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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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