Re: Chemistry of metals and glass - WarmGlass.com

Re: Chemistry of metals and glass

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Lynne Chappell
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Re: Chemistry of metals and glass

Postby Lynne Chappell » Wed May 28, 2003 1:50 am

OK, I'm hoping there are some scientist types reading the board. I have been using silver and lately copper with the glass, and every once in a while something really unexpected happens. Since I don't understand the chemistry involved, it is hard to analyze the factors in the different results.

When silver is melted into BE French Vanilla, there are two colors - a blue-grey, and the brown staining of the vanilla. Copper goes a greenish brown. I know that it is sulfur in the vanilla that makes it react but what is the compound that shows as brown? What is the composition of the blue-grey? I discovered that there is no staining when it is fired on fiber, and recently found that silver stringer pulled in the oxy/propane torch instead of the reducing Hot Head flame didn't stain the main body of the vanilla. At least I think that's what happened.

I figure the fiber must suck up any excess silver (pretty scientific, I think), and the oxygen must somehow bind the silver. Anybody have any clues to understanding this?

Brian and Jenny Blanthorn
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Re: Chemistry of metals and glass

Postby Brian and Jenny Blanthorn » Wed May 28, 2003 5:58 am

Lynne Chappell wrote:OK, I'm hoping there are some scientist types reading the board. I have been using silver and lately copper with the glass, and every once in a while something really unexpected happens. Since I don't understand the chemistry involved, it is hard to analyze the factors in the different results.

When silver is melted into BE French Vanilla, there are two colors - a blue-grey, and the brown staining of the vanilla. Copper goes a greenish brown. I know that it is sulfur in the vanilla that makes it react but what is the compound that shows as brown? What is the composition of the blue-grey? I discovered that there is no staining when it is fired on fiber, and recently found that silver stringer pulled in the oxy/propane torch instead of the reducing Hot Head flame didn't stain the main body of the vanilla. At least I think that's what happened.

I figure the fiber must suck up any excess silver (pretty scientific, I think), and the oxygen must somehow bind the silver. Anybody have any clues to understanding this?


U might try this one over craftweb

I think copper n silver can B inconsistant

But if U repeat stuff exactly then U should get close each time
Image

Carla

Postby Carla » Wed May 28, 2003 11:15 am

I'll take a try at this.

If you are using sterling silver it is an alloy. Copper has been added to it to make it harder. You might want to experiment with fine (pure) silver and see if it makes any difference....noting that fine silver is very soft.

Copper is known as a "dirty" metal ie: it reacts a lot. I personally love copper (as a jeweler) for exactly this reason. You can turn it all sorts of colors with different chemicals. And silver will react to many of the same chemicals because of the copper in it.

Liver of sulfur is a standard chemical for coloring silver and copper. Depending on how long it's been exposed they will turn a variety of colors. Both metals will tarnish over time, I suspect putting them in a kiln and heating them and fuming them also "tarnishes" them. Whatever things are in the glass can react with the metals.

Metallliferous in NYC carries a "deoxidized" sterling silver that is supposed to be resistant to tarnish. I have never worked with it. But it might be worth a try. They can be reached at: 1 888 944 0909

Hope this helps.

Carla

davebross
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Postby davebross » Wed May 28, 2003 7:27 pm

I might know just little enough to be helpful on this. I don't know the exact compounds formed.

All the wild and crazy metal reactions on glass seem to depend on depriving the glass/metal combination of oxygen, also known as as maintaining a "reducing" atmosphere around the hot glass. Raku pottery very much depends on this effect. More reduction = more effect

This is easy in hot glass or flamework , you just crank some more gas or less air into the glory hole or torch and the piece will reduce when you heat it in that oxygen deprived atmosphere.

Not so easily done in a kiln, although an electric kiln is usually somewhat reducing atmosphere anyway.

