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mmezalick
AGG member
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Sat Jan 13th, 2007 11:59 am |
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You know , inquiring minds what to know.
No, really, You had mentioned the acids in your response about the cause of oxidation, and I was just thinking.
I read somewhere that transporting HF could be done by using lead bottles.
Some years ago a studio I know was re-leading some clear glass windows that had become etched from the weather and some exterior sandblasting of the building. The process he used was to disassemble the window , soak the glass in a mild solution of HF and then relead the window. Again , I was wondering if he could have "dipped" the window if the lead was not effected.
Just wild thoughts, nothing serious, and I am not in the position to do real experiments, just some visual.
I know, I know not the best or even recommended, but just curious.
There will be no report paper out of this test.
Michael
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Ardbeg
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Posted: Sat Jan 13th, 2007 03:31 pm |
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"You had mentioned the acids in your response about the cause of oxidation, and I was just thinking. "
Michael - specifically, and crucially, acetic acid, in the presence of carbon dioxide, water and oxygen (air) will attack the lead catalytically
Last edited on Sat Jan 13th, 2007 04:04 pm by Ardbeg
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mmezalick
AGG member
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Sat Jan 13th, 2007 03:44 pm |
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Linda,
Does the acetic acid become affected thru this catalytic process or does it become reduced by other means, ( eveaporation etc.)?
Michael
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Ardbeg
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Posted: Sat Jan 13th, 2007 04:17 pm |
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OK, last one, I hope...
I'm doing this from memory,
- the lead + water + oxygen -> lead hydrate and water gets released
- lead hydrate + acetic acid + oxygen -> lead acetate and water gets released.
- lead acetate + carbon dioxide + water -> basic lead carbonate and acetic acid gets released.
Because the acetic acid is the catalyst, it remains, at the end of the process, unchanged, and free, in specific enironments as previously mentioned, to continue the process until there is no lead left to change.
Now, climate change has meant that we, in the West of Scotland, are feeling like Noah, it has rained continually for at least 3 months, and now the wind is driving hard. Our 80 yr old roof leaks, no longer able to cope with the saturation and never allowed to dry out, so I may be busy for a while....need to earn some money to buy a new roof and buckets!
Happy days
Linda
Last edited on Sat Jan 13th, 2007 04:58 pm by Ardbeg
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Ardbeg
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Sat Jan 13th, 2007 04:55 pm |
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The exact chemistry is as follows:
- 2Pb + 2H2O + O2 -> 2Pb(OH)2 + H2O (lead plus water plus oxygen -> Lead hydrate plus water)
- 3Pb(OH)2 + CO2 -> 2PbCO3.Pb(OH)2 + 2H2O (lead hydrate plus carbon dioxide -> lead carbonate plus water)
- Pb(OH)2 + 2CH3.COOH -> Pb(CH3COO)2 + 2H2O (lead hydrate plus acetic acid -> lead acetate plus water)
- Pb(CH3COO)2 + Pb(OH)2 -> Pb(OH)2.Pb(CH3COO)2 (lead acetate plus lead hydrate -> basic lead acetate)
- Pb(OH)2.Pb(CH3COO)2 + CO2 + Pb(OH)2 + H2O -> 2PbCO3.Pb(OH)2 + 2CH3COOH (basic lead acetate plus carbon dioxide plus lead hydrate plus water -> basic lead carbonate plus acetic acid)
The powdery, white basic lead carbonate is the last product in the chain of reactions and is stable and inert. The acetic acid is thus released, at the end of the process, to do further damage to the lead. If the environmental situation is not dealt with, all that is left, at the end of the process, is white powdery dust. This process has been known about for hundreds of years, and forms the basis of the pigment known as "White lead paint".
Linda
Last edited on Sun Jan 14th, 2007 11:55 am by Ardbeg
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mmezalick
AGG member
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Posted: Sat Jan 13th, 2007 05:07 pm |
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Thank You ,
Michael
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artfem
Administrator
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Mon Jan 22nd, 2007 05:06 pm |
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Linda:
Great explanation of the process, including the 4 part harmony and the descriptive as well as formulaic instructions. To add a comment, lead items displayed in museum cases can often suffer from the continued attack by the process so aptly described by Linda because the acetic acid is not drawn out from the case and the process continues on and on. This can also occur in unvented protective glazing situations where acetic acid is present. If one does not understand the chemical process, and most specifically that the acetic acid is a catalyst and exits the reaction unchanged, it would be easy, although incorrect, to assume that the lead carbonate continues to corrode the lead cames. This is another example of why it is most important when planning conservation to understand why the lead, structure, paint, etc. is failing before one chooses a solution to the problem.
