached from the principal melting furnace. This choice demands a profound reflection on the loss of heat this solution presents.

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