Glass Casting: How It Works and Where It Comes From
What is glass casting?
Glass casting is a way of making solid glass objects by pouring hot, melted glass into a mould. Once the glass cools down and hardens, it keeps the shape of the mould. People have been using this method since at least the 15th century BCE. It started in places like Ancient Egypt and Mesopotamia.
These days, artists and makers use a few different ways to do it. Some use kilns to heat the glass. Others pour it into moulds made of sand, graphite, or metal.
Glass casting in ancient Rome
Back in Roman times, they used moulds made of two or more parts that fit together. These moulds helped shape blank glass dishes. They added the glass in one of two ways. One way was to fill the mould with small pieces of glass, known as frit, then heat it until it melted. The other way was to pour in already melted glass.
At Pompeii, there's evidence that molten glass was being poured into moulds by the middle of the 1st century CE. After that, they let the vessels cool slowly in a process called annealing. Once the glass was solid, they would mount it on a lathe and cut and polish it on all sides to shape it.
Pliny the Elder, writing in the first century, mentioned in his book Natural History that lathes were commonly used to finish most glass items made during that time.
Where cast glass came from and how it looked
Early cast glass with many colors mostly came from Italy. But simple, single-color glass was more common in other parts of the Mediterranean. A lot of the designs were inspired by Roman metalwork, like bronze and silver items. Some bowls and dishes even took their shapes from Roman pottery.
By the end of the 1st century CE, there weren’t as many new styles being made, but casting didn’t stop completely. It continued into the second or third decade of the 2nd century. Clear glass bowls were everywhere in the Roman world by the late 1st and early 2nd century. These bowls were probably made in more than one place.
Casting made a small comeback in the 3rd or 4th century, but it didn’t last long. Only a few pieces were made during that time.
How sand casting works
Sand casting is one of the simpler ways to shape glass. It’s pretty close to how metal is cast. You pour molten glass straight into a mould made of sand.
To make the mould, you start with clean sand and mix in a little bentonite. That’s a clay that helps the sand hold its shape. You also add a small amount of water and mix everything well. Then you sift it and pack it into a shallow container.
Next, you press a template into the sand to leave an impression. The template can be anything solid - a wooden shape, a found object, or even something like a hand or fist. The shape it leaves behind becomes your mould.
You can add color by sprinkling glass powders or frits onto the surface before pouring. This gives the final piece a tinted or textured finish.
Once the mould is ready, you take molten glass out of the furnace. The glass needs to be about 1,200 degrees Celsius or 2,190 degrees Fahrenheit. That’s hot enough to flow smoothly. You pour it directly into the mould.
If you want, you can also drop other glass parts or small objects into the mould while the glass is being poured. These can end up looking like they’re floating inside the finished piece.
This quick and hands-on approach became more popular in the 1980s, thanks to Swedish artist Bertil Vallien. He helped develop and refine the technique.
What kiln casting means and how it’s done
Kiln casting starts with building a mould, usually out of plaster mixed with heatproof materials like silica. You begin with a solid model. This can be made out of wax, wood, metal, or anything solid. Once you’ve got the model, you create a cast of it. That part is called investment. After the mould sets, you take the original model out.
One popular method for making the mould is the lost wax process, also called cire perdue. You make a wax model and surround it with mould material. Once it’s set, you heat the whole thing in a kiln. The wax melts or burns out, leaving an empty space shaped like the model.
That empty space is where the glass will go. A funnel-shaped hole is added to the mould and filled with solid chunks or granules of glass. Then, the mould goes into a kiln. It gets heated somewhere between 800 and 1,000 degrees Celsius, or 1,470 to 1,830 degrees Fahrenheit. As the glass melts, it flows down into the cavity and fills it.
This process allows for some pretty large pieces. Artists like Stanislav Libenský and Jaroslava Brychtová from the Czech Republic used kiln casting to make big, bold sculptures. Today, kiln cast glass is widely used by modern glass artists. People like Clifford Rainey, Karen LaMonte, and Tomasz Urbanowicz have created major works using this method. Urbanowicz is known for the "United Earth" glass sculpture at the European Parliament in Strasbourg.
What is pâte de verre and how does it work
Pâte de verre is another way to kiln cast glass. The name means “glass paste” in French. It uses finely crushed glass mixed with a binder, usually a bit of gum arabic and water. Colorants and enamels are often added to the mix, too.
Once you have the paste, you apply it by hand to the inside surface of a mould. It forms a thin or thick layer, depending on how much you use. When the mould is fired, the glass particles fuse together. This creates a hollow piece of glass that can have very delicate walls or be built up into heavier forms.
The French crystal company Daum is known for making detailed pâte de verre pieces. Their work is highly sculptural and shows how flexible this technique can be.
Using graphite to shape hot glass
Graphite is another material often used when forming glass with heat. It holds up well under extreme temperatures and doesn’t stick to the glass. Moulds can be made by carving, machining, or stacking graphite blocks into the shape you need.
Once the mould is ready, molten glass is poured in. It stays in the mould just long enough to cool slightly and harden. Then it’s removed and moved into an annealing kiln, where it cools down slowly and safely. That slow cooling step helps prevent stress or cracking in the final piece.
