I wrote this article for the Soda Lime Times.

Glass has been around for thousands of years, yet little is known about how glass was first created. An old Roman story of how glass was first made comes from the historian Pliny. He wrote that Phoenician sailors landed on a beach near modern-day Israel. They were cooking food in a pot that sat on blocks of natron, which is a naturally occurring alkali. It’s said the heat from the fire melted the sand into a liquid which later hardened into glass. Although this isn’t really possible, it shows how mysterious glass can be.
The ability to make glass developed over a long time, but the ingredients haven’t really changed. Basic ingredients include sand (the chief ingredient), soda ash (an alkali that lowers the melting temperature of sand), and a stabilizer, usually lime (which keeps the glass from crumbling). Once all the ingredients are together they are heated to 2400°F. The glass melts in the furnace for a specific length of time before it is ready to pull. The melting time depends on several factors, including whether the glass is transparent or opaque and the color. The time can vary between 12 and 18 hours. As the glass cools, the atoms become locked in a disordered state like a liquid before they can form into the perfect crystal arrangement of a solid. Being neither a liquid nor a solid, but sharing the qualities of both, glass is its own state of matter.

Adding metal oxides creates the colors lampworkers love. Manufacturers add iron to create green glass, copper to make light blue, and gold for rubies. Manganese was used by early Egyptians when they created their purple glass. Chromium is one of the most useful coloring agents and is used to make dark green. It’s not easily soluble in glass and if not mixed correctly, black specks may form in the finished product. Chromium aventurine is interesting. The sparkling effect is caused by the formation of chromic oxide, which crystallizes out of the melt. During the cooling stage, the crystals orient themselves parallel to the glass, thus forming the glittering reflections we see.

Once the glass has finished mixing, it’s time to pull it into cane. Surprisingly, this process is still mainly done by hand, although the process has been mechanized. A glassblower takes the glass out of the furnace and creates a gather on the end of a thick rod. The rod is dipped and marvered several times in order to get a large gather to pull. Once enough glass on the rod, two blowers walk away from each other until they have the desired diameter cane.

Glass is categorized by its chemical composition and is known by its hardness. The most common manufactured glass is soda lime. It’s the cheapest to make and the least resistant to sudden changes in temperature. It’s known as soft glass.

Leaded glass is a second type of glass. When added to glass, lead makes glass brilliant, resonant, and heavy. Glasses containing a large percentage of lead are known interchangeably as crystal, lead crystal, and lead glass. It’s similar to soda lime in that lead glass will not hold up well to high temperatures or sudden changes in heat.

A third type of glass is borosilicate glass and it contains boron and is well known for its resistance to thermal shock. Known as hard glass, boro is used in cookware, lab equipment, and lighting. Boro was developed by German glassmaker Otto Schott in the late 1800s in a similar way to soft glass. In the beginning, most boro glass was colorless, but in 1986 colored boro was introduced to the market. Using silver to color the glass creates beautiful and sometimes surprising results. Because boro is stronger than soda lime glass, it’s used for creating figurines, pipes, marbles, and, of course, beads. Because of its heat resistance, borosilicate glass coated the insulation tiles on the Space Shuttle.
Other types of glass include aluminosilicate glass, 96% silica glass, and fused silica glass. These are more difficult to manufacture, but all can withstand temperatures higher than 1600°F.

From the beaches of ancient Israel to the pinnacle of space, glass is a valuable, yet mysterious, object. Without it, we couldn’t decorate our homes, our churches, or ourselves.