I have the great luck of having as a neighbor an authority on glass: Rob Cassetti, former Creative Director of the Corning Museum of Glass (Corning NY is about an hour from here) and before that Steuben glass in New York City.
Rob sat down with me last month at our local watering hole, Atlas Bowl in Trumansburg, NY.
He gave me a down-and-dirty introduction to glass-making. Although he claimed to be “unencumbered by the facts,” the science he shared was as interesting as his anecdotes.
What is a glass cabochon?
Most often, costume jewelry relied on the glass cabochon. A cabochon (cab-uh-shon) is a gemstone that has been shaped and polished rather than faceted. The term comes from the French word “caboche,” which refers to a dome-shaped cup. (The English translation is ‘noggin’ – an old-timey slang for ‘head’.) Cabochon gemstones generally have a smooth, domed top surface and a flat base. They highlight the rich colors possible with glass, and can be carved or etched. A cabochon may be made into any shape: ovals, rounds, baguettes, squares, etc.
Here are a few minusOne jewelry designs made with Art Deco era glass cabochons.
Of the colors, Cassetti explains, making blue is “dead-simple”. Reds are most difficult because they’re the least stable.
How do you tell the age of glass cabochons?
According to Rob Cassetti, there’s no easy way to date glass, at least not without the use of expensive equipment. When looking at a handful of glass cabochons, he said they could’ve been made in the 1920s or the 2020s. Telling the difference between an old and a new glass cabochon means having to develop an experienced eye for the quality of the glass’ surface, the colors, the style, and the complexity of the design.
For example, Rob affirmed a 1920s era production for the black pressed glass cabochons used in minusOne design stick pins.
A torch might’ve been used at the edges to melt-in inconsistencies. Plus, sharp points and details are more difficult to achieve.
And there is no true black glass – only very, very dark – because black is only a layering of colors.
I rely on an additional resource to date the glass cabochons I incorporate into earrings and other designs: where I am sourcing the glass. I source from defunct warehouses in New England where many jewelry manufacturers in the US established themselves in the 1930s and 1940s. American jewelry companies left behind glass cabochons dating back to the 1920s.
How were Art Deco glass cabochons made?
In the 1920s, the process for making glass cabochons was the same as it is today. Glass cabochons are made by cutting a piece of molten glass into a desired shape and then polishing it to create a smooth, rounded surface. All shapes, sizes, and colors were used in the 1920s and 1930s.
Rob advised to consider glass like the consistency of play-doh, like a squishy mold comparable to melted wax, a little stiffer than honey, being dropped in a dollop on a mold, and then pressed by an old-school embosser.
How glass gets its color
The color of glass comes from metal oxides. Coke bottle green, for example, is iron oxide (rust). Clear glass is simply colorless.
Add lead oxide to make glass sparkle, and it becomes the higher-priced “lead crystal”.
Processes like flashing – adding a silver finish to glass – is done by reducing the oxygen in the atmosphere around the glass, to insert the glass into a flame starved of oxygen.
Iridescence was once executed using gasoline, but Rob lamented, “that process typically kills people”. So they must have found another way.
Soda-lime glass and Coca-Cola soda bottles
One of the earliest methods of making glass involved heating silica, the primary ingredient in glass, along with an alkaline substance such as soda or potash. This mixture was then molded into shapes and allowed to cool. This process-product, known as soda-lime glass, was used to create decorative objects and vessels, such as bowls and bottles.
In fact, Cassetti shares, the green-colored glass that is the cheapest to make became a part of Coca-Cola’s brand. Coca-Cola decided on the color of it bottles – now such a large part of the history of its brand marketing – simply because it was the cheapest solution.
Soda-lime glass is also used for barware, according to Cassetti.
Rob also shared the mystery behind “trick glass” that dissolves in water: It’s the limestone (the “lime” in soda-lime glass) that stabilizes the structure. Keep it out of the process, and the “glass” will dissolve in your hand once liquid is added. Amaze your friends!
How was glass used in 1920s jewelry design?
The incorporation of colorful and clear glass into jewelry designs increased in the 1920s and 1930s with the growing production and consumption of costume jewelry. Jewelry designers started to think about how to manufacture affordable jewelry for the masses, and one way to do this was to use glass to simulate gemstones. Even high-end jewelry designers like Chanel featured glass.
Art Deco era tutti frutti designs by costume jewelers like Crown Trifari are good examples of how glass cabochons were incorporated. Many of the glass pieces featured in the tutti frutti style like the 1932 US-made Trifari bracelet shown here were carved, simulating ancient Chinese carved jade and other stones.
This Art Deco style silver stick pin features a simple blue glass cabochon.
Read about other American costume jewelry designers during the Art Deco era.
Art Deco era Gripoix glass used in higher-end costume and fine jewelry designs was made differently, poured directly into its setting in molten form, and used more often for fine jewelry or high-end costume jewelry.
