# Black Glass Patina



## obxcomber (Apr 30, 2018)

What causes this film? Is it a sort of Benicia iridescence? I've seen a similar patina on the outside of old black bottles, but in this shard it's on the inside curve. I'd appreciate any insight


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## obxcomber (Apr 30, 2018)

[video]https://www.facebook.com/kelly.merrell.92/posts/10214026113944614[/video]

Hope this links to vid...


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## Robby Raccoon (May 1, 2018)

Decomposition of the glass causes 'patination' or 'mineralisation'. A nice word for a rotting bottle. I believe it is the silicates in the glass that leach out due to acids or bases in the soil. When they leach out, other things take their place: much like bone that turns to a fossil. It goes to show that fossils can form in a few years. 
In this case, the broken bottle allowed the chemicals to due their work more strongly on the inside than they might have had it been intact. The outside probably sat on the sand more (assuming it a water find), which may have preserved the exterior better. Usually, staining is more on one side of a bottle than another, at least in my water-finds. 
If that was found in the water, though, you might try scraping it with your thumbnail to be sure it isn't an algal/fungal encrustation.


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## southern Maine diver (May 1, 2018)

It seems the Patina is thicker on salt water discovered old black glass like onion bottles. Then after the bottle is out of the water, it almost flakes off to reveal some nice early glass


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## obxcomber (May 1, 2018)

Thanks for the input, guys. This was a North Carolina Outer Banks beach find. Yes, with a bit of scratching the powdery patina will flake off a bit. This is the first iridescent black I’ve found, if I scrub away the patina will the iridescence remain? Is it cooler to keep the patina, you know, because it’s part of the story of the glass? I know, it’s not like it’s an actual bottle, but we sea glass folks appreciate the history behind the pieces, so I want to do the right thing. 

Also, do you think this could possibly be (or be as old as) an onion bottle? Is the patina an indicator of hundreds of years of this chemical breakdown?


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## sandchip (May 2, 2018)

A soak in muriatic acid will remove any black crud without affecting the patina.  What part of the bottle does it seem to be?  Looks like it may have been in a fire unless I'm just looking at it wrong.


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## obxcomber (May 2, 2018)

Sandchip, did you check out the link to the video? There's no black crud, just the goldish film. The curvature of the shard is suggestive of the body of a champagne type bottle, but my bottle experience is limited. My only experience with burned bottles is what we sea glass folks call bonfire glass, which is usually melted beyond recognition.


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## sandchip (May 3, 2018)

Wow, what a difference perspective makes.  In the still shot, the haze patina gives the illusion of it being wavy shaped.  Thanks for reminding me to check out the video.


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## NC btl-dvr (May 7, 2018)

I don't believe it's an onion or of that age. Most likely from mid 1800's- early 1900's. Mallet form bottle.
jay


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## Harry Pristis (May 7, 2018)

"Though glass is seemingly a highly resistant substance (and it is) it is still subject to the slow corrosion by water and chemicals.  This is a function of the specific composition of the glass as it relates to the chemistry of the soil/water that the glass resides in, as influenced by amount of exposure or contact time (Munsey 1970).  Patination is more common in bottles with a high soda (and low lime) content.  Water will gradually dissolve or leach out the soda component of the glass leaving . . . silica behind. . . . 

"Particularly susceptible bottles or those that have been in contact with water/soil for a very long time will exhibit pitting, which is the extreme corrosion of the glass surface.  Eventually glass will corrode completely away in any environment given enough time (Kendrick 1963; Elliott & Gould 1988)."

The "coating" of silica referenced above is the more durable, residual component of the glass as the more vulnerable soda (and lime) are dissolved away.  As the glass surface changes from smooth to micro-pitted, the light reflected from the surface changes from "shiney" to refractive,  Refraction breaks the light into different wave-lengths (like a prism or atmospheric moisture producing a rainbow).  This refractive surface may be more or less prominent depending on the chemistry of the original glass batch.  The micro-pitting (sickness) may advance over time until the bottle wall is compromised.
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