|
|
| | |
|
The "e;Unbreakable Glass"e; of Ancient Rome
In the reign of the emperor Tiberius (14-37 AD) a Roman glassmaker
demonstrated a remarkable new glass at the imperial court. Unlike ordinary
glass, it did not break: it must have seemed almost supernatural. The
event was recorded by contemporary writers Pliny and Petronius. They
called his glass vitrum flexile (flexible glass). The craftsman displayed
a beautiful transparent vase to the emperor and then dashed it to the
ground. According to the story, it dented but did not break. Tiberius
asked if the glassmaker had told the secret of unbreakable glass to
anyone else. When the answer was in the negative, the emperor had the
unknown genius put to death and his workshop destroyed fearing that the new
material would reduce the value of his imperial gold and silver.
Reports of this incident have doubtless been embroidered over the
years. One of them [Petronius] speaks of the glass maker repairing the dent
with a hammer [martiolum= Latin for small hammer]--though even today's
toughest shatterproof glass cannot be hammer-worked like silver/gold.
Some think that vitrium flexile was merely describing "e;bent"e; glass, and
that the man had designed some new style of hollow-handled vessel.
However, more than this would have been needed to impress the Roman
establishment, used as it was to vessels of great intricacy. A better
explanation would seem to be that the glass worker really had stumbled upon a
primitive kind of shock-resistant glass. [Note: Perhaps this is just a
tall tale; it doesn't have to be true!]
Ordinary soda glass is based on silicon dioxide (Sand) with sodium and
calcium as the metal oxides coming from sodium carbonate (soda) and
calcium carbonate (limestone). To modify this glass into relative
unbreakability requires a radical change to the formulation, but the essential
new ingredient is a few percent of boric oxide. So could our unknown
glassmaker have had access to either boric acid or borax, both of which
occur naturally?
In the Middle Ages borax was regularly imported into Europe from the
East to be used as a flux by goldsmiths. It came from a source in the
remote regions of Tibet. Could a little have found its way to ancient Rome
1500 years earlier? Maybe. There was a flourishing trade in those days
between the Roman Empire and the Indian sub-continent. If so our
glassmaker may have bought some borax and noted its remarkable effect when
added to the glass batch.
However there were two potential sources much nearer home. The steam
vents of the Tuscan Maremma north of Rome, contained natural boric acid.
Geologists did not establish this fact until the 1 820's, but it is
surely possible that our ill-fated glassmaker came upon some unusual
looking crystal salts in a dried-up Tuscan pool and decided to see if they
had any effect on glass. The Romans possessed even richer sources of
borates elsewhere within the Empire--in [what is now called] Turkey.
These, too, were unknown until the nineteenth century, but again it is
possible that boron-containing material from north-west Anatolia [modern
Turkey] found its way into the glassmaker's batch--mistaken, perhaps, for
some form of silica.
Boron has the wonderful gift of being able to change the number of
chemical bonds it can make. The boron-oxygen bond is itself astonishingly
strong, but true boric oxide can make only three bonds. However, if more
oxygen is added--sodium oxide, for example--it can make four bonds.
This imparts great three-dimensional strength. Add the compound to a
formulation and the response to thermal shock of the resulting glass is
crucially improved. It becomes heat-resistant, oven proof, and can be used
for cooking, chemical apparatus, thermometers, telescopes, and a
hundred other functions. The strong bonding also increases resistance to
water and chemicals, so boron containing glasses are ideal for medical
ampoules, laboratory instruments, floor and wall tile, even kitchen sinks.
Boron. Non-metallic element, fifth in atomic table, distinguished by
the symbol B, atomic weight 10.8. Boron does not occur free in nature but
always in combination with oxygen and other elements, notably sodium
and/or calcium.
Borax. Natural or refined sodium tetraborate with ten molecules of
water, i.e., Na2B4O7.10H2O. Borax also has pentahydrate (five molecules of
water) and anhydrous forms.
Borate. Generic term for boron compounds containing oxygen.
Boric acid. An acid formed reaction of borates with H2SO4. Formerly
called boric acid, its formula is H3BO3.
Borosilicate. General term referring to a range of glasses containing
silica (typically over 60%) and boric oxide (5-20%). Two specific
glasses cover most applications: low expansion Pyrex-type glass; and highly
durable 'neutral' glass. Glass fibers, ceramic glazes, and vitreous
enamels are also borosilicates.
|
| |
| | |
|
|
|