Reversing irradiation?

[JohnN]

I was looking at information about jars, and I came across this website: http://www.qnet.com/~glassman/info/b&e/primer.htm. In it, there was a section about irradiated jars. That's nothing special, but what got my attention was this:

One way you can tell if a jar has been irradiated is to bake it in an oven. A collector in Michigan set an irradiated jar in a 200° oven for 2½ hours and the color disappeared. (Placing your valuable old jars in an oven could cause them to crack, so be careful!)

Anyone have any idea if this is true, and if this really works?

 

[cyberdigger]

Let's find out!

We'll get some ACA's and bake em.. I'll bring the stuffed mushrooms.. []

 

[markh]

It is possible to remove the coloration in glass caused by UV light (or other irradiation) by heating (annealing). What the UV light does is "activate" an atom, called a color center, e.g. manganese, that has been added to the glass in small quantaties, so that the activated color center reflects a certian portion of the solar spectrum, e.g. purplish for Mn. Heating the glass relaxes the atoms to their original state. I've never tried baking any sun-colored bottles; you'd need to be careful on how fast you raise and lower the temperature so as not to induce too much stress.

Here are some articles going into the details:

http://www.nia.org/altered/index.htm

and a little more technical:

http://www.chem.colostate.edu/~glass/irradiation.html

from the 2nd link:

Q: CAN THE ACTIVATION OF COLOR CENTERS BE REVERSED? CAN THE COLORED GLASS BE MADE CLEAR AGAIN?
Yes. Glass, even at room temperature, is a liquid: the interatomic bonds are weak are and constantly breaking and reforming. These bonds are in fact an interaction of (or, sharing of) the valence electrons of adjacent atoms. (In metals this sharing of electrons results in electrical conduction. For example, in a length of copper wire, the individual electrons at one end of the wire will, in theory, eventually migrate to the other end of the wire, due to random motion and without the application of an outside force). The inter-atomic bonding structure within the bulk of a material places physical constraints upon the valence electrons. Therefore, if you activate a color center (by any means) the alteration in the valence shell will be either stable (unchanging with time) or meta-stable (will change gradually with time). The stronger the interatomic structure (or 'lattice' in the case of a true crystal which, unlike a glass, is a solid) the more stable the change. The change can however be reversed by weakening the inter-atomic bonds which will allow the formation of new, lower energy bonds (atoms will break their initial bonds and reform bonds with other neighbors). This can be accomplished simply by the application of heat. In practice, the temperature required for complete color deactivation in an amorphous material (such as a glass) is its annealing temperature. Therefore, simply annealing a piece of glass will deactivate the color centers.

Having worked extensively with researchers who conduct experiments which activate color centers by both of these means, I have frequently had need to deactivate color centers. An unexpected consequence of color center activation is the formation of strained inter-atomic bonds, and unless the piece is annealed, it may spontaneously fracture! As a rule, the more a piece is colored, the more strain the piece is under, and the more it needs to be annealed. Note that the color doesn't 'cause' the strain. Rather, color and strain are two entirely separate issues sharing the same cause.

Mark

 

[Plumbata]

Excellent topic, and thank you Mr. Mark H for that excellent rundown of the process! This is some particularly useful, quality information. Now if you can tell us how to chemically mimic the iridescent sheen seen on some old glass, i will be forever indebted. []

 

[Wheelah23]

I've gotta chime in here. Glass is NOT a liquid. This is one of the most common urban legends out there. Read this if you don't believe me. So that probably nullifies most of the "molecular" basis for de-irradiating glass.

However, I have heard that heating up glass can do that. I'm not willing to risk my oven by trying it, but I don't doubt it might help.

 

[AntiqueMeds]

Correct, glass is an amorphous solid. I believe the myth that glass "flows" over time originated from people seeing the irregular thickness of old window panes and they assumed the glass had sagged over time. If this was true you wouldnt find 2000 year old Roman glass still in its original form...busted...[] It makes me immediately skepticle when I see a statement like that in a response.

As far as reversing the purple effect in manganese glass, I know people who have tried with no sucess. I cant rule it out completely but I find it doubtful. My wife has a glass kiln so if people are really interested I could arrange an experiment. I can heat a bottle just to the point of melting (any hotter and the method would be useless[]). THe kiln is programmed to cool slowly so the glass wont shatter.

I guess the obvious question to be asked is... since there are a lot of very nice bottles out there virtually ruined by over irradiating, if it was as easy as put the bottles in a 200deg oven wouldnt you thing more people would be doing it?

 

[JohnN]

If you find a worthless bottle that you don't mind if it breaks, I don't see how experimenting can do any harm. I have some doubts about this method, but there is only one way to verify if it works.

 

[cyberdigger]

At least throw some shrinky dinks in the oven with it, so it's not a total waste of time.. [8D]

 

[markh]

Hi Plumbata,
glad you found this info useful. I've had the article on the irradiated insulators bookmarked for awhile. It looks like (fraudulant) color modification is much more of a problem in insulator collecting than for bottles.

