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Photo (c)
Eric Chu
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Murano glass
Venetian Glass: The Delicate Art
Written by: Jamie Sue Austin
The raw materials used in Venetian glass during the Renaissance differed
slightly from their competing Islamic counterparts. In this subtle
difference lies the transcendence of ordinary glass to remarkable art. The
base ingredient of glass is Silica. The most common form of Silica used in
glass making is sand. While sand has a relatively low melting point when
compared to pure silica it also comes instilled with many impurities which
can discolor or otherwise defect glass.
The production of clear, colorless
glass free of defects was difficult and as a result such pieces were highly
sought after. This glass, called cristallo, for its ability to mimic the
appearance of rock crystal, was the foundation of Venetian glass designs. To
obtain clarity of glass, glass makers strived to acquire only the best of
raw materials necessary. Ordinary sand was replaced by quartz pebbles
collected from the river beds of the Ticino and the Adige rivers.
These
pebbles, called cogoli, were a source of nearly pure, naturally occurring
silica for glass makers in Murano. The pebbles endured much scrutiny before
being chosen as material for glass. They had to be as colorless as possible,
without veins of mineral impurities, and they had to spark when being struck
by steel. After passing inspection these pebbles were heated until they
glowed then dipped in cold water.
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This process of heating and submerging
materials in water was a common practice and served to help remove
impurities that bled out of material. Following this treatment the pebbles
were crushed and ground into a fine powder. The finer the cogoli; the more
beautiful the end product.
The next critical raw material involved in glass making is a fluxing
agent. A fluxing agent is a material that serves to lower the melting point
of the glass, thus making it possible to melt glass in wood burning oven
made of clay. Fluxing agents were also added to enamels that would later be
applied to decorative glass to give the enamel a melting temperature that
was lower than that of glass itself. While many different fluxing materials
have been used in the manufacturing of glass, Venetian glass used "Levant
soda ash", called Alume catino, which they held the sole trade monopoly for.
This soda ash was created by burning the Salsola soda and the Salsola kali
plants that grew in the Levantine region. The alume catino underwent a
similar purification process as the colgoli. The ash was sifted until only
the smallest particles remained. These particles were boiled repeatedly to
distill the salts inherent in the plant matter. Then the damp material would
be put into shallow pans to air dry until it recrystallized as a white salt
which was powdered once more. This helped remove excess iron from the plant
material which could lead to discoloration of the glass.
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To further inhibit the colorization that occurs under the presence of
iron Manganese was added to final mixture. Manganese or Magnesium was often
called "glassmakers' soap" for its ability to counteract discoloration
caused by iron and other impurities. It was also instrumental in creating
other colored glasses and in lengthening the time that glass would stay
molten and pliable, thus allowing glass workers more freedom in their
designs.
Venetian glassmakers only used Manganese from Piedmont because of it
naturally lacked in substantial iron content. Manganese was not an expensive
component in glass making because little of it was required due to the
purification processes involved in preparing the colgoli and alume catino.
After these materials were refined they were mixed in together to form a
glass batch. A batch refers to the materials after they have been refined,
mixed, and are ready to be made into glass. The glass batch was prepared by
a conciatore whose sole occupation was to mix the ingredients appropriately
and modify the contents as necessary to produce the finest glass possible.
This batch material was then heated to create a frit. Frit is batch material
that is heated until it fuses into blocks.
Sometimes this included ground
cullet (pieces of broken or recycled glass) or additional manganese or other
agents to change the color and clarity of the glass.
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Photo (c)
another_penny_lane
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The frit was then re-melted and impurities were skimmed off the top.
During the final stages of glass preparation the glass frit material was
melted until molten and poured into cold water to remove further impurities.
This process could be repeated many times in the formation of crystal to
create as clear and colorless a glass as possible. To prevent bubbles and
striations in the glass molten glass had to be stirred constantly. Constant
stirring was necessary due to the lower temperatures in ancient furnaces. At
the same time the glass was being stirred manganese was added to clear up
poor coloration, furnaces were stoked with wood to maintain temperature. The
process of creating the crystal base for Venetian glass was as integral a
part of the beauty of the glass as the skill in which the molten glass was
worked.
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Phot (c)
FrogMiller
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The glassmakers' skill comes into play during the forming of the glass.
Molten glass was blown, twisted, and cut by expert hands. Trends of the day
dictated that finer pieces of glass would be thinner and more delicate than
utilitarian glassware. While thin glass had the additional benefit of not
showing the same degree of discoloration or bubbles or striations along with
being beautiful it also proved difficult to work with without breaking or
becoming misshaping. Good glass was deemed so not only because it was
delicate but because it was also perfectly balanced and proportioned as
well. Expertly created glass demonstrated clarity of color, universal
thickness of the rims, perfect symmetry and balance upon the stem, and
refreshing, unique, but constrained design. This required not only a steady
hand but an ability to create in real life something that existed prior only
in the artisans mind.
After a piece of glassware was produced it was then decorated. The
application of gold gilding and colored enamels was done not by the
glassmakers themselves, but by a separate guild of artisans who specialized
in painting glass. The enamels were formulated to allow durable, beautiful
color and design to be painted onto the glass. Enameling glass was a risky
business. After the glass was painted it had to be returned back to the
glass studio for a final firing. This final firing helped fuse the enamel to
the glass. The enamel melted at a lower temperature than the underlying
glass which allowed glassmakers to use heat as a fixative. However, this
process could have led to the cooled glass breaking from the rapid
temperature change. Great care and consideration was taken in the creation
of each and every piece of art glass.
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The glassmakers of Murano not only produced stunning works of beauty and
quality they also have the distinction of being part of a trend setting,
technologically advanced industry. Unlike other art forms during the
Renaissance glass making did not rely on inspiration from other outside
forces to develop fully. While painting and sculpture in the Renaissance
derived its inspiration from the Hellenistic and Byzantine eras, art glass
was unique to the area. It set the patterns in motion that other cultures
would follow for generations. While cristallo glass would not be surpassed
until the invention of lead crystal glass by the English the flooding of
Venetian styled fakes into the market and changing tastes in luxury goods
caused the glass industry of Murano to wane in popularity. Quality glass
goods from Murano are still being sought after till this day despite the
slump in sales and innovations in glass purity are still being discovered.
Even now, after hundreds of years since its glory days, Venetian glass is
still highly desired for the excellence in its design and execution.
Courtesy of::
Glassmaking in Renaissance Venice: The Fragile Craft by W. Patrick McCray
ISBN: 0-7546-0050-5
(c) 1997-2008 E. Massetti
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