[Work in Progress! Likely revised often in the next week. My tables were broken when published, I will revise them shortly - Brynn 8/7/18]
Components of Glass
Formers - The material that makes up the bulk of the actual glass. Most commonly this is Silica Dioxide (SiO2), as in soda-lime glass. Aluminum fluoride and zirconium fluoride are also options.
Fluxes - As in other trades and crafts, fluxes lower the melting point of the former(s) involved. Feldspar, soda, natron, and potash are all fluxes.
Stabilizers - These materials impact a variety of attributes of the glass, like ability to withstand weathering. Limestone (calcium carbonate) is a stabilizer. Sodium and Magnesium are water-soluble and can leach out in water without stabilizers.
Colorants - Optional components to add (or remove) color and opacity to the glass. Mostly metal oxides. Gold, silver, copper, nickel, cobalt, iron, and many others act as colorants. Lead acts as a decolorant. Tin and antimony are opacifiers.
Fining Agents - Optional compounds to remove bubbles, e.g. arsenic trioxide.
Types of Glass
Soda-Lime - The type we encounter every day, roughly 90% of all glass produced. Silica’s very high melting point is lowered by the presence of soda (Sodium carbonate). Different formulations are used for flat (or float) glass vs other containers.
Potash-Lime/Forest glass - Potassium Carbonate (potash) is used instead of sodium cabonate (soda). The potash comes from burning inland plants, hence the term Forest glass. The ashes are soaked in water (leached) to extract the needed substances.
Borosilicate - Glass with a very low Coefficient of Expansion (around 33), which is very resistant to thermal shock. This makes it useful for cooking ware, scientific glass equipment, and artisan smoking vessels.
Fluoride glasses - Glasses which do not use silica as the predominant ingredient. Aluminum fluoride and zirconium fluoride are both options. This glass is usually used for advanced engineering and scientific applications, such as fiber optic cables.
Lead glass (Lead “crystal”) - Originally glass with a very high lead content (potentially over 50%, higher than the ~30-40% silica content). This glass has a brilliant reflective quality and is very clear, making it the best choice for cut ‘crystal’ glassware. Due to concerns over the lead content, modernly barium, zinc, and potassium oxides may be used instead of lead.
Flint glass - An early form of lead crystal. 4-60% lead, flint was used as the source of silica originally, giving it the name.
Fused quartz/fused silica - Silica fused into glass without fluxes. This requires extremely high temperatures, which were not attainable until modern technology (1650°C/3000°F). This is often used for engineering/scientific needs, though rods are also used by lampworkers.
Milk Glass, Vaseline Glass, Carnival Glass - Mostly names based off specific colors, rather than actual differences in the glass.
Porcelain, thermoplastics, glass-ceramics, etc. - There are many other things which, in strict technical terms, are glasses. They are not what any of us laypeople think of as “glass” however and are ignored for our purposes.
Specific Glass Chemistry Concepts
Striking Colors - Colorants are added to the batch of raw glass, and at the high temperatures involved they disperse well throughout the material. The colorants are able to bond with other elements in the batch and, when quickly cooled, stay in those compounds. The glass is often clear when leaving the factory.
Once the color is introduced to a torch or furnace, the temperature is hot enough to cause the colorants to break free from the compounds and form oxides or colloidal particles. Colloidal particles mean the colorant is dispersed throughout the glass without actually bonding with/truly becoming part of the glass (clouds, mayo, milk, and many other things are colloids too). This temperature is also cool enough to prevent the colorant from forming the original bonds at the factory (and/or is cooled more slowly).
Silver glass is a specific form of striking color. Particles of silver are colloidal particles in the glass. At the working temperature, larger chunks form and cause the glass to get beautiful metallic sheens.
Coefficient of Expansion (CoE) - The CoE of a glass is something that often scares people. It is a value indicating how much the glass expands when heated, and generally speaking all glass artists need to know is that you can’t join two glasses which have more than a 1-point difference in CoE. Glaskolben (a glass bulb-and-tube used to make ornaments, which borosilicate lampworkers may also call a ‘point’) that are commercially available have a CoE of 89, but are still compatible with CoE 90 glasses which are commonly available at art glass suppliers.
