Sciencemadness Discussion Board

Oxidation and reduction in molten glasses (glassworking)

Junk_Enginerd - 18-8-2020 at 12:51

As a offshoot and some fusion of my chemistry and forging/casting hobbies, I just started dabbling with glassworking. It's fascinating and very satisfying.

There's this one thing that really bugs me that I don't understand though, oxidation and reduction of elements inside the glass.

I just got some pigments and colored glass that I wanted to experiment with, having so far only done it with laboratory versions of glass. Everything I did with them ended up black/brown/grey. Okay, so I eventually figured out the torch setup I'm using probably has a flame that's on the reductive end most of the time, so I'm getting more colloidal metal powders than I'm getting suble salts in the glass. It's not a glassworking torch, mind you. It's small gas welding torch that I've modified and am running on MAPP gas and oxygen from a zeolite oxygen concentrator. I can't quite squeeze out more oxygen from it at the moment without reducing fuel and thereby temperature, so I'm just thinking about other ways to attack the problem.

How does something like for example copper carbonate, a common glaze/glass pigment actually behave in glass? See, one thing I tried on a whim was to take a dull, gray brown failed piece of glass, and while almost red hot dip it in potassium nitrate. I wasn't seriously expecting it to work, but the colors in the glass just (figuratively) exploded in brilliance. How?? The pigments are enclosed in a bubble of glass! How would oxygen even reach them in there? Does it move through the molten glass in some ion form? Does molten glass act more like a liquid than a solid in this case?

And, could one extrapolate on the KNO3 route? Are there oxidizers that could be used to buffer some oxygen to make up for my torch's lack of it? Maybe something that holds on to its oxidizing potential a temperatures a bit higher than KNO3's 400°C ish?

DraconicAcid - 18-8-2020 at 15:18

I vaguely remember trying to make coloured glasses way, way, way back when. Cobalt was the only one that I found worked nicely, always giving a dependable deep blue.

If your glass is molten, then it is a liquid (although very viscous). Ions will diffuse through it.

violet sin - 18-8-2020 at 16:23

I've worked glass some. Boro pipes, bowls and stems. This was when I was fresh outta schooling studying chem and art.

The master artist teaching me had a lot of observations and false conclusions. They did not sit well with what I had learned. It sounded like justifications for his view on the science, which I don't believe he understood. The materials, he knew them well. Marketing, he was ok. All our glass and color sticks were purchased. We didn't MAKE the pigments. That's a whole other thing, milling and mixing, casting and drawing rod. It's like making watercolors or oil paints.

He was quite good and could tackle the touchy colors even in tight spaces, case dichro w/o burning it and generally manhandle large pieces of glass for bongs and such. His artistry with glass was great, but his curves and angles weren't pleasing to me. His specific style was not mine lol.

He did some colors inside out to preserve the brilliance. Would flare the boro tubing and apply to the interior, before gathering it up, stretching and blowing to shape. If you made latachino (sp.?) Sticks with color that bruised easily we'd get brown/grey on the exterior and nice colors on the cased portion. The hard part was getting enough heat, keeping it in the proper cone portion and not bruising colors.

The torch is pretty important. You want one of them with the big box head under a nice fine tip. Only way to heat it up, not stress it, and still punch holes where need be. We used oxy/propane.

Burning the colors out, or reducing them to nothing can look the same. We would fume silver or gold, color stick over, smooth it out, gather up one end to a big ball of glass( not solid just shorter and not stock thickness) then blow for contour, reheat and sink a bowl with graphite plunger cone on a graphite plate. If you were quick and half good, none of the silver was burnt off while doing the rest. But, beginners burnt off fume near carb/bowl holes and the mouth piece.

Silver would just fade, same with gold. Colors so that were reduced could be oxidised back with an excess of oxygen, but it was colder than a standard flame, would cool the piece. A cooler piece didn't take up the oxygen as easy... End up with a roaring flame for little work. Reductions were easier, just turn up the fuel. It cooled off some, but not nearly as much as unfetterd oxy line did.

All this may be of little help to you, but understand that there are a ton of headaches using prepared materials. I can't imagine expecting reliability from your attempts with out proper machines, torches and temp control. Reductive/oxidative blankets over glass color loafs. Grind and refire, grind and refire.

Best of luck

wg48temp9 - 18-8-2020 at 16:44

Glass containing lead becomes black or grey when heated with reducing flame. It would be very unusual for chemical laboratory glassware to contain lead. Light bulbs and florescent tubes are commonly manufactured from glass containing lead.

The hottest hydrocarbon flames are very close to stoichiometric. So if you are maxed out on oxygen flow reducing the fuel flow will make the flame hotter and smaller (faster burning). If your using a premixed torch you should be able to do that but many none premix torches must run fuel rich (reducing) for stability.

If the flame is reducing it will reduce any oxidising compound. as easily as the lead at the surface of the glass and at a depth dependent diffusion which depends on temperature and time.

