What is this stuff called 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, from simple containers to hold food or water, windows to see through yet protect us 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 quite complicated if exploring the topic from a purely scientific perspective.
WHAT IS GLASS MADE OF?
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 three 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’s the most common type used for most (but not all!) utilitarian bottles, jars, tableware, light bulbs, vases, window panes, art glass, etc.
After the ingredients are heated in a furnace to a molten state, and thoroughly 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.
The primary ingredient in glass is silica (silicon dioxide), a combination of silicon and oxygen, chemical formula SiO2, which is, in essence, quartz, agate, or “flint”. The hard rock commonly known as “flint” which was frequently used for fashioning arrowheads (projectile points) by earlier cultures such as the American Indian (Native American), 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. Most (not all) 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. Typical silica sand may be a light off-white, beige, tan or ivory color.
In an average “bottle glass” recipe, silica may comprise about 70 to 72% of the batch, 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.
CULLET
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 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 discarded 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.)
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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 and soda) along with cullet, other minor ingredients may be added to the molten “batch”. These include elements that increase strength and/or durability, such as Boric oxide to produce borosilicate glass – a very strong glass that resists breakage. Lead may be added for improved brilliancy. Other elements include 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.).
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, 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 will depend on the amount of iron present, in combination with other factors.
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. (You might wish to check out the links posted at the bottom of this page).
Very tiny amounts of iron may 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 sliver of glass is held up 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 “float 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 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 here: Glass – Wikipedia summary
Below are links to a number of web pages with more background information on glass and glass manufacturing.
The Chemistry of Glass (Corning Museum of Glass) https://www.cmog.org/article/chemistry-glass
Three common glass types and their properties and applications (Kopp Glass website – this is a superb article written by Adam Willsey)
https://www.koppglass.com/blog/3-common-glass-types-properties-and-applications
February 20, 1869, article about how common window glass is made, published in the Scientific American magazine.
https://www.scientificamerican.com/article/how-common-window-glass-is-made/
How glass bottles are made (Oberk.com website)
https://www.oberk.com/packaging-crash-course/how-glass-bottles-are-made
Float Glass (Wikipedia)
https://en.wikipedia.org/wiki/Float_glass
The four main types of Float Glass (modern mass-produced flat glass for windows, building construction and automotive use)
https://www.basystems.co.uk/blog/2016/10/glass-types/
The science behind amber glass – How these bottles protect liquids
https://www.airseacontainers.com/blog/the-science-behind-amber-glass-how-these-bottles-protect-liquids/
Color: The most obvious property of Glass
https://geology.com/articles/color-in-glass.shtml
The Chemistry of Colored Glass
https://www.compoundchem.com/2015/03/03/coloured-glass/
BOOK: “MODERN GLASS PRACTICE” (written by Samuel Ray Scholes) . This is a very good, detailed textbook on the manufacturing of glass. Many editions are out there. This book was first published in 1935 with many revisions up until at least 1975. I recommend this book for anyone interested in the background and practice of glass making from a more industrial viewpoint. The book can be expensive, but more reasonably priced editions may sometimes be found by searching for this title on Bookfinder.com. You may find a copy at your local library.
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Hi David,
I am very impressed with the countless hours and hard work you have put into this AWESOME ARTICLE, I could read all day.
I do have a question, although I totally understand if you do not have time…you have already shed so much light on glass manufacturers and years to possible values and how to locate prices.
However, all I know about the “decanter set” I have is that they are shaped like a genie bottle, with stopper, the rounded bottom has like a bubble pattern, favors hobnail but is not hobnail. The bottles are tall, not sure how tall. They are in storage and I need to get them out. They are a lightish, clear, see-through blue. I think I identified as Aztec Blue?
They have absolutely no markings on them anywhere to identify??
My mother in law left them to me along with other glassware of age, these two have me stumped. I have researched but can not find any like these 2.
I would greatly appreciate any information. If you have time, if not, fine. As I said this is a very AMAZING Research tool. You have worked hard and it has not gone unnoticed!!
Have a great Day!
God Bless You!
Millie
Hi Millie,
Thank you for your kind words about the site! When you have time can you please check my “Contact Information” page for my email address, and send me an email including photos of those decanters / bottles after you are able to unpack and take some pictures of them. Without photos I can’t be of much help.
Before seeing them, I am guessing they might be products of Italy (where many of those types of tall decorative bottles in many colors have been made), or perhaps Blenko Glass or another maker of art glass here in the US. But there are many, many different types of decanters and so-called “genie bottles” made around the world, so without photos this is just speculation. THANK YOU and best regards!
~David
I found a light purple bottle on a beach in Wilmington, NC about the size of a wine bottle. May have “WW” in script inside a circle on the bottom. Any info you can provide would be appreciated.
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?
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.
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!
~David