How is moonstone glass made?
The shimmering moonstone glow of some glass cabochons is made by acid-etching the dome of the cabochon so that it is matte instead of high-polished. Then, the back of the glass is mirrored. When light reflects off the mirror, the etching seizes rather than releases the light, causing it to bounce and “glow”.
Notice the moonstone glow of the faux uranium green glass earrings for sale at minusOne.
Art Deco era jewelers that used glass
In the 1920s and 1930s, and in the decades leading up to the rise of costume jewelry during the Art Deco era, many companies sold glass cabochons for jewelry making or manufactured jewelry that incorporated glass.
Some of the most well-known companies include:
- Swarovski. Founded in Austria in 1895, Swarovski is known for producing high-quality crystal components for jewelry making. They began producing glass cabochons in the early 20th century and continue to do so today.
- Miriam Haskell. Founded in 1926, Miriam Haskell was a popular American jewelry designer who used glass cabochons in many of her designs. Her jewelry was often sold in high-end department stores and boutiques.
- Czech glass manufacturers. Many glass cabochons used in American jewelry making during the 20th century were imported from Czechoslovakia, which remains a major center for glass production.
- Trifari. Founded in 1910, Trifari was a US-based Italian jewelry company that used glass cabochons in many of its designs. The company was known for producing high-quality costume jewelry that was sold in department stores and boutiques.
- Coro. Founded in 1901, Coro was an American jewelry company that used glass cabochons in many of its designs. Like Trifari, the company was known for producing high-quality costume jewelry that was sold in department stores and boutiques.
In addition to costume jewelry, Coro also produced a line of glassware called Coro Craft Glass. This line of glassware was produced from 1939 until 1952 using a technique called “flame-working” which involved shaping the glass by hand using a flame.
What’s the history of glass jewelry?
Bead-making was, in fact, the first form of glassmaking. “You can think of the history of glass as a string of glass beads stretching back nearly 4,000 years,” Cassetti poeticized.
An in-gallery exhibition at the Corning Museum of Glass (CMOG) provides an overview of various manufacturing techniques:
The history of making glass dates back to ancient times, with evidence of glass-making dating as far back as 3500 BCE in Mesopotamia. Over the centuries, glass-making techniques have evolved, and the use of glass has become widespread in various applications, from decorative objects to industrial and scientific uses.
Ancient Egypt and glass making
The ancient Egyptians were known for their advancements in glass-making, using it for both practical and decorative purposes. They created intricate glass beads and jewelry, as well as vessels for storing cosmetics and perfumes.
Archaeologists at the Metropolitan Museum of Art studied glass artifacts found in the tomb of Tuthmosis III, asking questions about the origins of glass production. The era of Tuthmosis III – nearly 3,500 years ago – is “commonly understood as the beginning of intentional glassmaking in Egypt”.
Glass-making continued to evolve throughout the centuries, with the invention of blown glass in the first century BCE. This process involved blowing air through a tube into a molten glass blob, allowing it to expand and take on various shapes and sizes.
Read more about how Ancient Egypt inspired Art Deco jewelry.
Italian glass and the Middle Ages
During the Middle Ages, the Venetians became renowned for their expertise in glass-making. They developed a technique for creating crystal-clear glass, which they used to make mirrors, chandeliers, and other decorative objects.
The Venetian glass-making industry thrived during this period, with glassblowers forming guilds to protect their trade secrets. Murano glass remains highly sought after decorative art and jewelry.
Bohemian Glass in the 19th Century
In the 19th century, glass-making underwent a significant transformation with the development of the continuous glass-making process. This method involved pouring molten glass onto a moving belt, allowing it to cool and harden as it moved through various stages of production. This method allowed for mass production of glass, making it more affordable and accessible for everyday use.
Boehmian glass is a part of this period of glass production and remains in production using the same methods today.
Here is a brief overview of the process:
- Glass cutting: The first step is to cut a piece of glass into the desired shape. This can be done using a glass cutter, diamond saw, or other specialized tools.
- Grinding and shaping: Once the glass is cut into the desired shape, it is ground and shaped using various abrasives. This can be done manually using sandpaper or a grinding wheel, or using a machine specifically designed for glass shaping.
- Polishing: After the glass has been shaped, it is polished to create a smooth, glossy surface. This is typically done using a polishing wheel and a polishing compound.
- Final inspection: Once the cabochon has been polished, it is inspected to ensure that it is free of defects and has a consistent shape and finish.
Shop all minusOne Art Deco jewelry for sale!
Read more about Art Deco jewelry on the minusOne jewelry through the decades blog.
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…or to learn the details of Rob’s theory of the lampwork bead economy in the 1990s.
Thank you, Rob! I had a blast talking with you.
