Alyssa Dumire, Director of Education
We’ve already tackled the two major “specialty” families of glass, so today we turn to the most common formula: soda-lime glass. Ninety percent of all manufactured glass is soda-lime, from windows to wine bottles, and it is also the most popular for glass artists. Its prevalence makes it tempting to think of soda-lime glass as basic or boring, but let’s remember the range of creative possibilities with this one material…
Sometimes called soda-lime-silica to include all three of its major components, it is a very malleable type of glass that includes sodium carbonate (soda) and calcined limestone (lime). This workability lends itself to a wide range of processes and types of manipulation, such that when you venture into the Glass Wing, most of what you see is soda-lime glass. Not only is it the most popular and widely-available formula today, as the first developed, it has held that title throughout the long history of the material.
Soda-lime glass far predates its more specialized cousins, lead and borosilicate, leading us all the way back to the third millennium BCE. The first man-made glass has been attributed variously to ancient China, Egypt, and Mesopotamia; the latter now most widely accepted by scholars. All glass contains silica as its main ingredient, but the high melting point of pure silica (over 3000 degrees fahrenheit) was prohibitive to mainstream production. Ancient glassmakers discovered that the use of a flux, in this case soda from plant ash or a mineral called natron, lowered the melting point to a more attainable 1800F. This was likely discovered as a byproduct of metallurgy or an evolution of silica-based ceramics. Soda alone would result in a glass that is water-soluble, but early glassmakers were fortunate in that their raw materials also contained calcium and magnesium, which stabilized the mixture.
Early glass formulations varied geographically based on the raw materials available, and were also subject to the impact of historical events (the fall of the Roman Empire, for example, stalled new glass technology for a time). Two developments set the stage for modern soda-lime glass. Venetian glassmakers on the island of Murano developed processes to further refine their raw materials, resulting in the superior clarity of their glass during the middle ages. However, it still included only two main components: silica from flint pebbles and plant ash imported from the Levant (naturally high in stabilizing calcium and magnesium oxides). Finally, the first glass that used lime as a significant, intentional additive was “chalk glass,” introduced in Bohemia (now part of the Czech Republic) in 1683.
Today, while the vast majority of glass used in both manufactured and studio-made objects is soda-lime, the exact batch mixture continues to vary. Glass for artmaking is “long,” meaning it remains workable over a wider range of temperatures. On the other hand, mechanical production of bottles requires speed and benefits from a glass batch that holds its shape more quickly as it cools. This is achieved by fine-tuning the amounts of soda, lime, and the other additives that give each type of glass its unique properties. Have you ever heard that glass, even when cool, is a liquid and it will–very slowly–settle over time? This is a myth, but it is rooted in the reality that additives to silica disrupt its orderly, crystalline molecular structure, turning it amorphous and irregular, resembling that of a liquid. So while the idea of room-temperature glass being a liquid is a myth, it’s not a solid in the traditional sense either.
Glass artists benefit from the malleability, relative affordability, and wide range of working temperatures afforded by soda-lime glass, but there is one major tradeoff. It has a high coefficient of thermal expansion, meaning it expands significantly when heated, contracting as it cools. This makes the controlled cooling of an annealing oven essential to avoid stress cracks.
So how can you tell soda-lime glass from the other varieties? Generally, the museum label will specify if a work uses lead crystal or borosilicate glass, but not if it’s made from soda-lime; its prevalence makes it essentially the default. “Colorless” soda-lime glass has a greenish blue tint like the sculpture above. It is also available in the widest range of colors and finishes–note the variety of surface textures in the three animals below, sculpted by William Morris, Shelley Muzylowski Allen, and Preston Singletary.
Shelly Muzylowski Allen, American, b. Canada 1964. High Noon. Blown and hand-sculpted glass, rock, steel, 2015.
Preston Singletary, Native American, Tlingt, b. 1963. Raven Steals the Stars. Blown and sand carved glass, 2008.
Its variety of applications, however, can make it difficult to pin down. Do you think you can identify the glass types in action in the gallery view below?
To see these and more glass works on display, visit the Glass Wing during your next visit to FWMoA!