Updated: Jan 31
Aside from the mental connotations the color red calls up (passion, mystery, heat, even danger), in the annals of glass history, in addition to those, red glass conjured up even more: secrecy, beauty, magic, and--at least for a time--a transmutational richness and value. It's a fascinating story that deserves at least this brief but incomplete retelling.
There is an old widespread legend that a nobleman once threw a gold coin ("Ruby glass"/Wikipedia) or ring ("Gold-Ruby" VGM) into a vat of molten glass, which, in an alchemical sort of way, produced the first batch of red glass ever. This is pure folklore; gold must be first dissolved in a highly-corrosive solution called aqua regia, a mixture of nitric and hydrochloric acid ("Gold Ruby-VGM"). The dissolved gold solution can then be added to a glass batch to ultimately produce red glass. In our technological and information-rich world, it sounds so simple.
But in ancient times and even as recent as about 250 years ago, it was anything but.
Corning Museum of Glass provides a comprehensive look at red glass, often referred to as "ruby glass" because of its deep-red color and also because it was so rare that it was afforded the same treatement and value as a precious new substance (par. 2). For the sake of space and holding your attenion, I am paraphrasing most of that entry, but the longer treatment is worth a good read--even if you're not a glass nerd like me.
"Gold ruby glass," according to Corning, has been around as long as glass production, and was thought to be a key elemental substance to extract or produce gold, the alchemist's dream. But the trouble is that it was so difficult to make that replicating the formula was nearly impossible, not only from batch to batch but over decades and centuries (pars 1-2). As another source notes, one reason it may have been so hard to discover in the first place and to reproducd consistently is that "the glass at first appears gray and turns red only on reheating" (Britanica). But a German phramacist, alchemist, and glassmaker, Johann Kunckel, (photo below, courtesy CMOG) came to Brandenburg in the 1670s and brought this glass out of obscurity.
The Corning article states Kunckel was hired by Frederick William I (who ruled Germany 1640-1688) and impressed the prince by exposing an imposter-alchemist who claimed he could make gold from silver (par. 3). The ability to expose this man undoubtedly came from Kunckel's previous experience 10 years earlier while poring over a vast library of alchemical writing while working under another ruler who wanted to know how to make gold (par. 4). It was ineveitable that Kunckel would apply this knowledge to glass-making.
During his own research, Kunckel commented on others' glass recipes for colorants (not just red), sometimes improving on them and sometimes denouncing their instructions, the Corning article goes on to say (pars. 5-10), and at one point, he even demurred giving his own successful recipe for gold ruby away stating he would have, "if [gold ruby glass] were not regarded as such a peculiar rarity by my gracious elector and master. Whoever does not believe that I can do it may come and see it. It’s true: for now it is too rare to communicate" (par. 11).
Kunckel's work with one glass formula in particular, Purple of Cassius, illustrates well just how difficult producing the exact ruby-like color of red so prized actually was. According to Corning:
This was the ideal raw material for gold ruby glass because it produces gold particles in the finest solution. When the finished product is reheated, the metallic gold forms nanoscale particles, which must be the right size and shape to convey a purple-ruby color through the absorption of light. If the gold colloids, as these particles are called, are too small, the glass remains colorless. If they are a bit too big and too much light is absorbed, the glass looks liverish (German, lebrig), or opaque brownish (par. 12). . . . It was one thing to make small samples of gold ruby in a laboratory, but quite another to blow an evenly colored vessel from this glass. How Kunckel and his gaffer managed to produce the large, impeccably colored goblets of varied thicknesses that we admire today remains an enigma. Developing the experimental creation of gold ruby into the production of vessel glass was Kunckel’s unique contribution. (par. 13)