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!

What is Lampworking?

What is Lampworking?

Lampworking is:

• an art
• a form of glassworking
• beautiful
• a lot of Fun!
• an expensive hobby
• the process of using a flame torch to melt and mold glass

Lampworking is a process used in the forming of glass, typically smaller glass objects like beads, marbles, small sculptures, laboratory glassware, etc (usually less than 1-2 inches in diameter). You may be familiar with glassblowing, another glassforming technique that involves molten glass on a much larger scale (vases, vessels, plates, large sculptures).

Whereas glassblowing uses a very hot furnace (a glory hole) to heat the glass to white hot (around 2,400 F or 1320 C), lampworking uses a smaller torch to melt glass.

Brief History

Lampworking has started as early as the Egyptians over 5000 years ago. Many Egyptian decorative beads made in that era survive today, a testament to how durable and strong glass actually is! These original beads are thought to have been made in cone-shaped furnaces instead of the modern torch.

Lampworking became very popular in Murano, Italy in the 14th century. In the early days, glass was melted using the flame of an oil lamp (which is why it is called "lampworking"). They would blow air or use a bellows to direct flame from an oil lamp to heat pieces of glass. Lampworking in Italy was considered a very close guarded secret for about 400 years.

Eventually American artists began learning lampworking techniques, experimenting, and developing new styles. Today, most people use torches that burn propane and oxygen or portable MAPP gass.

Types of Glass

Soda-lime glass

• the most common type of glass used in lampworking
• "soft glass" because it melts at a lower temperature than boro
• more brittle and prone to thermal shock than boro
• has a much broader range of color than boro
• can find unique colors including silver glass, metallics, opalinos, organics, webbing, etc
• COE ~104

Borosilicate glass

• "hard glass" because it requires higher temperatures (hotter, more expensive torches required) to melt than soft glass
• more thermal stable, less resistant to thermal shock, less prone to cracking when cooled (therefore more forgiving to work with)
• however, has a narrower working temperature
• colors are more limited in range, but this is constantly expanding over time as boro gains popularity
• some boro glass have very unique color properties not seen in soft glass (eg oxidizing or reducing glass)
• more expensive than soft glass
• COE ~33

(Other types of glass include Bullseye COE ~90, Spectrum COE ~96, Czech glass COE variable)

Beads, Marbles, Sculptures, and more...

When I say "lampworking," I usually refer to the process of making lampworked beads. However, lampworking is a versatile technique that can be used to make a variety of glass objects. Popular products include:

• Glass beads for jewelry
• Glass Marbles
• Small glass sculptures and figurines
• Hollow glass beads for storing small amounts of cremains, flower petals, dandelion seeds, small seed beads, sand, small shells, etc
• Glass vessels with stopper, able to hold small amounts of liquid like perfume, holy water, etc

There are also a very large number of styles and techniques to be mastered in the art of lampworking. Examples of different techniques include:

• Stringer application - for horizontal and vertical lines, scrollwork, surface decoration
• Dots, plunged dots - for surface decoration, making flower petals, internal trapped bubbles
• Raking - using a tool to manipulate and move surface glass
• Encasing - covering a bead entirely in a layer of clear glass
• Implosions - 3d flower, explosion, or vortex look inside clear glass, popular in marbles

Lampworking is a complex art, and one piece often requires mastery of many sets of skills.

Size Limitations

You can make many different types of glass objects using lampworking, but you are limited by size. Buying a larger and hotter torch can allow you to make larger glass objects, but there is an upper limit. You will never be able to make things as large as glass blowers with a furnace glory hole are able to (bowls, vases, paperweights, large sculptures).

On a Hothead torch, the maximum thickness is about 1 inch in diameter, assuming you have a digital kiln. Anything that big will need to be kiln annealed. A 1 inch marble is guaranteed to crack if cooled in a fiber blanket or vermiculite. Glass objects will be increasingly harder to melt on a Hothead as you approach the size limit. The hothead simply does not have enough heat to heat the glass completely through when it gets larger than about 1 inch diameter.

If you are interested in working larger, you must invest in a larger, hotter, and more expensive torch. You will also need to upgrade your fuel system to a propane and oxygen source. This comes with its own set of expenses (propane tank, fuel, propane regulator, fuel lines, check valves, flashback arrestors; oxygen tank and oxygen service if you want tanked oxygen, oxygen concentrator for an alternative, oxygen regulator, etc).

On this setup, you will be able to make glass objects up to about paperweight size <3 data-preserve-html-node="true" inches. I typically do not work so large. But if you decide to make large objects, understand that it will take longer to add glass and heat up to shape, so will require more and more patience the larger you go.

Additionally, larger objects must be carefully heated continuously. The parts of the object not directly in the flame will cool and be at risk of thermal shock if not evenly heated occasionally (insurance heat). For this reason, boro is typically favored for larger marbles and sculptures.

Larger sculptures can be created as long as no part of it exceeds 1-2 inches in diameter. However, legs or any long thin protruding parts are most at risk of thermal cracking and require insurance heating.

Lampworking is a very complex and intricate art, the mastery of which takes years of dedicated practice. However, it is very fun and rewarding, and soon you will have a beautiful collection of fine glass pieces of art!

What do you enjoy about lampworking? What techniques are you most interested in learning? Post links to your glass works of art below, I am interested in seeing everyone's work!

Additional Reading:

For more information on the Tools needed to begin lampworking, please continue on to my post "Lampworking Tools!"

For more information on the actual process of making a bead, please continue on to my post How to Make a Bead!

If you are interested in more chemistry and physics, look at "The Science of Glass!"