# What causes an iridescent color on old bottles?



## kjackson939

Hey all, 

While looking through a collection that was in storage, I noticed that some of the bottles have an iridescence to them- a sheen that reflects many colors of light, almost like an oil slick. 

It's not universal, and in some cases only one will be colored as such out of a pair (the other is just clear/blue/etc).

What causes this? A quirk of production? Something that was stored inside the bottle? Label residue? 

Thanks for any info!


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## Harry Pristis

Some collectors in the Southeast refer to this as "St. Augustine sheen."  Apparently, there is something in the mud at St. Augustine (and in many other places) that slightly etches the glass surface so that light is refracted into this rainbow effect.  This effect will not respond to simple washing; but, it can be tumble-polished away.


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## Robby Raccoon

From a letter I wrote to another guy:
"You will also see that the bottle is quite heavily stained, mostly white (some appears as iridescence, mostly seen in privy-dug bottles. The rest is dirt, which you can try to remove by shaking sand and water in it). I call it mineralisation. Most collectors call it ‘sick glass’. It results from both deposit of minerals in the ground, and the decomposition of the glass itself as some of what makes it up had leached out, and other compounds leached in to fill the void (much like fossils, as bone decomposes and minerals take the place of its pores). "
Iridescence is mainly seen in bottles dug from old outhouses and trash pits. The white or brown-orange staining also occurs in these, often being so heavy that it flakes off, but is also common of lakes and just having been tossed on the ground somewhere. 
It's a matter of molecules thick, usually. As said, tumbling will help remove this, but the bottles shown are not worth the cost of tumbling.


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## sandchip

Something I read once was that the soda and lime are leached out of the glass slowly over time, depending on the moisture and chemical conditions in the soil, leaving a silica skeleton behind.


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## sunrunner

I never mind about iridescence as long as it's transparent . With old bottles it looks cool.


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