The Science of Sea Glass

By Richard LaMotte

Frosty sea glass (Richard Lamotte).

Upon picking up that first extraordinary seaside gem, a simple piece of glass worn and frosted to perfection, one develops such a deep admiration for Mother Nature’s talent that can lead to a lifelong quest for more. The object has been strengthened during its long journey, yet its softly rounded edges provide it with an ethereal feel. How does this magnificent transition take place?

Well-worn blue sea glass (Richard Lamotte).

Lengthy exposure to a perfect blend of both moisture and abrasion provides essential tools in the palette for creating such masterpieces. Each shard requires just the right time and conditions. What most sea glass enthusiasts don’t realize is that the physical characteristics of the glass itself play a significant role in the length of time required for its metamorphosis. Once collectors comprehend all the forces involved in this intricate process, they can grasp just how unique their finds truly are—regardless of color.

Hydration

In addition to the construction of the glass, many people find it hard to believe that the surfaces of common glass objects are not hydrophobic—they are hydrophilic, meaning the surface of the glass can absorb water on a microscopic level. Glass does not repel every molecule of moisture that comes in contact with its exterior.

Multi-colored sea glass (Kirsti Scott). Glass under a microscope (wecarecarcare.com).

What appears to be a shiny and smooth water repellent surface actually has a texture that resembles that of a dish sponge—but obviously far less absorbent. Under a microscope one can see a complex field of nooks and crannies unimaginable to the naked eye. This is why common glassware at home can get stained and cloudy over time, depending on the mineral content within the domestic water supply.

The most common type of glass found by sea glass collectors is from bottles made with glass referred to as SLS, meaning its contents are primarily Silica, Lime, and Soda. It is these latter two components, the compounds of lime (calcium oxide) and soda (sodium carbonate) that begin to break down first. Silica is the inert ingredient providing far more of the structural integrity to the glass form. One way to think of silica is as the sand at the core of the formula that makes up about 75% of the batch. The lime and soda are added in as flux to make the batch melt faster, thus requiring less heat and making the glass more malleable, and therefore easier to shape. During production of the glass batch, if the ratio of soda added into the batch is too high, it can make the glass more susceptible to corrosion, especially if the ratio of lime is also too low. Those who bake a cake or mix a cocktail know very well that any critical element added in the wrong proportion can ruin a batch. Just as in baking or mixology, portion control is equally critical in the production of stable glass.

Antique bottles from the 1700s (Richard LaMotte). Dark green sea glass (Kirsti Scott).

One factor contributing to the strength of historic glass was the addition of excess metal ions. Whether added to or residing in the silica used when forming the glass, iron was a common element found in historic bottles. In the 1700s, when glass vessels had to be sturdy enough to survive sailing across the Atlantic in wooden crates, it was customary to add iron slag, which created bottles in almost opaque deep green or brown colors. These are often referred to as “black glass” by collectors. The dark color was considered helpful to protect its contents from spoilage due to sunlight, even though most of the beverage carried sufficient alcohol levels to protect the shelf-life well enough on its own.

Pieces of those black glass vessels are often found as large shards, sometimes even entire bottle bottoms, handblown by craftsmen during the Colonial Period and early 1800s. These containers were so robust they resisted over 200 years of hard knocks. Collectors find shards from these containers along the shores of Caribbean islands or coastlines and bays of the Eastern United States. Often these mouth-blown black glass bottles had very thick bottoms, and the extra iron in the molten forms made them far more stable for reuse over and over.

Leaded crystal sea glass (Richard LaMotte). Milk glass sea glass (Richard LaMotte).

There are other forms of sturdy shards that sea glass collectors notice as very slow to develop soft edges or frosted surfaces. One is leaded glass tableware, also called “crystal,” which is especially robust and tends to develop a hazy gray tint after decades of solar exposure. Another is milk glass jars and tableware which are normally found in white or cream colors, and sometimes in pale green or blue. It often appears that milk glass shards show little to no abrasion or wear on their edges. Since bone ash or tin oxide is added to create milk glass, it enables these shards to maintain their sharp edges far longer than typical SLS glass shards.

Lavender sea glass (Kirsti Scott).

Another issue with batch control appears when glassmakers, prior to 1916, added a bit too much manganese to create colorless glass. Subsequent exposure to sunlight caused the manganese within glass vessels to oxidize into colorful hues of soft lavender to purple sea glass. The more manganese added into the batch, the deeper its color becomes under the same solar exposure.

Honey amber sea glass (Claire Ferguson). Scottish sea glass and marble (Kirsti Scott).

