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mmezalick
AGG member
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Sat Jan 6th, 2007 10:09 am |
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Geoff,
Thanks for the descriptions. Over the years I have seen many windows with the white oxide. The simple process of easy brushing and re-puttying has always appeared to work, just like the photos you have posted. The brown oxide has been a bit more mysterious to me. Only seeing it once or twice before, it was hard to decide what to do. It would not come off as easy as the white oxide.
The project I have now has mostly the brown oxide. The windows are from the skylight of a historic spa (1915), the main steam room to be exact. Moisture galore. The lead is in great shape with zinc mixed in. No signs of any problems with either materials. The work is mostly repairing or replacing broken and missing glass. It's pretty straight forward.
Michael
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Ardbeg
AGG member
| Joined: | Tue Oct 24th, 2006 |
| Location: | United Kingdom |
| Posts: | 67 |
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Posted: Sat Jan 6th, 2007 11:46 am |
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Michael,
you say "lead with zinc mixed in". Is that not the answer.... in the moist environment of the steam room the zinc has received elecrons from the lead, and the lead has corroded sacrificially to the brown lead di-oxide as Geoff described so well? If there was no zinc present then I'm pretty sure that the usual white basic lead carbonate would have occurred.
It would be interesting to hear from others if zinc (or other metals) were present in other instances of brown lead.
Linda
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mmezalick
AGG member
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Sat Jan 6th, 2007 12:13 pm |
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Fabulous answer Linda.
Thanks,
Michael
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artfem
Administrator
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Posted: Wed Jan 10th, 2007 02:28 am |
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It appears as though much of the conservation of lead objects occurs in the field of marine archeology wherein lead objects have been immersed in fresh or salt water for long periods of time. The consensus opinion in this area of lead conservation is, “…the corrosion products of objects of lead, tin and their alloy, pewter, are stable. The corrosion products may be unsightly or even disfiguring, but they do not take part in chemical reactions that attack the remaining metal. The objects should be cleaned only for aesthetic reasons and to reveal surface details under the corrosion layers.”
It would appear that the issue is to do what is necessary to change the micro-climate that the lead cames are subject to stop further corrosion. There is no long term benefit of removing the products of corrosion. In the literature that I have read, if these products are removed for aesthetic reasons (for display purposes) it is accomplished with either electrolytic reduction or with the use of chemicals. The most common method of removing the products of corrosion is a treatment referred to as the “Caley Method”( Coatings and Incrustations on Lead Objects from the Agora and the Method Used for Their Removal Earle R. Caley Studies in Conservation, Vol. 2, No. 2 (Oct., 1955), pp. 49-54) wherein the lead object is immersed in a 10 percent solution of hydrochloric acid. In that this acid removes even well fired paint from glass, this approach would not be recommended for our purposes.
Electrolytic reduction involves the placement of the object into an electrolyte (acid) bath with a cathode and anode and flowing a current through the bath. This is also not recommended for leaded glass windows and would be impossible to do on site. The use of EDTA as mentioned by Linda is also mentioned, but I do not know what affect this might have on the paint or glass surface. None of the literature that I read mentioned the removal of the products of corrosion with mechanical means, either by wire brush or scraping with a tool.
In that the consensus is that these products of corrosion do not encourage or facilitate future corrosion of the lead cames and that the accepted methods of removing these compounds do not seem to apply to leaded windows, I would think that all of our efforts should be focused on altering the micro-climate of the window in question so that the corrosion is prevented.
Art Femenella, Director
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artfem
Administrator
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Wed Jan 10th, 2007 02:37 am |
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Linda:
In the article by Caley referenced above, he discusses the presence of lead dioxide on the articles found from the ship. He states his surprise at this finding in that lead dioxide is typically formed "...under high oxidizing conditions and is most readily produced by electro-chemical oxidation." Your theory that the electron flow from the zinc may be right on. He also reffers to the lead dioxide looking like an applied paint, similiar to the experiences of many that have joined this discussion.