This value is actually an average! The CoE is tested at a number of temperatures, generating a curve. The number glass artists (and glass scientists) usually refer to is the average, roughly. While we usually refer to it by a number, in scientific terms this number is expressed in scientific notation, 10-7 per degree (in Kelvin). It’s further muddled by the fact that the change is expressed as a fraction of the length, rather than a typical unit like millimeters.
CoE 90 = 90 x 10-7 K-1 = 0.000009 change per degree Kelvin
(The space between 100 degrees Fahrenheit is roughly 55 degrees Kelvin, for comparison)
Trivia: Ceramic artists and scientists use the same values, usually expressed in terms of 10-6 x K-1, so CoE 90 in “glass” would be CoE 9.0 in “ceramic” shorthand.
But HOW is it measured? - With a “push rod dilatometer”. A sample of a material is fixed in place, a rod with a known CoE is placed against it, and a VERRRRY sensitive device is on the other end of the rod. The sensor has to be able to detect very tiny changes. As the sample is heated it will push on the rod and thus the sensor. Then “math happens” to correct for the reference material’s CoE. Fused silica/quartz rods are often the reference.
Example Coefficients of Expansion:
Satake Glass
(Used in Japanese lampworking)
CoE 120
Effetre, CiM, Vetrofond, etc
(Common in western lampworking)
CoE 104
Spectrum & Wissmach fusible glass
CoE 96
Bullseye & Uroborus fusible glass
CoE 90
Typical window/float glass
CoE 82-87
Borosilicate glass
(e.g. Pyrex pans)
CoE 30-33
List of Colorants - Below is a list of common colorants, and the basic color(s) they give to glass. I’m using the default “Crayola 12” colors rather than trying to describe the different shades, such as the green from Iron verses that from Chromium. Most of these only need to be added in tiny amounts to color glass. .001% Copper Oxide can impart an emerald green, 2% Cerium Oxide gives a light yellow.
Some of the decolorants below are also colorants. This works because the resulting colors balance/cancel each other out (e.g. Manganese brings in purple which ‘fills out’ the green from iron impurities). Sunlight could change Manganese over time, turning clear glass into purple!
Red
Copper-Tin, Cadmium-Selenium
Amber/Brown
Copper, Nickel, Sulfur
Yellow
Uranium*, Cadmium, Sulfur
Green
Iron, Chromium, Uranium*
Blue
Copper, Cobalt
Purple
Manganese, Neodymium
Black
Iron, Manganese, Cobalt, Lead
White
(Opacifiers really)
Tin, Antimony, Arsenic, Bones**
Pink
Gold, Erbium
Decolorant
Lead, Manganese, Cerium, Sodium Nitrate
* - Yes, Uranium as a colorant is as bad an idea as you think it is! Mostly discontinued
** - Bone ash, specifically. Yep, skeleton bones!
Sources:
https://www.cmog.org/article/chemistry-glass
https://en.wikipedia.org/wiki/Fluoride_glass
http://www.compoundchem.com/wp-content/uploads/2015/03/The-Chemistry-of-Coloured-Glass.pdf
https://en.wikipedia.org/wiki/Forest_glass
Introduction to Glass Science and Technology, 2nd Ed. Shelby, J.E.
http://www.tainstruments.com/wp-content/uploads/BROCH-DIL.pdf
https://www.ima-na.org/page/what_is_feldspar
http://www.chemistryexplained.com/Ge-Hy/Glass.html
http://www.compoundchem.com/2015/03/03/coloured-glass/
https://www.lehigh.edu/imi/teched/GlassProcess/Lectures/Lecture04_Shelby_ColoredGlass.pdf
https://www.mountainglass.com/tips-and-tricks
https://sha.org/bottle/pdffiles/TheColorPurpleLockhart2006.pdf
http://www.bullseyeglass.com/what-are-striking-glass-colors.html
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