GLASS IS A PARADOX
This vitreous matter can be
transparent, translucent or opaque,
delicate or strong and
functional or decorative.
Throughout glass history, diverse glass formulae enabled artisan glass blowers, cutters and engravers as well as mass-producers of glasswares to enrich our lives.
Decorative and functional glass shapes are created by
mold or machine
and embellished with cutting, etching and engraving
by hand or machine in broad price ranges.
HOW DO WE KNOW IF GLASS WARES ARE GOOD?
Glass wares are as good as the raw materials
and production methods used to produce them.
Sand is fused with specific ingredients to become soda-glass, lead crystal or borosilicate glass (used in laboratories or as cookware). We cannot assess the quality of artisan or mass-produced glass without transparency of raw materials and production methods on all labels.
Non-leaded glass (most of our everyday drinkware) is made of various compositions of soda glass. Venetian soda-glass was ‘greenish’ until the middle of the 15th Century when a method of purifying soda ash was developed. The result was glass so clear and colorless it was called cristallo because it resembled rock crystal. Even though many bridal registries refer to all stemware as crystal—the word implies lack of color, it is not a formula of glass.
Melting pure silica requires temperatures higher than commercially practical. Alkalis, soluble salts as soda and potash may be added to reduce the fusion temperature of silica.
Ingredients commonly added to silica:
SODA: an alkali of sodium carbonate (usually ashes of dried and burned marine plants) which serve as a flux to reduce the fusion point of silica.
LIME: makes glass more stable, lighter, cheaper and cool faster.
POTASH: burned beech wood, oak or fern is substituted for soda to make glass harder, more brilliant and capable of being cut and/or engraved (not to be confused with lead crystal).
In 1676, English glassmakers introduced lead oxide as a flux in potash-lime glass to create glass with exceptional clarity and brilliance: marketed as lead crystal. Lead crystal is heavy, colorless, brilliant, and refracts and reflects light. It has a distinctive musical tone when the edge is flicked with a finger or struck with a pencil.
Lead oxides makes glass ‘softer’ — which makes it easier to cut and engrave but easier to shatter.
Producers of lead crystal usually list the percentage of lead oxide although exact formulae are guarded secrets.
To qualify as lead crystal, international guidelines require a minimum of 10% lead oxide. Steuben Glass (no longer made: still revered) used a platinum lined melting tank and stirring rod because platinum resists corrosive effects of molten lead crystal and imparts no impurities to the lead crystal formula.
Lead Crystal Quality Designations:
Half-lead requires a minimum of 24% lead oxide.
Full-lead requires a minimum of 30% lead oxide. (cristal superior)
Never store wines and spirits in lead crystal decanters.
Lead increasingly leaches from lead crystal into wine or spirits.
Made with boric acid, borosilicate glass, invented in the 1880s by German scientist Otto Schott, was used for heat resistant laboratory and cooking glass. It does not expand with heat or cold but cannot pass from freezer to oven. Silica content is usually from 55 to 80%.
In 1915, Corning introduced a clear, low-thermal-expansion borosilicate glass for laboratory glass and kitchenware; trademarked PYREX; (each letter is uppercase).
Spun off from Corning in 1998, World Kitchen, LLC, licensed the name Pyrex (not upper case) for kitchen products. Pyrex is not borosilicate glass.
World Kitchen’s Pyrex is *tempered soda-lime glass .
It is cheaper to make than borosilicate glass, not as
heat resistant and can break from heat stress.
*tempered glass is a safety glass of increased strength / as compared to normal glass.
N.B. Confirm HEAT–RESISTANT PROPERTIES for all borosilicate glass.
WE KNOW WHAT WE LIKE: HOW DO WE KNOW IF IT’S GOOD?
Pat Breen: EYEWITNESS TO QUALITY
MY NEXT POST: SHAPING HOLLOW GLASS.