I don't know why the glassblower from Co State called glass a liquid (quoted from the second link). It is an amorphous solid. I also think that when he says "In practice, the temperature required for complete color deactivation in an amorphous material (such as a glass) is its annealing temperature." he must mean the annealing point which is defined as: (from http://www.sgpinc.com/materials.htm)

"Annealing Point - This is the temperature at which glass will relieve stresses (either compressive or tensile) in a matter of minutes. For soda-lime glass, the annealing point is 546°C (1015°F).

When heat treating glass, all portions of the glass must exceed this temperature in order to prevent "cold-cracking" during quench.

When fabricating glass and to prevent warping or bowing due to unequal front and back surface stresses, the glass must be properly annealed. To anneal glass, whether during the manufacturing of the glass or after processing of the glass, all portions of the glass must be cooled uniformly form over the annealing point to under the stain point. (See Strain Point below)"


So if it takes 1000F temperature to remove coloration, this is not something you could do in a household oven. Maybe Matt could do an experiment in his wife's kiln.

I've never thought of how to produce iridescence on bottles chemically, but that's an interesting question. Doing a few searches here are some things of interest:

from: http://www.cmog.org/dynamic.aspx?id=1384

"Iris, the Greek goddess of the rainbow, lends her name to the word iridescence, a lustrous, rainbow-like play of color. Iridescence was admired by modern glassmakers but was not an intentional effect made by ancient artisans. The effect was found on pieces of ancient glass where burial conditions caused alkali (soluble salt) to leach from the glass and form layers that eventually separate and flake off. The remaining surface layers reflect light differently, resulting in an iridescent appearance."

from: http://www.ic.arizona.edu/ic/mse257/class_notes/iridescence.pdf

"b. Iridescence on degraded glass
i. As glass is exposed to water in its burial environment some of its components can be
dissolved by the water and carried away (leached out). This generates a thin surface
layer of glass that has a different composition than the undegraded bulk of glass.
Often there is a thin layer of air between the corroded surface and the bulk.
ii. The corroded surface layer on old glass acts as a thin film and can give rise to
iridescence."


So it seems if you could find the right acid mixture which would leach some of the alkali component from the surface of a bottle you might be able to produce iridescence. Don't know how long this would take at room temperature so heating may be required.

There are some receipes for producing iridescence on glass using a thin film coating and high temperatures.
From the arizona.edu link above:

"c. Intentionally developed iridescent surfaces
i. It is explained above that to produce an iridescent surface it is necessary to
superimpose a thin film over a bulk material. For a glass object, this can be
accomplished by exposing the glass on the surface at high temperatures with
compounds that will react with the glass and alter its composition and hence optical
properties.
ii. At the end of the manufacture of some Tiffany vases, a thin surface film was
produced by placing the piece in hot chamber with vapors that would react with the
glass on the very surface. This process now is referred to as chemical vapor
deposition. The vapor contained:
· Tin chloride, SnCl2, and/or
· Iron chloride, FeCl3
iii. These chlorides form oxides on the surface (SnO, Fe2O3). Tin oxide on its own gives a
more silvery color, while iron oxide gives a golden tone to the iridescent effect it
produces on the surface of the glass. Mixing them gives something in between. The
size of the oxide crystals is very small and the thickness of the layer that they form
on the glass surface is on the order of one micron (10-6 m). This is a film thin enough
to give rise to iridescence."

and from: http://www.warmglass.com/making_your_own.htm

"IRIDESCENT GLASS

Caution: This process can be hazardous. Do not attempt it without appropriate safety equipment and ventilation.

Purpose: To produce an iridized surface on the glass. Glass iridized with a stannous chloride solution will not lose its iridescence at fusing temperatures.

How to use: Spray solution on glass at fusing temperature.

Ingredients: Stannous chloride crystals, muriatic acid, distilled water. Muriatic acid, also used to clean brickwork or increase the acidity of water, is available at hardware or swimming pool supply stores. Stannous chloride is more difficult to find, but is stocked by some ceramic/pottery suppliers.

Safety precautions: Both muriatic acid and stannous chloride can be hazardous. Wear complete safety equipment, including a respirator and gloves. This process creates significant fuming, so kilns should be well ventilated. Care should also be taken to minimize overspray from the solution onto floors and other surfaces.

How to make: Use equal parts muriatic acid and stannous chloride and from two to three parts water. Place the crystals in a glass jar and add the muriatic acid. Add the water and mix the solution thoroughly. The amount of water required can vary depending on the kind of sprayer used. Start with two parts and increase as necessary.

Use a glass sprayer. Plastic may melt in the heat of the kiln and the acid will corrode metal. Heat the glass until the temperature is around 1500 degrees Fahrenheit. Turn off the kiln, and spray the solution on the glass. Use a fine mist.

After applying, close the kiln and heat for a few moments. Then cool and anneal as normal.

Additional note: This formula and technique was adapted from Boyce Lundstrom's Advanced Fusing Techniques."


There must be some chemical receipes out there to induce iridescence on glass without using these very high temperatures. If I run across any I'll let you know.

Mark

 

[AntiqueMeds]

"Annealing Point - This is the temperature at which glass will relieve stresses (either compressive or tensile) in a matter of minutes. For soda-lime glass, the annealing point is 546°C (1015°F).

thanks, that gives me a target to work with.
now the trick is to see if I have any crappy SCA bottle laying around. I think I gave most of them away.