Of course, there are also more modern forms of glass, like borosilicate glass, which is stronger than SLS and will take far more time to become frosted, well-worn pieces of sea glass. But the good news is that those are rare finds since the vast majority of sea glass found today comes from softer SLS glass bottles.

Italian sea glass (Kirsti Scott). Blue sea glass (Kirsti Scott).

The slow “weathering” of glass surfaces is sometimes referred to as “hydration,” as moisture helps the soda-based components in the glass to be the first to get leached out from the glass surface. In higher pH levels such as seawater, above 8.0, the process is more rapid. As this pitting begins, the robust, solid, silica form remains intact, but tiny, open pockets vacated by soda and lime leave space for elements like chlorides to infiltrate the surface and form crystalline precipitates.

English sea glass (Kirsti Scott). English sea glass (Kirsti Scott). Icelandic sea glass (Kirsti Scott).

When dry, these can almost appear like sand on the glass as they refract the light, leading the once transparent piece of glass toward translucency and later to near opacity. This process can take decades to show obvious signs unless, of course, the surface of the shard is also getting impacted by a coarse seaside environment. Then things start moving a bit faster.

Surface attrition from physical conditioning that doesn’t reach the inner crevices of a sea glass Coke bottle (Richard LaMotte).

The Rub

The most active force for reducing the transparency of a new glass object is from direct surface abrasion. This physical conditioning process is subject to the forces of wave action, and the substrate beneath the shard of glass is exposed as the waves move it in and out along the shoreline, often in zig-zag patterns for years. The formal name for the primary force moving sand and pebbles across the beach is “longshore drift.” Geologists refer to the action of wearing down rocks and pebbles on a shoreline as “attrition.” Those studying coastal sediment transport call the area where waves are landing on the beach the “swash zone.” They refer to the waves coming in as the swash and the receding wave as the backwash.

Longshore drift transport of natural coastline particles (VectorMine/Shutterstock.com).

Among this endless fury of waves, the strong silica structure on a piece of sea glass will be ultimately worn down—thus creating those desirable soft edges. Currents just offshore can cause shards to migrate short distances, but not as far as sealed bottles or plastics. Tides and varying wind directions play obvious roles as they shift continuously, leaving the littoral zone as nature’s workshop to make everything it contacts a bit smaller. Coarse strata on the beach in the form of pebbles and stone increases the attrition rates of glass objects while soft sands tend to require longer periods to sharpen rough edges into softer shapes. Studies have been performed using marbles with RFID tags on beaches to study abrasion loss. One study in Italy showed a 2.4% weight loss in two months while another beach showed an 8.5% loss over the same period. The study noted that some marbles settle into areas away from the swash zone and of course had far less abrasive weight loss.

English sea glass (Kirsti Scott).

The question so many collectors have asked is, “How long does it take to make an ideal piece of sea glass?” It truly depends on contact time, years in the swash zone, and what type of abrasive surface is in that zone.

Sea glass marble (Kirsti Scott). Well-worn shards (Richard LaMotte). Black sea glass (Richard LaMotte).

Freshwater versus saltwater

Veteran collectors on the Great Lakes know their beaches often produce sea glass in small bean-sized forms, as opposed to larger shards commonly found along sandy coastal and bay beaches. They may also notice their shards have slightly less frosting on the surface. While most ocean waters have one hundred times more chloride than freshwater, that differential is shrinking in the Great Lakes. Salt runoff has been adding considerable levels of chlorides over the past fifty years. Back in 1950, a piece of sea glass found on Lake Erie was likely a bit clearer on the surface, despite being well-worn by the attrition of wave action on the pebble-rich shoreline.

Scottish sea glass (Kirsti Scott).

Chloride-rich water is far more corrosive than clean freshwater. Thus, salt in seawater is more destructive to glass surfaces and many other material surfaces. In regions where one might expect freshwater in local waterways, run-off from de-icing of roads can have detrimental effects on surface water. One key example is what happened in Flint, Michigan back in 2014. The city switched from Lake Huron water to water from the local Flint River, which happened to be eight times higher in chlorides. That, along with a lower pH level, led to rapid corrosion within pipes made of lead or welded with lead solder. The result was a health catastrophe begun by escalated corrosion.

Anyone fortunate enough to live near an ocean has observed far more cars and metal fixtures with obvious signs of corrosion and rust than on similar items further inland. Those manmade items are better kept away from a saline environment. Meanwhile, the salt air that draws so many of us to the shore is busy creating nature’s gems.

This article appeared in Beachcombing Magazine Volume 49, the July/August 2025 issue.