Glass Chemistry Lesson

Written by Cathy Charles

Soda Lime Glass

Bullseye and Moretti glass are known as soda lime glass, which is one of the most commonly produced glass types in the world. Commonly available forms of soda-lime-silica glass are windows and glass bottles.

The name is derived from its chemical makeup, of silica, soda and lime. There is no fixed combination of these items as glass is considered a solution or liquid. A couple of articles reference different compositions such as 70 percent silicon dioxide (silica), 15 percent sodium oxide (soda), and 9 percent calcium oxide (lime), with much smaller amounts of various other compounds (Encyclopædia Britannica http://www.britannica.com/) or 6 parts washing soda; 2 parts limestone; 7 parts silica (http://cavemanchemistry.com/slides/tables/foil104.html).

While Silica forms the majority of the glass, the other ingredients influence the behaviour of the glass. The soda content acts as the “flux”, which lowers the temperature at which the silica will melt. This in turn affects the annealing temperatures of the glass. The Lime acts as a stabilizer, which makes the glass strong and water proof. Without Lime, water and humidity would dissolve the glass over time. (http://www.cmog.org/index.asp?pageId=738). Glass without Lime is termed “Water Glass”.

Colours

The variations in colour, which lampworkers love, comes from the addition of oxides. The smallest additive of iron gives the green tinge you find in window glass. As with painting, if you add a large amount of the colorants together and you will end up with black. The following table illustrates what colours some oxides create:

*COLORANT*	*GLASS COLOURS*
*Iron*	Green, brown, blue
*Manganese*	Purple
*Chromium*	Green, yellow, pink
*Vanadium*	Green, blue, grey
*Copper*	Blue, green, red
*Cobalt*	Blue, green, pink
*Nickel*	Yellow, purple
*Uranium*	Yellow, brown, green
*Titanium*	Purple brown
*Neodymium*	Purple
*Gold*	Red
*Selenium*	Pink, red
*Antimony Sulphide*	Red
*Praseodymium*	Green
*Cerium*	Yellow
*Carbon & Sulphur*	Amber, brown
*Cadmium Sulphide*	Yellow

* “The colour depends on the state of oxidation of the colorant, the type of glass in which it is used, and thermal treatment”(http://www.glassglobal.com/forum/topic.asp?FID=4&TID=41)

To create the opal or opaque style of glass, fluoride or phosphate compounds are added to the silica and they produce a crystal growth, also known as devitrification.(http://www.glassglobal.com/forum/topic.asp?FID=4&TID=41).

For those interested in the colour composition of glass an excellent reference is Coloured Glasses by W.A. Weyl, 1951 (fifth reprint 1999), 216 mm × 137 mm, 558 pages, black and white illustrations, ISBN 0-900683-06-X.

Borosilicate

Borosilicate glass is defined as any glass that has at least 5% boric (or boron) oxide in its composition. The Boron acts as the flux holding the silica together, like the soda in soda-lime glass (http://www.solsticeglass.com/about_borosilicate_glass.html). It’s the third most common glass type in the world behind Lead glass, and is used in everyday items like cooking plates and the more unusual items like test tubes in Laboratories. The glass is characterized as having a lower thermal shock level (due to its low COE) and is more resistant to chemical corrosion. (http://www.glassonweb.com/articles/article/41/)

October 2004

Return to the Articles