Getting back to raku and hot glass tricks. I've read about hot glass workers throwing M&Ms (the candy) into a glory hole to get a reducing atmosphere for a short while as the candy burns up. The Raku folks smother the screaming hot pot in something like leaves or sawdust that will burn and use up the oxygen. I don't think glass would survive the smothering trick (although that would make a great demo show!)but tossing things that burn into the kiln might get interesting, perhaps in more ways than one. As in don't smoke yourself out or start any really big excitement.

Perhaps hitting the hot glass with a reducing (fuel rich) flame from a torch would work in kiln forming?

Another thought....

Glasses melted with copper or silver in them as part of the batch will also react with each other (and glasses that have sulphur) in strange and interesting ways in a reducing atmosphere. I'm sure some of the greens and blues you use have copper in them but the only silver bearing glasses I'm familiar with are the overlay bars made for blowing color that were designed to reduce. The overlay colors (particularly the ones with lots of lead in them like most of the transparents)might work with something like spectrum system 96, many (not all!) overlay bars are designed to fit in that expansion range. You can also buy all sizes of frit in the overlay colors. As always, your mileage may vary....testing required.

David Williams

Postby David Williams » Thu May 29, 2003 1:15 am

A few things: Most all of the silver-bearing kugler reducing colors (k218)fit spectrum. Also all the Gaffer silver colors I've tried. You can reduce some of them in the kiln with an exact torch. It has to be one of the really good ones like 218 or you'll be blasting them over and over for awhile to bring the metal up. Coppers, cad/sel, gold bearing lead colors, all can have interesting effects with silver bearing colors and each other. Mostly "outlining" or edge effects, but there can be other effects as well. Most of the really good silver colors seem to be in the amber, gold brown family. Also the blue-green shifting with reduction in silvers like Kugler Iris blue is very cool. Most of the effects take some practice to duplicate.

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Postby Barbara Muth » Thu May 29, 2003 7:57 am

davebross wrote:I might know just little enough to be helpful on this. I don't know the exact compounds formed.

All the wild and crazy metal reactions on glass seem to depend on depriving the glass/metal combination of oxygen, also known as as maintaining a "reducing" atmosphere around the hot glass. Raku pottery very much depends on this effect. More reduction = more effect



That's interesting. I have been getting some awesome metal reactions on glass with silver in a vented kiln, hardly what I would call a reduction atmosphere. But I know you are right Dave because it's what all the lampworking beadmakers are doing, pulling their stringer in reduction flames. So I am stumped. Why is it working for me. Of course now that I have said this, it will definitely stop working for me. <grumble grumble>

Lynne, try turning down the oxygen in your torch and you should get the fuming effect on your stringers.

Barbara
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davebross
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Postby davebross » Thu May 29, 2003 9:08 am

Barbara,

If it's sterling it might be the copper in the silver causing reactions with the silver and the glass.

Some glasses have reducing arents added to them in the form of something like black tin. Not that many of them though, so if you're getting it with a lot of colors that's probably not it.

I'm just beginning to get a small handle on glass color chemistry and one of the major things seems to be "where/when/how much is the oxygen"

David,

Oh good! I was hoping someone had some "hands-on" experience on that. Thanks!

David Williams

Postby David Williams » Thu May 29, 2003 12:17 pm

David,

Oh good! I was hoping someone had some "hands-on" experience on that. Thanks![/quote]

Its not part of my regular work but I've farted around alot with it. The exact torch or a similar high btu single fuel torch is the best way I've found to get spot reduction in the kiln. You can flash a really big piece in short order. I can bring the metal up out of 218 with two 5-10 second blasts. I used to use it on reduction colors at the bench too but now I just turn down my air at the glory hole for a few seconds. BTW Steinert has a new hands free igniter for trigger torches like the exact.

