Soft glass is the common name for soda-lime glass, the most widely produced type of glass in the world. It makes up roughly 90% of all manufactured glass and is what you encounter in everyday items like windows, bottles, light bulbs, and drinking glasses. The name “soft” refers to its relatively low melting point and the ease with which it can be shaped compared to other glass types, not to any physical softness you’d notice by touching it.
What Soft Glass Is Made Of
Soft glass is built on three main ingredients. Silica (from sand) makes up 70 to 75% of its weight and provides the glass structure. Soda, or sodium oxide, accounts for 12 to 16% and acts as a flux, lowering the temperature needed to melt the silica. Lime, or calcium oxide, contributes 10 to 15% and stabilizes the mixture so the finished glass doesn’t dissolve in water. Small amounts of other compounds like magnesium oxide and aluminum oxide round out the formula, usually totaling less than 5%.
This recipe is ancient. Roman glassmakers used a mineral called natron as their source of soda. Around the 9th century AD, glassmakers across Europe and the Middle East shifted to using ash from soda-rich plants as the flux instead, partly due to supply disruptions and political instability in the regions where natron was sourced. That plant-ash tradition carried forward into the famous Venetian glassmaking workshops, where soda-lime glass became the foundation of an art form that still thrives today.
Why It’s Called “Soft”
The term is relative. Compared to borosilicate glass (the type used in lab equipment and high-end cookware), soda-lime glass melts at a lower temperature and becomes fluid and workable more easily. Glassblowers and lampworkers call it soft because it flows readily when heated, allowing them to sculpt, stretch, and shape it with less energy input. Borosilicate, by contrast, requires significantly higher temperatures to reach a workable state and is often called “hard glass” for that reason.
This fluidity is a double-edged quality. It makes soft glass ideal for the sweeping, organic forms you see in furnace-blown art glass, but it can be tricky for beginners learning on a torch. The glass moves quickly once it’s hot, demanding confidence and speed.
Soft Glass vs. Borosilicate Glass
The biggest practical difference between soft and hard glass comes down to how they handle temperature changes. Soft glass has a relatively high coefficient of thermal expansion, meaning it expands and contracts significantly when heated or cooled. That makes it vulnerable to thermal shock: expose it to a sudden temperature swing, and the uneven expansion creates stress at the boundary between the hot and cool layers. Cracks start at that stress point. Even a modest temperature change applied rapidly can produce enough force to fracture the glass.
Borosilicate glass, with its lower expansion rate, handles extreme temperature shifts without cracking. That’s why it’s the standard for laboratory beakers, ovenproof baking dishes, and scientific equipment. Soft glass would shatter under the same conditions.
On the other hand, soft glass is lighter and far cheaper to produce. It’s also easier to recycle, which is one reason the bottle and container industry relies on it almost exclusively. Borosilicate is harder, more durable under thermal stress, and slightly denser, but those advantages come with higher manufacturing costs and more demanding processing requirements.
Common Products Made From Soft Glass
If you can see through it or drink from it, there’s a good chance it’s soft glass. The list of everyday products includes:
- Windows and flat glass panels
- Bottles and jars for food, beverages, and pharmaceuticals
- Drinking glasses and tableware
- Light bulbs
- Mirrors (the glass substrate behind the reflective coating)
- Art glass objects, from Venetian vases to handmade beads
Soda-lime glass is relatively lightweight compared to leaded crystal, which is denser and more brilliant but also more expensive and less practical for mass production. When you pick up a standard wine glass at a restaurant, the lightness in your hand is a signature of soft glass.
Soft Glass in Art and Lampworking
Soda-lime glass is the original formula behind traditional Venetian glassblowing, and it remains the most popular choice for artists working in hot shops and at the torch. In furnace-based glassblowing, a gather of molten soft glass on the end of a blowpipe responds quickly to gravity, breath, and tools. Artists can pull, twist, and inflate it into shapes that would be much harder to achieve in borosilicate.
In lampworking (also called flameworking), artists melt rods or tubes of glass over a bench-mounted torch to create beads, figurines, pendants, and other small objects. Soft glass is the go-to material here because it flows at temperatures a standard oxygen-propane torch can easily reach. The wide range of colors available in soft glass rods also makes it a favorite for decorative work. Borosilicate lampworking exists too, but it requires hotter torches and a different skill set, and the color palette has historically been more limited.
Handling and Limitations
The main thing to understand about soft glass, whether you’re working with it or just using products made from it, is its sensitivity to rapid temperature changes. Pouring boiling water into a cold soft glass tumbler can crack it. Placing a hot glass dish on a cold countertop creates the same risk. The faster the temperature shifts, the greater the internal stress, and cracks initiate where the temperature difference between adjacent layers is steepest.
For artists, this means soft glass pieces must be annealed after they’re shaped. Annealing is a controlled slow-cooling process in a kiln that relieves the internal stresses built up during shaping. Skip this step, and the finished piece may crack hours, days, or even weeks later as those locked-in stresses slowly release. Properly annealed soft glass is durable for everyday use, which is why your windows and bottles hold up fine under normal conditions.
How to Tell Soft Glass From Other Types
Without lab equipment, distinguishing soft glass from borosilicate or leaded glass takes some observation. Soda-lime glass is relatively lightweight. If you’re comparing two similar objects and one feels noticeably heavier and throws more sparkle, that’s likely leaded crystal, which is denser and has a higher refractive index. Borosilicate glass has a slightly different feel as well, often described as more “clinical” in appearance, with less of the warm, fluid quality you see in soft glass art pieces.
For glassworkers, the behavior under flame is the clearest giveaway. Soft glass glows a bright orange as it heats and becomes fluid quickly. Borosilicate stays stiffer longer and glows at a different temperature range. The two types also can’t be mixed in a single piece, because their different expansion rates mean they’ll crack apart as they cool.