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.