How to Fake an Emerald in 1653

I study how alchemists understood nature during the Early Modern Period—about 1450–1700. As a research fellow at the Science History Institute, I’ve been exploring how alchemists created fake gemstones.

In the Early Modern Period, people wore gems not just because they were beautiful and displayed your wealth. Gemstones were also thought to hold hidden virtues. Wearing a gem could give you the stone’s special powers.

Alchemists believed that physical appearance held clues to a gem’s hidden virtues. “Amethist is in the colour like wine, and it keeps from drunkenness,” wrote Giambattista della Porta in his book Natural Magick in the late 1550s.

But there’s a surprise. Many gems from the early modern period—including the ones on this chain mail shirt—are not genuine. They are artful fakes, made of high-quality glass tinted with complicated ingredients.

Trying to follow those old recipes is a fascinating way to enter into the world of early modern alchemy. It helps us discover what alchemists knew about chemical reactions, and how they might have worked in their laboratories.

I joined the Vitreous Alchemy Collective to learn more. We’re a group of historians and glassmaking students. Working in a studio at the Rhode Island School of Design, we endeavored to re-create a 1653 recipe for a glass emerald.

Our reconstruction used silica quartz powder. For safety, we substituted cream of tartar (potassium bitartrate) for salt of tartar (a dangerous eye and lung irritant). We also added soda ash as a flux, which helps the mixture vitrify (become glass) by lowering its melting point.

Next, French describes how to color the glass using metal and mineral pigments.

“Common copper makes a sea-green,” he instructs, while “copper made from iron makes grass-green.” To get sea-green, “a plate of copper must be made red hot, and then quenched in cold water.”

French then adds 5–6 "grains" (.33 grams) of the pigment to the powder mixture, placed in crucibles “half full.”

“There melt them in a strong fire until they become like glasse.”

We put our crucibles inside a modern glass kiln, which can reach a temperature of up to 1,700°F (927°C). The vitrification (glass forming) process takes several hours. We had to wait overnight for it to cool.

1) industrially produced copper oxide
2) copper oxide prepared according to French’s instructions
3) copper made from iron

The crucible containing industrially-produced copper oxide, however, produced a dark glass. Broken open, the crucible revealed that the copper pigment had settled into a thick base of red at the bottom of the crucible. However, a thin line of ‘emerald’ green formed at the top!

This result may be related to copper’s sensitivity to pH. While the salt of tartar called for in French’s recipe is alkaline, the safer cream of tartar we used is acidic.