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Postby Tony Serviente » Thu May 29, 2003 12:24 pm

Dave-I,m curious. Why would an electric kiln have any kind of reducing atmosphere at all. I could see that a gas kiln would, but what is in an electric kiln to consume oxygen? Since the density of a gas decreases as temperature increases that conceivably could be the cause, but to what degree, and wouldn't the leakage of the kiln be a signifigent factor to consider. I was thinking that if you had a hypothetical kiln with a perfect seal and an infinitely expandable shell, as temperature increased the gas density would decrease proportionally as long as the kilns volume could increase,but as soon as you limited the volume the density would be unable to decrease. Since my kilns are all very leaky, there must be an equilibrium with the outside atmospheric pressure that limits the density differential between inside and outside. What do you think?
Only thing I ever considered to create a reducing atmosphere in my kilns was to try and draw a vaccuum(technically too challenging for me to pursue right now), or to pump nitrogen in, or burn something up. I haven't tried any of these...yet!
Not so easily done in a kiln, although an electric kiln is usually somewhat reducing atmosphere anyway.

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Postby Brad Walker » Thu May 29, 2003 12:37 pm

A reducting atmosphere is one in which the kiln is starved of oxygen. This does not normally happen in an electric kiln. It's possible to create a reducing atmostphere in a kiln by throwing in a combustible material (charcoal is commonly used), but this will negatively impact element life.

Here's an article on the subject. It was written for ceramic kilns, but the principles translate to glass kilns also: http://www.ceramicstoday.com/articles/reduction.htm

davebross
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Postby davebross » Fri May 30, 2003 9:45 am

It did't make any sense to me either that an electric kiln would be reducing, but certain very sensitive colors will reduce in my electric. I would give Bullseyes steel blue opaque as an example of a common fusing glass that will definitely reduce in an electric. It's probably just marginally reducing, but it's enough to get some of the colors.

I wondered about this for a while and was thinking it was a unique situation until I got talking with some potters who use fan powered vents to evacuate their electric kilns to avoid reduction.

I've heard of cheap aquarium air pumps and stainless tubing being used on smaller kilns for the same reason.

Lynne Chappell
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Postby Lynne Chappell » Sat May 31, 2003 12:02 am

I'm not sure your experience with steel blue is reduction in the usual sense. If it is covered by clear glass and therefore not exposed to oxygen, it stays blue - this is reduction right? If it is on the surface and exposed to the air then you get the metallic surface forming (depending on temperatures of course). The same thing happens with copper foil, if covered by glass it turns burgundy (the reduction color of copper) and if the covering glass isn't flat down on it, air gets at it and it turns blue (or just chars). Except of course, if you put copper leaf (not foil) on white and cover with clear, it absorbs into the white and you get blue. And puke green on french vanilla.

Funny thing is I can get copper leaf to burn in blue on my oxy/propane torch and silver and french vanilla will make brown stringer, but when I put the silver stringer onto a piece of french vanilla and put it in the kiln, it doesn't stain. It certainly does when made in the other torch. Maybe it isn't really the torch but something else at work. I haven't had time to do a scientific test to make sure it's the torch.

So, does anyone have any clever theories on why firing on fiber paper affects the reactions?

David Williams

Postby David Williams » Sat May 31, 2003 1:33 am

[quote="Lynne Chappell"]I'm not sure your experience with steel blue is reduction in the usual sense. If it is covered by clear glass and therefore not exposed to oxygen, it stays blue - this is reduction right? If it is on the surface and exposed to the air then you get the metallic surface forming (depending on temperatures of course). The same thing happens with copper foil, if covered by glass it turns burgundy (the reduction color of copper) and if the covering glass isn't flat down on it, air gets at it and it turns blue (or just chars).

No. Its sort of apples and oranges what you are talking about. No reduction reaction is possible if the metal or metal-bearing color is covered. Its different than preserving leaf. What you get when you cover say, copper or silver leaf is you just prevent it from oxidizing. That's not the same as reduction. If you do that you are protecting it from any interaction with the atmosphere once its sealed. The typical reduction effect with silver bearing colors is when you bring the metals to the surface creating that silvery mirror look. So you have it pretty much backwards. If the color stays the same it wasn't exposed to reduction. Lots of other effects are possible by reducing then "erasing" or putting the metal back in solution by raising the temp again. This way you can progressively shift silver bearing colors all over the place. Blues to greens to creamy opals and finally mud ugly grey. Remember though, if you want to keep the silver effect you have to reduce on the way down. Don't raise the temperature beyond 1100 or so or you'll lose it. You can flash it in your slump, that's the best way. Or toss in your mints or whatever if you need to do it the hard way.

davebross
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Postby davebross » Sun Jun 01, 2003 12:41 pm

Davids explanation is great.