[Edited on 8/19/2020 by wg48temp9]

draculic acid69 - 18-8-2020 at 17:51

Quote: Originally posted by violet sin

 
I've worked glass some. Boro pipes, bowls and stems



Just say bongs and pipes

violet sin - 18-8-2020 at 21:57

I didn't get to bongs; only did borosilicate pipes, bowls and down-stems. That's a factual representation of a tiny moment of my history. I'm not sure if you think I'm trying to church it up, or what exactly. Perhaps I ramble, unhelpfully?

One of the local shops he sold to did a lot of art glass, in house in front of customers, figurines and such. Quite a bit of sodalime glass there. why I mentioned it wasn't soft glass. Much of what he produced was sold in head shops but there wasn't much of an Internet sale at the time. Yep, it was a while ago.

I also did the color mushroom in clear drop pendants for friends, flower stakes with butterflies for my stepmom, and got to play around creatively making a stick of silver fume glass only. Repeatedly drawing it out, fuming it, gathering and repeating till I got a nice hazy/milky color. That was pretty cool layer down over a fresh fume with Aurora spiraled on too. The latachino sticks were fun mixing a few colors and clear, spiraled and drawn. The moss green was easy burnt, came out white, couple reds that went brown, amber/violet was somewhat touchy. Cobalt blue was solid.

It was fun and easy to learn. After years of doing that, I'd only been doing it a couple months, the guy up and moved, changed to making glass knitting needles and such. Haven't seen him since. He never did smoke weed. Chain-smoked basic light 100's. But from a few internet searches, he's doing aces these days in an unrelated profession.

Junk_Enginerd - 18-8-2020 at 22:14
Quote: Originally posted by DraconicAcid  
I vaguely remember trying to make coloured glasses way, way, way back when. Cobalt was the only one that I found worked nicely, always giving a dependable deep blue.

If your glass is molten, then it is a liquid (although very viscous). Ions will diffuse through it.


I have actually tried Cobalt oxide a little bit. While it has been the easiest so far, it has been far from idiot proof. I've made grey brown with that as well lol.
Quote: Originally posted by violet sin  
I've worked glass some. Boro pipes, bowls and stems. This was when I was fresh outta schooling studying chem and art.

The master artist teaching me had a lot of observations and false conclusions. They did not sit well with what I had learned. It sounded like justifications for his view on the science, which I don't believe he understood. The materials, he knew them well. Marketing, he was ok. All our glass and color sticks were purchased. We didn't MAKE the pigments. That's a whole other thing, milling and mixing, casting and drawing rod. It's like making watercolors or oil paints.

He did some colors inside out to preserve the brilliance. Would flare the boro tubing and apply to the interior, before gathering it up, stretching and blowing to shape. If you made latachino (sp.?) Sticks with color that bruised easily we'd get brown/grey on the exterior and nice colors on the cased portion. The hard part was getting enough heat, keeping it in the proper cone portion and not bruising colors.

The torch is pretty important. You want one of them with the big box head under a nice fine tip. Only way to heat it up, not stress it, and still punch holes where need be. We used oxy/propane.

Burning the colors out, or reducing them to nothing can look the same. We would fume silver or gold, color stick over, smooth it out, gather up one end to a big ball of glass( not solid just shorter and not stock thickness) then blow for contour, reheat and sink a bowl with graphite plunger cone on a graphite plate. If you were quick and half good, none of the silver was burnt off while doing the rest. But, beginners burnt off fume near carb/bowl holes and the mouth piece.

Silver would just fade, same with gold. Colors so that were reduced could be oxidised back with an excess of oxygen, but it was colder than a standard flame, would cool the piece. A cooler piece didn't take up the oxygen as easy... End up with a roaring flame for little work. Reductions were easier, just turn up the fuel. It cooled off some, but not nearly as much as unfetterd oxy line did.

All this may be of little help to you, but understand that there are a ton of headaches using prepared materials. I can't imagine expecting reliability from your attempts with out proper machines, torches and temp control. Reductive/oxidative blankets over glass color loafs. Grind and refire, grind and refire.

Best of luck



Yeah the part about artists and the science is one of my worst issues with this. They make... systems of pseudoscience, that work well within their own set of rules as a tool to understand how to get the results you want, but it doesn't hold up under scrutiny. Even worse this way of thinking tends to box you in, so that as soon as you want to do something new or unconventional, everything just stops making sense. This is usually true for any area where science and art or craftmanship meet, e.g. glass, pottery, welding(try to find information on a welding arc's electrical characteristics) etc.

Okay, so I thought the colors part would be simpler and that my torch was just holding me back... Really it helps a lot just to know it's a skill in itself and that it takes some care to achieve good results.

The oxidize-grind-remelt part has actually come up as a solution, so knowing that it is a standard method confirms for me that it would be worthwhile to give it a go. Thanks.


Quote: Originally posted by wg48temp9  
Glass containing lead becomes black or grey when heated with reducing flame. It would be very unusual for chemical laboratory glassware to contain lead. Light bulbs and florescent tubes are commonly manufactured from glass containing lead.

