If you leave a shard of glass in a dry place, it will retain its smooth, shiny surface practically indefinitely. It is one of the most stable materials you can find in a natural environment, and that's why at beach sites of very old dumps, when everything else (wood, metal, etc.) has completely broken down and disappeared, glass, or natural sea glass is what remains.

However, glass does decompose or "corrode."  Under natural conditions when exposed to water, and especially to salt water, chemical changes occur that produce the unique "frosted" or "etched" surface that is the hall mark of beach glass, including real sea glass.

When put under a high-power microscope, the surface of a newly-made sheet of glass actually appears potted and webbed, kind of like a photo of the surface of the moon. These "potholes" collect contaminants from the surrounding environment that do eventually break down the surface of the glass and give genuine sea glass its unmistakeable patina.

Glass is composed of:

1 - Silica

2 - Alkali (usually either soda  ash or potash derived from wood ash), 

3 - Lime  

The combination of soda and lime has been the most common way to make glass down through the centuries and is still the most common glass today.

In old glass, soda glass is characteristic of southern Europe, where it is made from crushed white pebbles and soda ash from burnt marine vegetation. 

Soda glass, which is often used for the manufacture of cheap glass, is twice as soluble in water as potash glass.

So it is possible that a piece of real beach glass that is deeply etched may not be not be as old as the natural sea glass right next to it that is less deeply etched, depending on the whether soda or potash was used in the original glass.

Although glass is very durable, any glass, 17th-century glass in particular, can undergo complex disintegration.

Potash glass is more characteristic of interior Europe, where it is made from local sands and potash derived from wood ash and burnt inland vegetation. A little salt and tiny amounts of manganese are added to make the glass clear, but potash glass is less clear than soda glass. Most early glass is green because of iron impurities in the materials.

As long as the original glass mixture was kept in balance, the resulting glass will be stable. But problems come up when there is too much alkali and/or too little lime in the mixture. When this occurs, the glass will be especially affected by moisture leading to a deeper, rougher appearance seen in some instances of real beach glass.

In all glass, the surface of the glass absorbs moisture from the surrounding elements. The absorbed moisture and exposure to carbon dioxide causes the components to change to sodium or potassium carbonate. Both of these components are highly susceptible to moisture. In water, especially salt water, the sodium and potassium in leaches out, leaving only a fragile, porous hydrated silica network.

This process gives the surface of real beach glass a frosty appearance.

At the present state of knowledge, the decomposition of glass is imperfectly understood, but most glass technologists agree that glass decomposition is due to preferential leaching and diffusion of alkali ions across a hydrated porous silica network.

Real beach glass often has iridescent layers on the surface that are formed by the leached silica layers.

The dissolving of genuine beach glass takes place at a constant rate. Of course, the water itself, with its differing components (fresh water, salt water, surrounding runoff and minerals) affects the rate and depth of this leaching process and the resulting look of the mature genuine sea glass.

Have you wondered about where your real sea glass came from, or how the natural beach glass got its colors in the first place?

Next - More information on the origins of real sea glass colors.

See also - 

What is sea glass?

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