The Stretch Glass Society gratefully acknowledges the author, David Adams, for granting us permission to share this article with you. This article first appeared in the National Depression Glass Association’s News & Views Newsletter in the December 2017 / January 2018 Issue, Mark Buersmeyer, Editor. We thank Mark and the NDGA for their assistance in obtaining David’s permission for us to include this article among our technical information posts.
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What goes into the making of glass?
By David Adams
I am fascinated by the subject of glass and mesmerized by watching glass artists create freehand objects in glass. If you haven’t ever seen the process in person, check around where you live – there are over 40 Universities in the United States that offer glassmaking as an elective. You’ll be able to watch demonstrations. The Corning Museum of Glass, Corning, NY, has daily exhibitions of the art of glassmaking.
I have heard, over the years, that most glass is made primarily of sand, soda ash, and either lime or lead. OK, but what the heck is “soda ash” and while I put a lime in my margarita, what kind of lime do they use. And lead is a heavy silvery-gray metal, so how does that figure in making beautiful crystal glass?
One more thing: The term lead crystal is, by technicality, not an accurate term to describe lead glass, since glass, an amorphous solid, lacks a crystalline structure. The use of the term lead crystal remains popular for historical and commercial reasons.
Here are some of the answers:
First ingredient: sand. The main raw material used to make glass is sand. To make clear glass, a special sand called silica sand is used. This fine white sand is needed because it is very pure and does not contain other unwanted chemicals. It is “washed” to remove as many impurities as possible, as impurities can adversely affect the clarity and color of the glass.
Next ingredient: soda ash. This is a compound very much related to baking soda. Soda ash is chemically called sodium carbonate (Na2CO3). Soda ash plays a vital role by reducing the furnace temperature necessary to melt the silica used, thus reducing the energy required to produce glass. For some glass formulae (notably lead glass), potash (K2CO3) is used in place of soda ash.
Third ingredient: usually either lead or lime. Lime is a shorthand term for limestone, which provides both calcium and magnesium to increase durability and control viscosity during the formation of glass. Consistent quality and low trace elements (i.e. iron, chromium) are essential. Approximately 90% of the world’s glass production is soda-lime glass.
Lead glass (commonly known as lead crystal) is used to make a wide variety of decorative glass objects. It is made by using lead oxide instead of limestone, and potassium carbonate (potash) instead of the sodium carbonate (soda ash).
There are many other types of specialty glass formulae, using ingredients other than lead or lime. One well-known example is borosilicate glass – which we know as “oven glass” or “Corning Ware.” In this case, boron trioxide (B2O3) is substituted for lead or lime as the third main ingredient. Borosilicate glass is known for being resistant to thermal shock, more so than any other common glass, so it can tolerate rapid changes in temperature without damage.
Other ingredients: Regardless of which type of glass was being made, arsenic was added to the glass formula. That's right -- arsenic. It was specifically used to reduce the bubbles that formed as the ingredients were being melted.
Colored glass: If the manufacturer is making colored glass, then there were additional chemicals added to the glass mixture. Some of these include compounds of the following elements: Chromium, Iron, Cobalt, Nickel, Selenium, Neodymium, Manganese and Sulfur. Some glass formulae even required the addition of pure Gold.
Toxic material: You might recognize Arsenic and Selenium as toxic materials. How about Uranium? That’s pretty toxic and it sounds somewhat scary. Yet it was (and still is) a common ingredient used to produce colored glass (and pottery). Is there anything to worry about with these toxic materials? The answer is a resounding NO.
Where a formula for glass might call for 1,400 pounds of silica sand and 540 pounds of soda ash (these numbers were taken from an actual glass batch formula), the coloring agent for the mix might be something like 4 pounds of uranium oxide. Four pounds of anything mixed in a ton of other stuff is a pretty insignificant amount. The amount of arsenic added to the same formula might amount to about 5 pounds, and all of it is essentially used up in the process of removing the dissolved gas bubbles before the pot of glass is ready to be used.
Similarly, small amounts of the coloring agents are used for the various colors. To make lead glass, using the same 1,400 pounds of sand, the company might use up to 600 pounds of lead compound. That doesn't mean that you have anything to worry about - the lead is part of the glass itself, and the lead compound is what gives the glass its clarity.
Glass colored by Uranium will set off a geiger counter if the counter is placed directly next to the glass. Move the counter a foot away and you will get no reading at all. The glass is not dangerous to humans unless you hold it next to your body 24 hours a day, 7 days a week for about 15 years or so, after which you might show signs of radiation damage.
Lead in glass can come out a little, for example, if you leave alcohol in a lead glass decanter for 15 or 20 years, there is a chance that some of the lead might have infused the alcohol. Leave it in for 5 years and the likelihood is almost zero. And why didn’t you drink it before then?
The bottom line is that the various compounds used to manufacture glass and to color it, are not harmful to humans and serve only to make the glass more enjoyable for us all.