Art Femenella, Director
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Ardbeg
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Posted: Tue Jan 22nd, 2008 12:56 pm |
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To continue with this conundrum....
We're restoring a church with the brown lead corrosion (lead oxide), as previously discussed, is present throughout the windows. Otherwise, the lead is strong (almost hard), but the joints are failing and the windows are badly buckled.
Curiously, each panel is set inside a cast iron frame (curious for here, that is, maybe common else where).
Is it possible that the iron work is affecting the lead. If so, in the occasions where the lead has turned brown, has each panel been set inside ironwork?
Just a thought. Cast your minds back to where you've come across the brown patches on the lead - could the iron work be to blame, just as wood aids the promotion of white lead carbonate?
Linda
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mmezalick
AGG member
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Thu Jan 24th, 2008 10:22 am |
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Linda,
The only real brown I have seen, when referring to the metal frame, was the rust left on the lead. More of a dark golden brown and just around the perimeter.
The frame was pretty well rusted and it would make some sense that this film was from that aspect .
I did find this while searching. I don't know if it makes the correct assumptions as I do not pretend to have the depth of knowledge but I'm learning ...
Michael
The causes of lead-containing pigment conversion in mural paintings of the Church of St John The Divine in Rostov the Great (Yaroslavl region) were studied. The degree of bacterial contamination of the mural paintings was determined. Bacteria were revealed to participate in the conversion of white lead to lead sulphide. It was proved that oxidation of white lead resulting in the formation of brown-coloured lead dioxide is a biogenic process. Microorganisms isolated from the mural paintings are capable of oxidizing, along with white lead, other bivalent lead-containing pigments (massicot, minium).
Microbiological Degradation of Lead-Containing Pigments in Mural Paintings
Julia P. Petushkova, Natalie N. Lyalikova
Studies in Conservation, Vol. 31, No. 2 (May, 1986), pp. 65-69
Last edited on Thu Jan 24th, 2008 10:23 am by mmezalick
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mmezalick
AGG member
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Posted: Thu Jan 24th, 2008 10:52 am |
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More ideas,
Manual of Chemical Technology
By Johannes Rudolf Wagner
Lead Peroxide. - When red-lead is treated with moderately strong nitric acid,
there are formed lead nitrate and peroxide of the metal a brown-coloured
powder largely used in the composition of the phosphorus mixture for Lucifer matches.
My thoughts;
Red-lead being used in the putty mixture and nitric acid being used in the refining process of the lead.
Or simply the reaction of the lead to the nitric acid during the refining process.
Michael
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Ardbeg
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Thu Jan 24th, 2008 01:56 pm |
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Food for thought, Michael - thanks for these.
I'm not sure about the bio-organisms, as it's so "bloody arctic" where these specific windows come from, but the nitric acid in refining sounds interesting. The lead is increadibly hard, as if it has a lot of impurities in it, or has somehow changed chemically over the century? When we cleaned it with the dremmell to get back to the clean lead, it still resisted the solder, as if, somehow, although visually clean it was still oxidised.
Something is "wrong" with the lead, although it's very strong and secure, and does not need to be replaced. I'd just like to know what it is!
The putty does not contain the bright red-lead that I've often seen before - in this case it's cold and grey and rock hard.
I'd be very interested to hear from others on this one, as I don't know what's caused the lead to go "splodgy brown" and hard.
