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Glass: Anything but Clear (Part 1)

During Intraterm, I had the pleasant and enjoyable opportunity to play with, craft, and harness the power of glass. I learned too much through that experience to avoid further researching and sharing it with you. So, here we go, a new series all about glass:

Episode 1 ⁍ Chemistry

Glass is made of your standard, molten, opaque sand. Sand is a silicate from its high composition of silica dioxide (SiO2). Silica sand melts around 1,723 ºC or 1996.15 K. Once cooled, glass adopts a new, clear, fragile, but strong form, quite different from its gritty, sandy origin. However, glass cools with a new chemical structure void of consistent crystalline patterns making it—well neither solid nor liquid. Categorized as an amorphous substance, glass behaves like and resembles both states of matter. Standard glass is an amorphous silicate, really, but with some slight modifications.

Glassmaking involves three ingredients: formers, fluxes, and stabilizers. The former is the base to all glassy substances, usually silica (SiO2). Other formers include boric oxide (B2O3) and phosphorus pentoxide (P2O5) with melting points ranging 400-600ºC. However, their glass products easily dissolve in water, rendering them quite useless. Regardless, pure silica is difficult to melt, being that the melting point is 1,723 ºC. To lower it, chemicals called fluxes are added, reducing the melting point to 800-900 ºC. Soda (Na2O), potash (K2O), and lithia (Li2O) are all fluxes, frequently incorporated via carbonate substance (i.e. with appended carbon dioxide), creating CO2 emissions—so not entirely eco-friendly. But again, glass with only former and flux has poor durability and the troublesome property of being water-soluble so, stabilizers like lime (CaO), magnesia (MgO), baria (BaO), or litharge (PbO) are added. The most common glass is made with a silica former, soda flux, and lime stabilizer—AKA soda-lime glass.

Typical glass is soda-lime glass, a combination of metal oxides like soda (Na2O) and lime (CaO) with the silicon dioxide in ordinary sand. It is inexpensive and easy to melt and shape, giving it the substantiality to become the most widely used glass (e.g. windows, bottles, light bulbs, jars). However, it yields poor durability, no chemical resistance, and poor thermal shock resistance as limitations.

In 1664, Ravenscroft discovered that incorporating plumbous oxide (PbO) in a 20-30% ratio to silica glass yielded much better malleability. Lead glass is now used artisanally in exquisite goblets and bowls.

In the 1900s, an age of mining and railroads, glass lanterns were engineered to withstand thermal shock by replacing some of the soda in glass with boric oxide (B2O3). Borosilicate glass, containing 5-13% boric oxide, could withstand a 200ºC temperature variation with added chemical durability, making it a great choice for scientific and culinary glassware.

Aluminosilicate glass with 5-10% alumina (Al2O3) was crafted with even greater thermal resistance and durability. However, it implicated more of the tradeoff between malleability and thermal resistance than borosilicate.

High-silica glass, 96.5-100% silica, is the glass most difficult to make due to its high melting point. Vycor is used to make 96.5% silica glass while fused quartz has enough pure silica to make 100%. Both possess outstanding durability and chemical/thermal resistance with the ability of blocking ultraviolet light. Spacecraft windows of 100% silica glass can withstand temperatures as high as 1,200ºC.

Now, all the colors in the glass are not just standard food dye. They are made primarily with other metal oxides, a different one for each color. Cobalt produces blue, chromium produces green, manganese violet, cadmium sulfide yellow, selenium pink, gold for ruby-red, etc. Just standard chemical colors. However, other factors like the glass composition and the reaction (oxidizing or reducing) in the glass furnace affect color as well. Copper alone can produce blue, green, or opaque red, depending on melting conditions.

Glass is an incredibly intricate substance, study, art, science, and story. Chemistry is the first step to fully understanding the mystery. Despite the molecular complexity of the substance, I hope you will have just as much fun as I will reporting the next thrilling aspect of glass.


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