patina question

[stefan]

I have always wondered what exactly causes the patina on a bottle. Does this only happen to bottles that have been buried, or does the glass react over time to the air?

If only when buried, what chemicals are reacting with the glass, and what is the chemical make-up of the patina?

 

[oz-riley]

I will have a go at this one!
Patina is a word used to describe ware on antiques in general.

From the dictionary:
Patina. n. - Incrustation, usa. Green on surface of bronze; Gloss produced by age on Woodwork.

Patina is not really a word I would use to describe bottles, but that is just me, Patina is a nice way of saying damage due to age of an item that has an ascetic appeal. I don' t believe there is a chemical make up of Patina as such, but there are chemicals involved in the different processes that cause glass damage or breaking down of the glass in general.

Let’s break down the different types of glass damage that can happed.

I have a bit more information here, this page is about how to clean these things but it is relevant here and has picture of the listed glass damages below. Cleaning Bottles

Leaching - This is when the lime in the glass begins to break down due to being buried with household rubbish, this rubbish breaks down and creates acids and other chemicals which can attack the glass and cause a chemical reaction in the glass itself causing the lime to leach out.

Contents Hazing - This is caused by the contents of the bottle, usually it is the sugar in the product that causes the hazing but some food acids have the same effect. So this can happen even if the bottle was not buried, this happens most when there is still some contents in the bottle when it was thrown away and give the sugar or acids time to work on the glass.

Pitting - Again this usually happen to bottle that were buried, items buired with the bottle break down and cause acids and other chemicals that attack the glass directly.

Manganese Oxide- is the black substance that forms on the outside of the glass on bottles found in creeks and rivers, I' m not sure exactly why it forms but it is easily removed.

Rust Staining - Caused by bottle being buried with metal that rusts and can eat into the glass, can usually be removed from glass but does leave pitting on the glass, very hard to remove from stoneware.

Pressure - Pressure damage is a funny one, you can have a bottle that was mint when you dug the bottle and then 12 months later you could here a tink and a large crack has formed in the bottle. This is pressure damage and is caused by the pressure the glass was under when buried.

I don' t think you would get damage to glass from exposure to just air, not for at least 1000 years anyway. What you call Patina is generally caused by what I have listed above.

Anyone else have any comments on this subject?

Thanks.
Chris.

 

[IRISH]

Hi stefan,
I agree with Chris that patina is not really a term for what you see on some bottles.
As for Leaching some of it is just as Chris wrote above, but in some strongly alkaline tips ie full of ash (mostly Potassium biCarbonate) and lime you also get a surface devitrification of the glass, this gives the cloudy dull look.
heat as in a burnt tip also makes this a lot worse.

Manganese Oxide is just precipitated out of creek water on the surface of the glass it does not react with the glass at all that' s why it' s easily removable.

Hope that makes sense
IRISH

 

[IRISH]

It is amazing what can suvive in the ground, the oldest labels I' ve seen dug were from 1850s tip in Melbourne.
the odd thing about those bottles is that they were very sick and pitted exept were the labels are. the same thing happens in the 1890s tip I dug in on the way back from the nat. bottle show a few weeks ago, half the bottles still have labels or bits of label on them.
its all in the chemicals in the tip [] .

 

[Mark Potten]

Hi Stefan - this is what I beleive happens when glass is left buried for a long period of time.
The majority of glass bottles recovered from old dumps were manufactured from soda glass - a mixture of soda-lime and silica. Soda and lime are water soluble substances and when glass items are buried for long periods under adverse conditions - i.e. dampness combined with alkalinity - the two soluble constituents become leached from the surface, leaving a film of insoluble silica. This film gives the surface an appearance of being frosted or etched, a condition usually referred to as ' sickness' among collectors.

 

[stefan]

Thank you. That makes sense. Does it however address the rainbow effect? Does the silica refract light?