The Mystery of Sea Glass Strength

japanese fishing float

By Meg Carter

I have often wondered about the strength of sea glass. As a jeweler, the strength of each piece is something I test often. Sometimes great amounts of pressure are put upon the glass while I am setting them into jewelry. From time to time, a piece will snap or chip, which is followed by a deep sigh from me. My curiosity about strength deepened when looking at my collection of glass and considering the different colors.

strength of sea glass

Why is it that some colors seem to be found in larger pieces like white, black, and gray, while other colors like cobalt seem to be mostly smaller pieces? Is this just because these colors were commonly made thicker and harder to break, or is there another reason? I was determined to find answers to these questions.

Little did I know, this would lead me down an interesting rabbit hole mystery.

Customers often ask me when considering a sea glass jewelry purchase, “Will the glass break?” It’s a valid question when you ponder the material. We often think of glass breaking. At a young age watching cartoons on a Saturday morning, we became accustomed to the shattering sound of one character smashing a mirror over another character's head. Glass breaks; it’s a fact. But glass can also be incredibly strong. (Google “Prince Rupert’s Drops")

But why is there a difference when it comes to colors? I wanted to test my theory. Taking a hammer to pieces from my own sea glass collection to see how they broke clearly was not an option. I needed to talk with someone who tested sea glass strength regularly. Many artists drill sea glass, which is a good test of its strength.

this tiny oceanSuzie Thomas of This Tiny Ocean makes beautiful minimalist sea glass jewelry. She is well known for her bar necklaces. These necklaces feature six tiny pieces of sea glass that have been drilled to make bead-like elements. Having made tens of thousands of these necklaces, it is easy to do the math. Suzie has drilled A LOT of sea glass.

“All colors are susceptible to breaking,” Thomas said. “I can even hear a unique squealing sound of the drill when I know a piece is going to break.” She continued, “I find there to be a distinct difference when it comes to drilling different glass. Specifically, art glass with multiple colors tends to break very easily.” Both of us assume this is because of the mixing of the colors. Possibly, making layers of different compositions would create weak separations in the glass. Interestingly, Thomas said, “Shades in the color family of teal to juniper are the hardest to drill, not because they break easily, but because they are physically harder.”

I found this point to be particularly thought-provoking; sea glass pieces in the teal to juniper family can be found in larger and smaller sizes. So why is it hard to drill? I wondered.

I needed to investigate further.

I figured that speaking with a glass expert would be the next step in my research. I called Ed Streeter, owner of Conway Glass (not far from me in Conway, South Carolina). Ed has over 40 years of experience in the glass industry, building a business and studio with his wife, Barbara. I told him about my theory of the different colors having different levels of strength.

I asked, “Do the different compositions of color make the glass stronger and weaker?”

His response was a confident, “Absolutely.” Streeter went on to say, “Colors that contain iron in them will be stronger and when they do break, they will break into larger chunks.” (White, gray and black all contain iron, the same colors that I noticed often break into larger pieces.)

“White—which also has tin and titanium in the composition—cools very quickly in comparison to other colors of glass. The quick cooling results in a stiffer end product,” Streeter explained.

When asked what color would be the softest and break into smaller pieces, Streeter answered, “Blue would be the softest color glass. Cobalt blue is made by adding cobalt oxide. This additive makes the glass heat up quickly, cool more slowly, and stay soft longer when working with it as an artist.”

So not only do the chemicals making the colors in the glass influence the strength, also the rate at which the glass cools contributes to the strength of the glass. It was interesting to hear that my theory had science behind it.

Before ending my call with Mr. Streeter, I recalled what Suzie Thomas had told me about the teal and juniper glass being so hard to drill. I asked Streeter about these particular colors and the possible reason behind the abnormal strength. My theory was that pieces from this color family could possibly originate from Japanese fishing floats. His thought was that, “The original composition of the glass may be the cause of the hardness.”

My rabbit hole grew deeper. 

glass fishing floats

Starting around 1910, Japanese fishing floats were primarily made using recycled sake bottles. (See The Glass Floats.) The manufacturing of these floats was quick and cheap. Visual aesthetics were not the goal, but rather functional floats for the intended use.

Considering the original bottles that were used, I searched to find the manufacturer of these bottles. One of the largest and oldest companies in Japan producing alcoholic bottles is Nihon Yamamura Glass Co., Ltd. In the early 1900s, their manufacturing was done out of Nishinomiya City. According to their website, they purchased the silica for their manufacturing from Yamamura Shoten. This silica sand was mined from the foot of Mt. Rokko.

It is only in the past few decades that the soil of Japan has really been investigated by scientists. The silica sand sold by Yamamura Shoten to Nihon Yamamura Glass is a type of soil now referred to as andisols. Unique Properties of Volcanic Ash Soils,a paper by Masami Nanzyo of Tohoku University, studied the elemental makeup of the soils in Japan. Nanzyo’s paper states, “The unique chemical properties of Andisols are basically due to their aluminum-rich elemental composition.”

My curiosity was piqued about aluminum in a glass mixture. Further investigation brought me to Corning Glass in Corning, New York. For decades, Corning Glass has been on the cutting edge of glass manufacturing. One of their latest products, Gorilla Glass, is groundbreaking and one of the strongest on the market. According to Glass Works: How Corning Created the Ultrathin, Ultrastrong Material of the Future, an article written by Bryan Gardiner for Wired, “Gorilla Glass is said to withstand 100,000 pounds of pressure per square inch and if you own an iPhone, you most likely have a piece of this glass. It is used on 5 billion cell phone devices worldwide.” Gorilla Glass derives its strength from a specific key component: aluminum oxide. Mystery solved!

While enjoying your beachcombing, if you come across a large piece of cobalt blue, remember not only is blue a rare find, but a larger-sized piece is particularly lucky. Alternately, when you find a tiny piece of gray or black, this is also a rare find. If you dropped a piece of teal and thought it would break but it doesn't, or you try to drill a piece of teal and it takes a bit longer than other colors, you now have an idea why.

Learn more about floats

articles and information about glass fishing floats found on the beach

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    This article appeared in the Glassing Magazine November 2017 issue.


    Very interesting. Great information.

    carole webber May 25, 2021

    Thank you for your explanation. Now i understand .

    Dao N Howell August 03, 2020

    Wow, very thorough explanation. Thank you so much.

    Raisa January 09, 2020

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