Glass didn't start out as its own thing. For thousands of years before people created glass windows, containers, or figurines, they used the substance as a strong, translucent coating for pottery. It took a few more centuries for until Egyptians learned to forge glass -- with its own unique properties -- into novel shapes. And then their products were slowly spread through Europe by Phoenician traders.
The art of making glass probably grew from the field of pottery, most likely in Egypt from about 4,000 B.C., towards the end of the Neolithic era. Glass is made from heating a clean mixture of soda, lime, and sand until it melts into a liquid.
Objects made solely from glass didn't start appearing until about 1,500 B.C., the products of what we now know as glass-blowing. While Romans created beautiful mosaic glasses and bowls for the emperor's table, but it was Syria that perfected the art of glass-blowing, first with molds and then freehand. Reheating the glass, a practiced blower could blow and spin the molten substance into any shape. The practice quickly spread across the Roman empire, and even to the "outer barbarians" from Afghanistan to the northern highlands of Scotland.
Molding the glass to desired characteristics can be tricky. The glass had to be cooled slowly -- too quick and the gas bubbles won't escape, or it could crystallize. The three basic ingredients when mixed will produce a clear glass, but even trace elements of other minerals could change the color of the resulting product.
Much of the glass we see today comes with greenish-brown color, thanks to the iron present in much sand. Other impurities produce other tints: Copper for green-blue, silver for yellow, cobalt for blue (duh). But to make glass extra clear? Manganese dioxide, or 'glass-maker's soap.'
Red was always a problem. The Assyrians used traces of gold to make a ruby colored glow. But it was the famed Murano glass makers who cracked the code, by adding copper-oxide and heating without air. By the 14th century, glass-makers of northern Italy's Murano got rock star treatment--they could wear swords, were immune from state prosecution, and even marry their daughters into prominent families.
Just to the south Venice became a prominent glass making center, and its influence spread across Italy. Stained glass windows flowered as an art form in this time period as well. By 1226, glass-houses in the forests of Surrey and the Weald started to pop up.
The art of glass coloration gradually evolved over the centuries, driven by people's ever-insatiable need for luxury goods. Glass-artisans felt along by intuition and lucky alchemy, until chemistry came along in the 17th century to explain the underpinnings. The coal-powered glass furnace was born of necessity, when, in 1615, England banned the burning of trees for glass making,in an effort to keep timber economical. One immediate benefit? The hotter, 1,000+ degree temperatures these furnaces offered.
--Summarized from "A Short History of Technology: From the Earliest Times to A.D. 1900."