Glass: The Old, The New, The Natural

Glass is made by melting pure quartz sand, grains of which are made of silicon dioxide, also known as silica. Glass and quartz are made of the same material but they do not have the same molecular structure. In natural quartz the molecules of silicon dioxide are arranged in a regular pattern. Molten glass is cooled quickly so the silicon dioxide molecules are arranged randomly; this characteristic of glass affects how it behaves and is used.

The Egyptian’s were the first to create a glazed glass layer on ceramic vessels as early as 3500 BCE. Glass making began about 3300 BCE, at the start of the Bronze age, and glass was traded across Egypt, Mycenaean Greece and Mesopotamia (modern day Iraq and surrounding regions). In 100 BCE Syrian craftsman developed the first pipe for blowing glass which allowed glass vessels to be made more quickly and easily and less expensively.

‘Clear’ glass often has some iron impurities within its structure that gives it a slight green tinge. Small amounts of manganese oxide is added as a decolouriser to counteract the green tones and was used as early as 200 BCE. If a higher concentration of manganese oxide is added to a mix, it creates a pale purple coloured glass.

In 2000 BCE cobalt oxide was used in Egypt to create a dark blue coloured glass. It gained popularity during this time because the blue glass was similar in appearance to the highly valued gemstone lapis lazuli.

Scientists are studying the properties of fulgurites to collect information about the lightning strikes that create them. This will help them better understand the energy needed for this process to occur as well as how the amount of energy varies as lightning passes through the sand. Fulgurites have even been used for forensic work in legal cases to determine if a fire was caused by power lines or a lightning strike.

Obsidian is not chemically stable in the long term. Over time, parts of the randomly arranged glassy rock rearrange into regular patterns, creating a quartz-type mineral called cristobalite. This change in structure is known as devitrification. When polished these white inclusions in the obsidian look like snowflakes in the black rock, so this is called ‘snowflake obsidian’

Obsidian has and continues to be used by Indigenous people around the world for many different purposes. Obsidian was used by the Rapa Nui people as part of the eye design for the Easter Island heads called Moai. Pre-Columbian Mesoamericans used obsidian for decorations, in trade and, most commonly, to make tools and weapons.

Obsidian tools were first used as medical blades in Egypt and Turkey during the Bronze age. Modern surgeons use medical grade steel scalpels for surgery but some doctors prefer an obsidian bladed scalpel; they have a sharper edge than steel and do not leave any trace metals in a wound. Obsidian scalpels are also much cheaper than other specialty scalpels (like diamond scalpels), however, they are much more fragile and require special care to prevent them breaking.

The force of a large meteorite hitting the ground can cause the surrounding rock to melt due to heat, pressure, or shock-waves. If this melted rock cools rapidly then glass can form, and if this glass remains around the impact site it is called impact glass or impactite.

Some meteorites cause the rock at the impact site to not only melt but also the molten blobs are splashed high up into the atmosphere, sometimes even into space. When this molten material falls back through the atmosphere to Earth ‘tektites’ are created.

Natural and human-made glass is all around us and has been used for thousands of years to better all our lives.

You can test your knowledge from viewing this exhibit by completing the 'Glass Crossword' on the Geoscience Australia website.