What is Glass??

          WHAT IS GLASS??   An Overview

So, just what is this familiar, much loved (but mysterious) substance we know as “GLASS”?   An incredible variety of objects and uses have been found for this material, all the way from the most simple containers to hold food or water, and windows to see through yet protect from the outside elements, to durable glass building blocks used in construction.

Sophisticated technological developments in recent years have spurred the widespread use of thin LCD (Liquid Crystal Display) glass plates as components in the manufacture of devices including mobile smartphones, tablets such as the iPad, notebooks, computer monitors, microwave display panels, television screens, etc.   The “Google Glass”, a wearable computer with an optical head-mounted display similar in general appearance to a pair of eyeglasses, includes a tiny glass LCD display panel, although the main frame, because of durability and weight considerations,  is actually made of a durable plastic and titanium.

The general term “GLASS” actually covers a very wide variety of compounds, and the subject can be extremely complicated,  looking at it from a purely scientific perspective.

HOWEVER, for the purposes of this website, in simplest terms, glass is a hard, brittle material, usually transparent (less commonly translucent, or entirely opaque), formed by the melting together of 3 main ingredients: sand (silica or quartz),  lime (calcium) and  soda.   This type of glass is the most frequently made of all glasses, and is called “soda-lime glass”.   It is the most common type of glass used for most (but not all!) utilitarian bottles, jars, tableware, light bulbs, vases, window panes, art glass, etc.

After the ingredients are heated to a molten state, and are sufficiently mixed together, the mixture, typically of the consistency and appearance of a hot SYRUP,  is then cooled just fast enough so that it reaches a “solid” state but without acquiring an orderly, crystalline arrangement of the atoms that make up the glass.

Cobalt Blue candy dish (Indiana Glass Company); Red "Crackle glass" vase (Rainbow Glass Company); Aqua Telegraph line Insulator (Brookfield Glass Company)

Cobalt Blue candy dish (Indiana Glass Company); Red “Crackle glass” vase (Rainbow Glass Company); Aqua Telegraph line Insulator (Brookfield Glass Company)




The primary ingredient in glass is silica (silicon dioxide) which is, in essence, quartz, agate, or “flint”.  The hard rock known as “flint” which was commonly used for fashioning arrowheads (projectile points) by earlier cultures such as the American Indian, is really nothing more than a form of quartz or silicon dioxide.  Silica is a very hard substance, and is typically the primary constituent of ordinary beach sand. Sand beaches are the end result of long decomposition of silica-bearing rock. Much of the softer, more soluble components of the rock has long since eroded away, leaving silica, in the form of tiny grains, as the primary remaining ingredient.  Much of typical silica sand may be a light off-white, beige or ivory color.

In an average “bottle glass” recipe, silica may comprise about 70 to 72% of the glass, lime (calcium oxide) approximately 9 percent, and soda (sodium oxide) about 15 percent. These proportions will vary somewhat depending on the exact type/application of the glass involved.


Also, in actual practice, virtually all container glass contains some percentage of cullet, which is crushed glass from recycled bottles, jars , etc.  Many recycling programs around the United States collect used glass containers (along with metal, cardboard, plastic, etc), which is sent to materials processing plants for sorting, separation and crushing, and eventually that crushed glass is sold to be used by glass manufacturing companies.  The percentage of cullet used in glass formulas varies widely, ranging from 10% to 50% or more.  In general, the more cullet used in a particular glass batch, the easier and faster the batch will melt, saving on energy costs, and this decreases the cost of manufacturing the finished product.

(In many European countries, the rates of recycling glass containers are much higher than in the United States, reaching 90%, compared to around 33% in the US.  Of course, this means that in the US about 2/3rds of new glass containers are thrown away in the trash after use, ending up in landfills and dumps instead of being recycled.  Much of this disparity is because of the very high cost of processing – the separation of glass from other materials and contaminants in the waste stream.)

