Answered By: Aprille Nace
Last Updated: Sep 17, 2016     Views: 312

David Whitehouse explains the difference between the terms glass and crystal in the "Glass Glossary" (found on the CMoG website http://www.cmog.org - Research, Teach and Learn - Glass Glossary:

Crystal
A popular term for colorless lead glass which has a high refractive index and consequently is particularly brilliant. Today, the word is often used to describe any fine glass tableware.

Brilliantly clear "crystal" was originally produced to imitate “rock crystal" or colorless translucent natural quartz.

According to Harold Newman in An Illustrated Dictionary of Glass (London: Thames and Hudson, 1977), some countries establish a minimum percentage of lead in crystal. For example, in England crystal "must contain at least 24% lead oxide...and that with 30% lead is called 'full lead' or 'cristal superieur.' Recent regulations of the European Economic Community (EEC) forbid any glass to be called or labelled 'crystal' unless meeting such standards, and a special label is provided for each class." The British Glass website provides a current definition of lead crystal: http://www.britglass.org.uk/AboutGlass/TypesofGlass.html#2

There are no standards in the United States. "Crystal" tends to be a popular or marketing term and may be used to describe any fine glass tableware. It may or may not contain lead.

What gives glass the green tint on the edge of window glass?
The slight green color is generally caused by iron oxide which occurs naturally in glass, "from the sand, other batch materials, or from the pot or tank in which the glass was melted." (Bray, Dictionary of Glass, 2nd ed., p. 94).
Window glass is generally made of soda-lime glass. The lime acts as a stabilizer while the silica is the glass former (see the British Glass website for more information about its composition). To make glass colorless, a mineral must be introduced "into the batch which neutralizes the coloring effect of other materials. Traditionally, the decolourising agent used over many years was manganese dioxide.... In the process, the manganese and the iron each absorb the light which the other transmits. Selenium and a mixture of selenium and cobalt have also been used in a similar context." (Bray)