To answer the other two questions...

What sort of flame on the torch, oxidising, neutral or reducing? Different effects possible with any of the three. Note Davids comment on pushing the silver colors around.
An example...I do an effect that brings up the copper from a copper green with a reduction flame, them burn some of it (the edges) back into a green color with an oxidising flame. It's a tenths of a second timing thing to avoid scumming the glass on the reduce and then not turning it all to green on the oxidation.

The fiber has a binder in it. Same idea as tossing M & Ms in there. The binder burning out can reduce the color by using up the oxygen in the binders combustion. The old-timey glass melters used to throw sugar, flour, charcoal, and all sorts of other things in their melts to get the the right conditions for the colors that had to have reducing.

Lynne Chappell
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Postby Lynne Chappell » Wed Jun 04, 2003 1:19 am

I really do appreciate the input from you guys that have played with these effects, but...

1. BE steel blue turns silver when exposed without any fancy reduction work - just the regular oxidizing atmosphere of an electric kiln. If you lose the silver by fusing too high, it comes back on the slump (I guess only sometimes, it's a bit unpredictable). I don't think this is a "silver" effect as in using the metal silver, but something else in the glass. Although it looks like the effect you get with reduction frits. There may be something in the archives on steel blue, I should check.

2. The fiber had been fired before, so there weren't any binders left, and this has happened consistently. Actually, the last little test I did, you could see a little yellow staining on the fiber board beside the glass piece. And the French Vanilla had no brown staining from the silver stringer. It's really kind of weird and I've never heard anyone mention this anywhere.

Lynne Chappell
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Postby Lynne Chappell » Wed Jun 04, 2003 1:24 am

OK, I checked the archives about the steel blue and this is a post from Lani of BE:

"Ron, sorry that I can't tell you the composition. It's proprietary (and everyone at BE reads this board so I'll surely get busted by the production mgr if I tell)

I can tell you that there is no lead in this glass. The color change is related to atmosphere: in a reducing atmosphere it will maintain the blue color (just cover it with overglaze, clear frit or stringers, or sheet); in an oxidizing atmosphere it will turn silver-grey - but usually only within the range of 1300-1450F.

At elevated temps (full fuse or above) it will frequently revert to its true-blue self after having passed through the silver-grey stage. Don't ask me why. I am not a glass chemist. And the one we have tries to keep one or two secrets to himself. "

David Williams

Postby David Williams » Wed Jun 04, 2003 12:51 pm

Lynne Chappell wrote:OK, I checked the archives about the steel blue and this is a post from Lani of BE:

"Ron, sorry that I can't tell you the composition. It's proprietary (and everyone at BE reads this board so I'll surely get busted by the production mgr if I tell)

I can tell you that there is no lead in this glass. The color change is related to atmosphere: in a reducing atmosphere it will maintain the blue color (just cover it with overglaze, clear frit or stringers, or sheet); in an oxidizing atmosphere it will turn silver-grey - but usually only within the range of 1300-1450F.

At elevated temps (full fuse or above) it will frequently revert to its true-blue self after having passed through the silver-grey stage. Don't ask me why. I am not a glass chemist. And the one we have tries to keep one or two secrets to himself. "



Sounds like silver. It can't react to what it isn't exposed to. If you cover it there won't be any reaction, it'll stay the same. Far be it for me to disagree with Lani, but that's not a reducing atmosphere. If you could get a reduction reaction that way life would be way easier or more complicated depending on your perspective. I don't know the color but I'd say what is happening is the changes when it is exposed to the atmosphere are from some slight reduction in the kiln. Like Dave said.


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