The soda helps to lower the temperature at which the silica melts (thus lowering production/energy costs) to the neighborhood of 1000 degrees C. (1,800 F), making it easier to manufacture the glass. However, since the soda also causes glass to become more water-soluble, the lime in the formula counteracts that, causing the glass to be become much more stable, wear-resistant and virtually non-soluble in water.

Besides the “main three” (silica, lime, soda) along with cullet,  other, minor, ingredients may be added to the molten “batch”.  These include various elements that increase strength and/or durability: Boric oxide for “borosilicate glass”;  or brilliancy (such as lead), or those added intentionally to affect the color of the glass (such as selenium for pinks or red; manganese for purple; cobalt for rich cobalt blue, etc).



The many elements used to color glass, and the many slightly different shades of color that can result (depending on a HOST of variables), is an an art and a science in itself, and the interested researcher can find a lot more in-depth information on this aspect of glass manufacturing on the internet, if desired.

Most “natural” glass (that is, glass in which no extra ingredients have been added specifically to affect color or masking of color) will exhibit some shade of light blue, aqua, or green tint, and this results from that fact that almost all sand has a tiny percentage of iron in it as a naturally-occurring impurity.  Iron causes the greenish or bluish color, and the exact resulting shade of color can depend on the amount of iron present, in combination with other factors.

Very tiny amounts of iron might result in a faint aqua tint to glass (as might be seen along the edge of home or automotive window glass panes), and very heavy amounts of iron in the sand could result in glass so dark as to be called “black glass”, which was common for a lot of the crude utilitarian containers and bottles that were made throughout most of the 1700s and 1800s.  Much of the so-called “black glass” made with iron is actually a very dark olive green or olive amber color.  (Black glass made with manganese as the “darkener” appears as very deep purple when a thin section is held to the light).

Typical soda-lime glass can be manipulated by molding into a virtually unlimited variety of possible shapes and forms. Glass can be blown into a mold (either by mouth or by machine), blown “free-form”, pressed into molds (as is common for most modern mass-produced tableware, drinking glasses, etc), drawn out into very fine diameter cylindrical fibers, formed into sheets of perfectly flat “plate glass”, and many other forms and uses.

Glass has been made and used for many thousands of years, and no one knows for sure when it was first discovered, or manufactured by man. However, there is some evidence uncovered by archaeologists that suggest the first glassmaking may have developed approximately 3500 B.C. (5,500 years ago) in Syria, Mesopotamia, or ancient Egypt.  Beads are believed to be the first type of glass object made by man.

Obsidian, a naturally-occurring type of glass, very close in composition to man-made glass, and usually an intense black color, is formed from lava erupting from volcanoes, and has been utilized by man for thousands of years.

In our modern technological age, the uses of this “simple”, “old-fashioned”, yet ultra-sophisticated substance known as glass are advancing in ways no one would have dreamed of just a few years ago!

Please click here to go to my website HOME  PAGE.

Click here to go to “Page one” of the Glass Bottle Marks pages.  (A listing of manufacturers’ marks seen on glass bottles, jars, insulators, containers, tableware, etc).

For a more involved discussion on glass and some of the various types, you can check out Wikipedia’s webpage on Glass here:  Glass – Wikipedia summary


3 Responses to What is Glass??

  1. Noel Kennedy says:

    I have a Green Tinted glass bottle with some numbers on the side and the bottom and the bottom also has a keyhole symbol. I’m trying to date it back anyone have an idea of how old it might be?

  2. Kathy says:

    Hi. Who is the maker of the green glass (bottom) is shown on your web home page. It looks like an eye? Also has number left right top and bottom.

    • David says:

      Kathy, that is the first (earliest) mark used by Owens-Illinois Glass Company. That is easily the most frequently seen mark on mid-20th century American-made glass containers. I get more queries on that mark than any other. For some more info please see my page on Owens-Illinois Glass Company.
      Hope this helps!

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