SHAPING HOLLOW GLASS©

glassware header

 ANCIENT GLASS SHAPING TECHNIQUES:

Egyptians poured hot liquid glass around a core of clay or sand to create hollow vessels to hold precious oils and perfumes as early as 1,500 B.C.  All glass was translucent or opaque. Clear glass was not developed until the 1,500’s A.D.

Blowing air through a pipe to inflate molten glass originated in Syria in the first Century B.C. Making hollow glass vessels began to flourish throughout the Roman Empire. 

SHAPING GLASS TODAY:

At glass factories, silica and other glass ingredients are sieved and mixed with scrap glass called cullet.  Cullet comprises 1/4 to 1/2 of a new batch of glass because it helps ingredients fuse faster and is used as a starter to save fuel. This ancient technique of using scrap glass is an example of true recycling.

All ingredients are mixed in large pots and placed in furnaces in the afternoon to achieve proper temperature and fusion for the next morning’s production. Fusion takes place at 2,700-3,000º Fahrenheit.  In Murano, workers begin their work day at 6:00 A.M because factories get very hot in the afternoon. The first glass furnaces heated with wood, (the reason glass factories were near forests): oil replaced wood,  then methane, natural gas and electricity.

glass furnaces

Over 400 large glass-melting furnaces in the U.S currently employ an energy-saving technology of oxy-fuel firing.

FREE BLOWN:

scan0042 (2)

The gaffer (master glass blower/artisan) blows air through the blow-pipe into the molten-metal and presses simple iron paddles and calipers against the billowing taffy-like glass to define a shape. The blow-pipe is constantly rotated to prevent sagging due to gravity.

glass furnace (2)

As molten glass is shaped, it’s reheated frequently to maintain a workable temperature which the artisan team  judges only by color.  Finished shapes are cooled slowly to prevent crystallization.

shattering glass

Watching a team spend more than 30 minutes developing a large mouth-blown shape, the gaffer shattered it because it was not up to his standard of excellence. The blown shape probably had large bubbles or striations  (visible streaking). Minute bubbles in mouth-blown glass are quite acceptable, large bubbles are not.

 MOLD BLOWN:

venice for blog 5.MHT (2)

Molds are used to create uniform shapes and sizes for soda glass, lead crystal or borosilicate glass. Any of these molten glass formula can be mouth blown or machine blown into molds for mass-production.

Metropolitan Museum glasses

Products shaped in cast-iron molds usually have mold marks whether mouth or machine blown. Mold marks lessen the quality and value of glassware. Manufacturers of better soda-glass products usually grind mold marks away;an acceptable practice.

mold blown glasses

Soaked wooden molds are used to shape better quality glasses because hot molten glass in soaked wood creates steam—and steam prevents mold marks from developing.

SAFEDGE:

SAFEDGE

Mouth-blown tumblers can be mass-produced by mouth-blowing molten metal into iron molds, used to shape tumblers one-at-a-time.  Where a tumbler is attached to the blowpipe, the closed end is cut open by machine, leaving a very sharp edge. 

As glasses pass beneath an intense flame with the cut edge face-up, the sharp-edge quivers, melts and thickens to become a smooth ‘safedge’.  Glasses with a safedge are inexpensive; they require no polishing by hand or machine.

PRESSED GLASS:

Pressed glass was patented in 1825 by John Bakewell, an American, who developed a way to make pressed glass knobs for furniture.  Mechanical glass pressing machines soon led to the global mass-production of inexpensive decorative glassware.

Gathers of molten-glass are plunged into molds to form the outside shape or bottom of a product. Before the molten-glass solidifies, a second mold presses down to shape the inside or topside of the glass shape. This process allows for mass-production because it requires little or no hand labor for production or finishing—and making it affordable.

venice for blog 6.MHT (2)

Molds impart texture to help offset inevitable mold marks.  Pressed glass replicates the texture of ‘cut glass’ but has its own charm— even with mold marks—although some manufacturers grind the mold marks away (adding to the cost).

Antique Colonial and Depression pressed glassware is highly valued by collectors but mold markings are essential for authenticity. Mold marks are retained in replicas of Colonial and Depression glassware but labels must state that they’re reproductions.

scan0017

A soda-glass cake plate is being mass-produced as calibrated amounts of molten glass drop onto a rapidly rotating mold.  A top mold is pressed down to control the shape and texture of each plate. A conveyor belt carries each hot plate through an annealing oven where it cools slowly to prevent cracking.

Texture eliminates any need for polishing and the plates can be immediately boxed for shipping.  Costly technology is offset by minimum hand-labor and volume sales;  an example of economies of scale .

Pat Breen: EYEWITNESS TO QUALITY

MY NEXT POST :  COLORED AND CASED GLASS