Obsidian: The first use of glass
The history of glass manufacturing keeps getting rewritten as archaeological discoveries find earlier and earlier evidence of people making things out of glass. The latest find shows that glass tools existed before humans did! A find in Africa shows that hominids were making obsidian hand axes 1.7 million years ago, which predates the first human tools by 500,000 years.
Let's start with what obsidian is. It's a glass that is naturally formed by volcanoes when conditions are right. The requirements are that:
Lava must be cooled rapidly enough to prevent crystals from forming.
The lava has a low water content.
The lava has a high enough silica content and high enough viscosity to resist flow.
Because of this, obsidian is only formed by a small fraction of volcanoes (less than 10%). There are a couple of similarities between obsidian formation and modern glass melting. For one thing, the silica content is over 70% with an alkali (sodium and potassium) of about 5 to 10%, alumina 2 to 10%, and then contaminants like iron. This is not far off the composition of some modern industrial glasses. It forms under the Earth’s surface where temperatures can exceed 1000°C. It also has the advantage of being under high pressure, which helps the melting process. That pressure becomes apparent on eruption, which spews glass all over the place.
Next, why was glass used to make hand axes and eventually arrow and spearheads? It can be formed into a very sharp blade in the process called knapping. This is a process, still used today, of whacking a knocking stone, or bopper, against the edge of a piece and chipping flakes off it. Glass works in this application because it breaks with a "conchoidal" fracture. The intersections of all these smoothly broken surfaces make a blade sharper than a razor blade.
An obsidian hand axe
The hand axe has been called the first invention. Even today, rock collectors make these. You can read accounts online of people mishandling obsidian and causing some serious bodily harm. Early people used these to scrape animal hides and whatnot. So glass has been a part of our lives for as long as we have existed as humans. Today you can order obsidian pieces online, and it's used for some surgery because it can make thinner cuts than a steel blade.
