The Science of Glass

The Science of Glass

Glass is an amorphous solid, meaning it is a solid with no crystalline order. However, when heated it transitions from the hard solid state that we are familiar with, to a molten liquid like state. Glass is normally transparent, reflective, and refractive. It has many unique physical and optical properties. It can be molded into any imaginable shape. Because glass is transparent to visible light, yet is able to reflect to refract light, it is commonly used in optics such as microscope or Camera Lenses. The transmissivity of glass also allows it to be used in light transmitting fiber optics.

Color and Composition

The most common type of glass is soda-lime glass, which is made of silicon dioxide (SiO2), sodium oxide (Na2O) from soda ash, lime (CaO), and other additives for color and other properties. When glass has metallic salt impurities, it becomes colored. Glass can also be colored by nanometer sized colloids or colored inclusions. Other methods include dichroic coatings or colored coatings.

• Ordinary soda lime glass appears colorless, with a faint green tint due to iron oxide impurities.
• Additional iron (III) oxide --> bluish-green (eg beer bottles)
• Iron (III) oxide & Chromium --> richer green (eg wine bottles)
• Sulfur & Carbon --> yellow to dark amber
• Sulfur & Boron --> blue
• Sulfur & Calcium --> deep yellow
• Sulfur & Cadmium--> deep yellow
• Sulfur & Cadmium & Selenium --> orange to bright red
• Manganese --> amethyst
• Cobalt --> blue
• Copper oxide --> turquoise
• Nickel makes --> blue to purple to black, depending on the concentration
• Chromium --> dark green to black
• Chromium & Tin oxide & Arsenic --> emerald green
• Titanium --> yellow-brown
• Uranium --> fluorescent yellow-green (slightly radioactive, carcinogenic if glass powder is inhaled)
• Lead --> deep red
• Didymium --> green or red
• Selenium --> pink, red
• Selenium & Cadmium sulfide --> brilliant red (Selenium Ruby)
• Copper --> very dark opaque red
• Gold --> ruby (Ruby Gold or Rubino Oro)
• Silver --> yellow to orange-red, color changes depending on the way glass is heated and cooled
• Tin oxide & Antimony & Arsenic oxides (colored inclusions) --> opaque white (milk glass, imitation porcelain)

Crystal Field Theory

The bright colors exhibited by many coordination compounds of metals can be explained by Crystal Field Theory. If the d-orbitals of such a complex have been split into two sets, when the molecule absorbs a photon of visible light one or more electrons may momentarily jump from the lower energy d-orbitals to the higher energy ones to transiently create an excited state atom. The difference in energy between the atom in the ground state and in the excited state is equal to the energy of the absorbed photon, and related inversely to the wavelength of the light. Because only certain wavelengths (λ) of light are absorbed - those matching exactly the energy difference - the compounds appears the appropriate complementary color.

Because different ligands generate crystal fields of different strengths, different colors can be seen. For a given metal ion, weaker field ligands create a complex with a smaller Δ, which will absorb light of longer λ and thus lower frequency ν. Conversely, stronger field ligands create a larger Δ, absorb light of shorter λ, and thus higher ν.

For more in depth information on crystal field splitting, read this wiki article or take a chemical bonding class.

What is COE?

COE = Coefficient of Thermal Expansion

COE describes how the size of an object (in this case glass) changes when the temperature changes. Because glass requires very high temperatures to melt and be shaped, it necessarily has to cool back down to room temperature at the end of your torch session. When the glass cools, it will shrink in size ever so slightly. It is not noticeable by the naked eye, but is significant on the molecular scale.

For soda lime glass with a typical COE of 104, this means that the glass will expand 0.0000104 inches for each degree C increase in temperature, and it will contract the same amount for each degree C cooled.

For borosilicate glass with a typical COE of around 33, this means that boro will expand and contract much less than soda lime glass over the same temperature range.

Because of these differences in expansion and contraction, glass of different COEs are INCOMPATIBLE with each other. They cannot be mixed or fused, otherwise your entire work will crack in pieces. If you use multiple COEs of glass, you must always keep your glass separated in your studio to avoid accidentally mixing them. This is critical because in many cases you cannot tell by sight whether a rod of glass is soda lime or boro. It's best to keep them always separated and clean up your bench in between uses.

