Glass Chemistry

[Redhotsal]

A student emailed me this morning asking if there were any books on the chemistry of glass - colours reactions and so on. Since I emailed back rather a long reply I thought I may as well share it here. I'm not claiming it to be super accurate but it does try to answer the basic questions like "what is reduction" and so on and there's a few good links which might be useful to someone:


Colouration:

All glass is pretty much the same basic "stuff" but you can get the different colours by adding different oxides. These are usually metal
This link has a good table of colours:

http://geology.com/articles/color-in-glass.shtml

You get reactions because some of these oxides will react when they are mixed together. So, a typical reaction is that between turquoise glass and ivory/yellow/some reds.

I've always understood this to be a selenium/copper reaction. So while some colours will sit happily on top of each other without a reaction, others will form a dark line where the two colours meet.

I think Corina Tettinger had a good book out on colour reactions:

http://www.amazon.com/SPOTLIGHT-BEADS-VOL-REACTIONS-Lampworking/dp/B003K2GM16

The other two processes you need to be aware of in beadmaking is REDUCTION and STRIKING.

You can now buy lots of reducing or striking glass rods. People tend to clump them together but the two process are very different.

Reduction happens when the glass is heated in an oxygen starved flame. So you can either turn your oxygen down on a dual gas torch or you can work at the bottom of the flame or even cover the air holes over with foil on a hothead to get reduction.

When this happens the oxides in the glass tend to be stripped of their oxygen which results in a colour change to the glass. If there is a lot of metal to begin with you can get a metallic lustre forming on the surface of the glass. Double Helix "Triton" is a good example of this. As you can see from here, there are lots of others:

http://www.doublehelixglassworks.com/reductioncolors.aspx

If you add oxygen back to the gas mix you will usually (not always!) reverse this process and the lustre will vanish.

So, it's a good idea to reduce glass right at the end of the bead process, after you've finished all the shaping and decorative bits which require a neutral flame.

Reduction is essentially a CHEMICAL process.


Striking is a thermal process. Some glasses are strikers. That is, they start of as one colour and then change permanently to another colour. Usually, transparent reds are strikers. They start our as a straw yellow colour and then change to a deep red. Some of the fancier "designer" glasses also do this - like Double Helix again:

http://www.doublehelixglassworks.com/strikingcolors.aspx

Striking is essentially a thermal process as it involves getting pigment crystal to form - they only do this at a specific temperature. Usually this is lower than the melting temperature. So in effect - you make your bead, let it cool down for a few seconds and then reintroduce it to the top of the flame where the heat is lower. This usually causes the glass to "strike". Sometimes putting the glass into a hot kiln will actually do the same thing and beads will often strike - or change colour in the kiln. BUT, as with reduction - striking can be reversed so if you heat the bead up again quite fiercely the colour pigment will "redissolve" back into the glass and it will become the same colour it started out as.

More on striking here: http://en.wikipedia.org/wiki/Glass_coloring_and_color_marking

Hope that answers a few of the usual questions!  

[garishglobes]

Thanks, there are some good links there

I find glass chemistry really fascinating. I think it may also play a greater part in boro reactions, which is another reason for my interest (obsession...). The Glass Alchemy guide to boro colour contains a lot of interesting information on boro striking and boro chemistry generally - http://glassalchemy.com/index.php/resources/user-manual

...and the Corning Museum of Glass has quite a bit of glass chemistry information (http://www.cmog.org/dynamic.aspx?id=264#science_technology) as well as an interative kids version of glass chemistry on the Corning site ("heellllooooo there, glassmakers!!"  - http://www.cmog.org/glasschemistry/ )

[Ian Pearson]

Excellent info here, thanks.

Society of Glass Technology is aimed at furnace work but their site is worth a visit I feel
http://www.societyofglasstechnology.org.uk/cgi-bin/open.cgi?page=index&sessionid=36754110

Ian

[helenfc]

thanks Sal, that was a really interesting 

[Dickie]

Sal, please feel free to add this to the Wiki...

[Redhotsal]

Um, how'd I do dat? 

Will you do it for me.......? Blinks innocently.......

[alchemist]

Sal
thanks - really interesting post

[Hamilton Taylor]

'Coloured Glasses' by W A Weyl

Sean

[garishglobes]

Oh for goodness sake!! Talk about not being able to escape your past....

I went to search for that Weyl book on Amazon and actually found myself thinking this one on the Science and Archaeology of Materials looked really interesting... http://www.amazon.co.uk/Science-Archaeology-Materials-Julian-Henderson/dp/0415199344/ref=sr_1_15?ie=UTF8&s=books&qid=1276271641&sr=1-15

as does this one  http://www.amazon.co.uk/Archaeological-Chemistry-Paperbacks-Mark-Pollard/dp/0854042628/ref=sr_1_16?ie=UTF8&s=books&qid=1276271641&sr=1-16 on Archaeological chemistry.

Aaaaagh, metallurgy!!!!

[Sulis (Hazel)]

One of the things I love about Lampworking - it's got that 'Alchemy' feel to it 

This is really fascinating stuff Sal, thanks for this!

Hazel  xx

[Hotglass28]

Good stuff, I like a bit of geeckery. lol

[noora]

That's great info! This is the first time anyone managed to explain striking to me. So far everyone I've asked have just said "oh, I don't know, some chemical process I guess". I like understanding why things happen, not only learning that they do.

[★★Terri★★]

Wow Sal......you ever thought of writing a proper book?

This is just the type of info I have been looking for in countless books.  I do love to know the nitty gritty of what I am doing and why I am doing it.

This forum is just great for this type of stuff

[Veebee]

Wow, this is great Sal, thanks!

[Redhotsal]

Wow Sal......you ever thought of writing a proper book?

Yeah - but Joan beat me to it! LOL Only kidding  . Well, I started writing a book a few years ago but it got to over 200 pages. I dont' write very concisely so I have to force myself to rein in a little. Then I made some DVDs and then loads of people came out with various e-tuts and so on so the book project got a bit shelved. Would like to revisit it sometime but never quite found the time......

Glad it's helped a few people.