The hottest hydrocarbon flames are very close to stoichiometric. So if you are maxed out on oxygen flow reducing the fuel flow will make the flame hotter and smaller (faster burning). If your using a premixed torch you should be able to do that but many none premix torches must run fuel rich (reducing) for stability.

If the flame is reducing it will reduce any oxidising compound. as easily as the lead at the surface of the glass and at a depth dependent diffusion which depends on temperature and time.

[Edited on 8/19/2020 by wg48temp9]


So I've heard, and noticed as well. I actually made a bit of lead glass to see how it behaves, just dipping a hot rod in lead oxide. I can see why it was used for so long, because it really plays nice. Fascinating to see how it just "disappears" into the glass in oxidation, and in reduction you can end up with straight up metal blobs just magically appearing on the glass surface. I suppose it is because lead is easier reduced than most metals, while sodium which there is plenty of in most glass, isn't gonna let go quite so easily.

wg48temp9 - 19-8-2020 at 01:29

If your into glass you may find a youtube video by Applied Science interesting. Its about making photochromic glass at home.

https://www.youtube.com/watch?v=mUcUy7SqdS0

Chemetix - 19-8-2020 at 16:12

I'm a glassblower, and do artistic stuff with the glass as well, chemistry is a big part of precess when it comes to getting colour effects and not screwing up the colour you want.
When it comes to the mechanisms of redox with glass it is mainly confined to the outer layer. Where flame chemistry and temperatures are the main driving factors behind the colour changes.

One of the biggest problems with beginners is they use too much heat. And going for the hottest flame isn't your best friend in this regard. Get rid of the MAPP gas and go for LPG. You don't need the high energy high carbon fuel that MAPP creates, you need a flame that will have a gentle gradient of reductive(not fully burnt fuel) to fully burnt and then a soft heat gradient in the back of the flame. You need the flame to be adjustable with a range of sizes and still preserve the range of flame chemistry. Mapp is either super hot and boils the glass or reductive with unburnt high carbon fuel and the very narrow zone of right temperature and right flame chemistry. Terrible for glass blowing and colour control.

Glass has a matrix of cations like silicon sodium and calcium and with borosilicate glass, boron. All linked with oxygen molecules. The oxygens can behave like the oxygen in water where a charge can surround a cation and make it soluble. Just like in water the charged species can create a colour in a solution. Then there are pigments where a large molecule like an insoluble sulphide can be wrapped in the glass matrix, cadmium colours form strongly coloured opaque glasses in this way.

If you get the glass hot enough and you give the glass a reducing flame, the carbon will take an oxygen that was once paired with a metal oxide trapped in the glass and leave the metal returned to it's metallic state or with a lower oxidation number, the reduced species no longer is soluble in the glass or no longer give the same colour that the oxidised state did. Generally this is the grey scummy look the glass gets when reduced or boiled. Sometimes a hot oxygen rich flame will return the colour, or an oxidant like KNO3, as you observed, can supply the oxygen. It will also supply a cation such as potassium and will locally change the formula of the glass. This could lead to the glass no longer bonding with itself as the different formula will have a different coefficient of thermal expansion and crack when it cools down.

Hot glass conducts electricity and so electrons can move their charge through the glass matrix so when there are cations wanting to donate or grab an electron they can do it when the temperature is right. So even some species within the glass can be affected by the reducing or oxidising environment. Some formulas of coloured glass will play off one species against another. Silver iron and copper with their very low E- nought values don't take much energy to dislodge their electrons and so subtle changes in the glass with temperature can be created. Amber glass made with silver reacting with residual iron in the glass formula is one of the oldest coloured effects used in glass, the depth of colour controlled by concentration of the silver, the temperature and the time to allow the colour to develop.

A modern coloured glass using germanium and silver makes a glass that can be made to give a range of colours from deep ambers to rich royal blue and strong vibrant purples. Holding the glass in the right oxidising flame at the right temperature then judging the right point at where the glass has cooled enough to allow the right reaction to occur then holding that temperature by returning it to the right part of the flame is where the colour is developed. A lot of skill with a knowledge of the chemistry at work helps.

Another great use of reduction with silver is when the silver can migrate into the glass and be made to return to it's metallic state in nano particle clumps of the right size and spacing. The light reflects certain wavelengths back giving rich iridescent blues and greens. Control of concentration and temperature and ultimately reduction is a delicate art.

There's plenty of images and artists on the web showing off their skills. My insta is @ptlabservices where I show off my glasswork. A mix of scientific and art (ok there's some bongs in there too)

wg48temp9 - 20-8-2020 at 02:37

Quote: Originally posted by Chemetix  

snip
There's plenty of images and artists on the web showing off their skills. My insta is @ptlabservices where I show off my glasswork. A mix of scientific and art (ok there's some bongs in there too)


You have some great looking and to me very original flask/bong forms there. Fantastic work.

draculic acid69 - 7-10-2020 at 20:26

Excellent vid to watch https://m.youtube.com/watch?v=mtUnsfqMlsg

Also love the wine glass beaker