Thanks Michael
Linda
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artfem
Administrator
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Posted: Thu Jan 24th, 2008 08:03 pm |
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Linda, so good to hear from you again. Hope you are having a great New Year. I came across a conservation article about an old lead necklace (I think it was from somewhere in Scotland) on which a lead corrosion product called pyromorphite was found (Pb5(PO4)3Cl). I have since read that this pyromorphite can further "morph" into mimetite (Pb5(AsO4)3Cl) in the presence of arsenic. Both of these compounds can form on lead in mild acid environments (ph 6). While they typically run yellow to green both can be brownish red in appearance. The compounds can only be verified through chemically testing. I will see if I can scare up the name of the article. Cheers,
Art
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mmezalick
AGG member
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Thu Jan 24th, 2008 08:09 pm |
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Uses of mimetite
Industrially, mimetite is a minor ore of lead, especially when found in relatively large quantities. The chief use of mimetite is as a collector's specimen, often creating very attractive botryoidal crusts on the surface of the specimen. Though mimetite is also found in prismatic crystal forms, it is not used as a gemstone due to its softness. The best of these prismatic forms have been found in Johanngeorgenstadt in Saxony and Wheal Unity in Cornwall, England.
Ironic that the best comes from the UK, or should I not be surprised?
Michael
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Ardbeg
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Posted: Fri Jan 25th, 2008 12:06 am |
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Art. Michael,
thanks for your thoughts, and a good new year to you too. I've checked out "mimetite" on the web, but it looks too crystaline. "My" brown deposit looks just like the photo that Geoff supplied on the 18th Nov.
It doesn't remove easily for a visual clean. My main concern, however, is that the lead seems to be affected internally, as it's just not taking the solder well at all. But yet, the lead is so strong and hard. We've tried hot bolts and cool bolts, we've scraped the lead clean and wiped it with acetone, but to little avail, it's still very difficult to solder.
I'm coming round to the conclusion that there are lots of impurities within the lead which are causing it to "act strange".
I really want to know why the lead is the way it is - yes, it can be cleaned up, but unless I know why it's turning brown and hard, then I can't prevent it happening again.
Do you think the ironwork is a factor (along with the driving rain and the acidic, peaty moorland?)
Linda
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gwsg
AGG member
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Mon Jan 28th, 2008 02:10 am |
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G'day Linda,
The brown oxide in my photo occurred in a masonry installation that contained no iron work apart from internal saddle bars so I don’t think that the iron necessarily has any effect on lead oxidation. In fact we have all seen heavily corroded iron vent frames where no brown oxide has occurred on the lead. The window in question though had been covered with unvented double glazing for 30 years.
I do recall another instance where the brown oxide occurred quite extensively on the weather side of a church in Kyneton, a very cold, wet, boggy district of central Victoria, subject to heavy driving rain in winter. Not quite arctic but bloody cold. In that instance the (originally) galvanised wire screens had corroded to the point that the wires were just crumbling away and had left severe rust stains down the wall. At the time I did wonder why this was occurring only on one side of the church but could not come up with an answer but did note that was the direction that the prevailing weather came from. It was dramatically apparent that this aspect had a very corrosive atmosphere for some reason and you could almost see the wire screens fizzing when they got wet.
At that time I was not aware that peaty moor land was acidic but the severity of the corrosion had me thinking that the location and prevailing conditions were having some kind of effect on the iron however I could see no link between the iron and the lead. The brown lead oxide was hard and protective of the lead underneath, not at all corrosive. I have always found the lead underneath brown oxide to be in good condition.
The difficulty you are having with the lead taking solder is unlikely to have anything to do with impurities in the alloy as it was obviously able to be soldered when it was new. Try shaving the surface off the lead to make sure all of the oxide is gone and not just pushed down into the lead which is what will occur when abrading.
Geoff
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Ardbeg
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Posted: Mon Jan 28th, 2008 10:36 am |
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Hi Geoff,
that's a really helpful reply, thanks.
I agree with you, that the lead's in really good condition under the brown, nice and strong. It could be the aggressive environment after all.
I'll try using a different blade on the dremmell, one which will cut away rather than sand, and see what the affect is on my soldering.
Many thanks
Linda
PS. Where else have others seen this type of corrosion in situ - what were the conditions like?
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mmezalick
AGG member
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Mon Jan 28th, 2008 11:43 am |
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Linda,
I have used the Dremel 570 1/8" Carbide Grout Removal Bit.
The sides of the bit are designed to cut as well, not just the tip.
As for other examples, you may remember that I posted a similar situation where the windows were from a Spa ceiling. The panels were made form both lead and zinc.
Rethinking that project it could have been the acidic atmosphere that was more the cause of the brown lead.
Michael
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