Annealing Process

Annealing is the process of slowly cooling glass to relieve internal stresses. Because glass is heated to molten hot during the process of glassforming, it must subsequently be cooled. If glass is not annealed, or not cooled slowly enough, it is subject to cracking and shattering if exposed to a temperature change or physical shock. Unannealed beads are therefore very weak and easily shattered. They will often break apart spontaneously.

All beads must therefore be carefully annealed to maintain strength and integrity. Beads are best annealed in a digitally controlled kiln, where you can program in a specific annealing program. In the annealing process, glass is heated until the temperature reaches a stress relief point. This is a temperature where the glass is still hard (will not change shape in the kiln), but the molecules are soft enough to slowly relax and relieve stresses. The glass is soaked at this annealing temperature until the temperature is even throughout the piece (larger diameter glass will need to be annealed longer). The glass is then slowly cooled until the temperature is below a strain point temperature, after which it can be cooled more rapidly to room temperature.

My Annealing Protocol

I will share my annealing protocols for soft glass (COE 104) and boro (COE 33). I sometimes also use Bullseye glass (COE 90), and I use the same protocol I do for soft glass. I've included modified versions for batch annealing (start at 0 deg C and have to ramp up to garaging temperature), silver glass (usually more sensitive, so I like to anneal at a lower temp). My program 2 is for boro glass annealing, either with kiln striking or no striking (meaning I strike in the flame then don't want the glass to change in the kiln). You will notice that boro requires a higher garaging temperature and higher annealing temperature than soft glass.

I may be on the conservative side, as I like to ramp down at a fairly slow rate. This could probably be sped up, but I have not experimented with that at all. I usually bead in the evenings and let the kiln anneal overnight, so it doesn't matter to me how long it takes. I'd rather be over-annealing than under-annealing and risk fractured pieces.

I use a GlassHive Regular Guy kiln, and I highly recommend it. I may do a separate review in the future.

Program #1 Soft Glass, COE 104, 96, 90

Garage 925
Ramp to 950        15 min
Hold 950              1 hr 30 min
Ramp to 700        4 hour (1.04 deg/min)
Hold 700              15 min
Ramp to 500        1 hour (3.33 deg/min)
Hold 500              15 min
End
Total: 7 hr 15 min

Change Program #1 for Batch Annealing Soft Glass

Garage 0
Ramp to 950        3 hours  
Hold 950              1 hr 30 min
Ramp to 700        4 hour (1.04 deg/min)
Hold 700              15 min
Ramp to 500        1 hour (3.33 deg/min
Hold 500              15 min
End
Total:  10 hrs

Change Program #1 for Silver glass

Garage 900
Ramp to 920        15 min  
Hold 920              1 hr 30 min
Ramp to 700        4 hour (1.04 deg/min)
Hold 700              15 min
Ramp to 500        1 hour (3.33 deg/min
Hold 500              15 min
End
Total: 7hr 15 min

Program #2 (Boro Glass with color strike)

Garage 975
Ramp to 1000       15 min
Hold 1000             30 min
Ramp to 1150        10 min
Hold 1150              30 min
Ramp to 1050       30 min
Hold 1050             1hr 30 min
Ramp 750             3 hr (1.66 deg/min)
Hold 750              15 min
Total:  6 hr 45 min

Change Program #2 for batch anneal Boro Glass with color strike (eg restriking in kiln)

Garage 0
Ramp to 1000       3 hr
Hold 1000             30 min
Ramp to 1150        10 min
Hold 1150              30 min
Ramp to 1050       30 min
Hold 1050             1hr 30 min
Ramp 750             3 hr (1.66 deg/min)
Hold 750              15 min
Total:  9 hr 25 min

Change Program #2 for  Boro Glass with no strike

Garage 975
Ramp to 1000       15 min
Hold 1000             30 min
Ramp to 1150        10 min
Hold 1150              30 min
Ramp to 1050       10 min
Hold 1050             1hr 30 min
Ramp 750             3 hr (1.66 deg/min)
Hold 750              15 min
Total:  5 hr 40 min

I hope this information is helpful. The science of glass is quite interesting, and the way colors are made by transition metal complexes are quite complex. If you would like to learn more, please consult external sources. As someone who has completed a Chemistry major, I highly recommend learning more about chemical bonding if you found this interesting.

I also hope that my annealing programs presented are useful. I have done many searches on optimal annealing protocols, and I compiled my own based on several sources. If you found my information helpful, comments are always appreciated! Thanks!