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Glass Identification - Post here for all ID requests => Glass => Topic started by: David E on September 19, 2012, 11:38:13 AM

Title: Iridisation of glass - can it occur naturally?
Post by: David E on September 19, 2012, 11:38:13 AM
I know that red wine(?) can cause iridisation to the inside of decanters and carafes, but does anyone know if this effect can occur naturally to the outside of glass? The reason I ask is because I have an item of glassware where it should not be present, but it exists very faintly. So was this deliberately done, or a natural cause?
Title: Re: Iridisation of glass - can it occur naturally?
Post by: Lustrousstone on September 19, 2012, 11:42:52 AM
Could be the dishwasher...
Title: Re: Iridisation of glass - can it occur naturally?
Post by: David E on September 19, 2012, 11:51:55 AM
Hmm, possible... but then I would have expected the gilt bands to have worn badly.
Title: Re: Iridisation of glass - can it occur naturally?
Post by: chopin-liszt on September 19, 2012, 11:55:27 AM
The dishwasher will do it; I don't know about the gliding - maybe that takes longer to come off?
Burying it for a thousand or so years in soil containing appropriate minerals would do it too. ;)
Title: Re: Iridisation of glass - can it occur naturally?
Post by: Ivo on September 19, 2012, 12:34:31 PM
Metal salts in earth and sewage will do it quite fast, it does not need hundreds of years to achieve a lustre effect.
Title: Re: Iridisation of glass - can it occur naturally?
Post by: johnphilip on September 19, 2012, 03:22:09 PM
OK David i offer my body and organs up for research into red wine and its effect on glass if it will help , just bury me beside a bottle,,,,  make sure its a good year and full . :D not near the sewage outlet please . :( jp
Title: Re: Iridisation of glass - can it occur naturally?
Post by: chopin-liszt on September 19, 2012, 03:37:47 PM
oooh, Ivo -
"Metal salts in earth and sewage will do it quite fast, it does not need hundreds of years to achieve a lustre effect."

now you've given me ideas......... ;D
Title: Re: Iridisation of glass - can it occur naturally?
Post by: David E on September 20, 2012, 09:52:33 AM
I always thought gilding, particularly on vintage (or should I say vintique (https://www.glassmessages.com/index.php/topic,49478.0.html) :) ) glass was highly prone to wear in a dishwasher. I would have thought it would take many, many washes to achieve this level of iridisation and then the gilt would have vanished.

So is there some air-borne chemical, perhaps a naturally occurring one, that could achieve this - I assume the presence of metallic salts that normally produces iridised glass. Perhaps it was left in a garden shed or something?

If not, then the only conclusion I can draw is that it was done deliberately as an effect.
Title: Re: Iridisation of glass - can it occur naturally?
Post by: Ivo on September 20, 2012, 10:19:43 AM
I've seen the combination of very light iridescence and gilding in Polish tableware.
Title: Re: Iridisation of glass - can it occur naturally?
Post by: johnphilip on September 20, 2012, 10:21:00 AM
David i am sure the guy who makes the repros for museum shops can help with this one i believe his name is Mark Taylor he has a partner who works with him .
Title: Re: Iridisation of glass - can it occur naturally?
Post by: Pinkspoons on September 20, 2012, 11:18:24 AM
Long-term storage in dusty places prone to condensation seems to have a knack of causing iridescence to the surface of glass - I've bought old, but ostensibly 'new', glass from closed down retailers in Italy and Denmark in the past, and iridescence was present on quite a few pieces that had been left to their own devices for 30+ years.

I assume the dust makes water in the air stick to the glass longer than it otherwise would - items in more sheltered areas were unaffected. Also, being wrapped in paper exacerbates iridescence far more than glass left in the open - presumably as a result of the paper being more absorbent than dust.
Title: Re: Iridisation of glass - can it occur naturally?
Post by: Lustrousstone on September 20, 2012, 11:25:45 AM
Possibly because paper is quite acidic; acidic free paper is what conservators specify
Title: Re: Iridisation of glass - can it occur naturally?
Post by: Pinkspoons on September 20, 2012, 11:30:20 AM
This would also account for it.
Title: Re: Iridisation of glass - can it occur naturally?
Post by: David E on September 20, 2012, 12:32:57 PM
I've seen the combination of very light iridescence and gilding in Polish tableware.
That's interesting, except it is British glass :D

Thanks Nic for that thought, which could quite possibly be the answer. Old newspaper would certainly account for this and I suspect that's what was used. The iridescence is right up to the rim, so unlikely to be a liquid so, again, the wrapping could be the reason. I can't determine whether there is iridising on the inside though - very difficult to tell.

BTW, what are the metallic salts used to iridise glass?
Title: Re: Iridisation of glass - can it occur naturally?
Post by: chopin-liszt on September 20, 2012, 12:41:51 PM
I would strongly suspect different metals are used for different finishes - but I know that IoWSG sprays a lustre finish on using Stannous Chloride. (Tin)
This isn't an overly mirror-like shiny surface as some are, but somewhat matt.

I've got a "Balloon" vase, which has a layer of stannous chloride sprayed inbetween casings - it gives a subtle, slightly glittery, rainbowy effect inside.

In Wedgwood Fairyland lustre, the lustre can be a gold or green shade, Carltonware used a goldy red (on Rouge Royale), Zsolnay use an eosin glaze with a gold coloured lustre.....