Art Femenella, Director
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Ardbeg
AGG member
| Joined: | Tue Oct 24th, 2006 |
| Location: | United Kingdom |
| Posts: | 67 |
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Posted: Wed Jan 10th, 2007 10:03 am |
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Art, Michael, thanks.
EDTA is a chelating agent and is used in glass conservation to remove corrosion products. However, as with many conservation chemicals, it's use for one type of situation (archaeological glass vessels) is highly usuitable for painted architectural glass. Therefore it's use in stained glass is not well documented outside of the conservation of glass literature. EDTA's use in cleaning lead is also well documented in metalurgical conservation literature. I added it into the conversation, because, yes, it effectively removes lead corrosion products, but, again, it's use was on lead objects, not leaded glass, and it's suitability for one material (lead), in our case, is disaterous for the other (glass).
My reason for mentioning it was that the discussion was going on to discuss methods of deposit removal from lead, and if chemicals are used for this purpose, they are going to have a knock-on effect on the glass. Any amount of EDTA on susceptible glass will cause a similar effect, over time, to that of hydrofluoric acid as it seeks out the calcium within the glass network.
I completely concur with Art, in that it's far better to work out why the lead is deteriorating and solve the cause, than simply to remove the symptoms, which may, themselves, act as an unsightly, but protective coating. Further deterioration should be slowed down, or prevented, in a stable environment. That is, afterall, why we use lead - it is a very stable, flexible, maleable, low m.p. metal, with just the right amount of strength vs fatigue from a metalurgical point of view for its use in architectural glass.
Linda
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mmezalick
AGG member
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Wed Jan 10th, 2007 10:31 am |
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Linda, Art and Geoff,
Thanks for the most informative responses to my questions. Better than any other class I had taken.
The vast knowledge that is apparent is impressive and I can't be more grateful for the willingness to share.
I will take this information and try to apply it to my work routine.
I do have a few minor items I would like to ask. Geoff mentioned the range of colours that the oxidation shows up as. The red lead has a red to brick red colour. The brown oxidation is , well, brown. without being too anal, there may be a fine line between the two colours.
What we have , I believe is the brown oxidation. I have tried sample testing on some small sections and have found what everyone has suggested, an acid based product works the best, but is potentially damaging to the glass.
I even tried an deck cleaner. It showed the best results but will not use it because of the Oxalic Acid it contains. I know that this type of acid is used to clean mildew but it did have an effect on the lead and on the glass. Nonetheless , we'll keep the oxidation.
In my final report to the client, I would like to include the information each of you has presented. I would also like to give you the credit, because I certainly did not learn this on my own.
Again, my most grateful appreciation,
Michael
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gwsg
AGG member
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Posted: Wed Jan 10th, 2007 09:27 pm |
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Michael,
Red oxide really is bright RED, or orange/ red. It used to be used as an additive to leadlight cement and sometimes you will see deposits of it inside the channel of the came as you disassemble a window. In my experience, its occurrence on the surface of the came is extremely rare but when it does occur it is soft, powdery and corrosive like white oxide. Conversely, brown oxide forms a solid, ‘paint like’, protective patina. Hope this helps.
Geoff
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mmezalick
AGG member
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Wed Jan 10th, 2007 10:13 pm |
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Absolutely, It's much clearer. We have the brown.
Thanks,
Michael
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Roberto
AGG member
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Posted: Sat Jan 13th, 2007 04:31 am |
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The discussion on lead oxide has been very interesting to me. Here in New England, I have come across the brownish/red oxide mentioned and shown by Geoff in his photos, although not as dark. (see photos "light brown oxide 1 & 2). What I have come across a few weeks ago was again very interesting.
See photos "black oxide 1,2&3"). Selective areas or pieces of the interior leads in these Connick windows from the late 1930s, have turned black. Although it really looks like paint, it is not. What I have noticed (and I could be wrong) is that the lead that has turned black, seems to be of a slightly different profile and size of the abutting leads. Could the different leads contain different metals? And what about the solder joints? Maybe the lead that has turned black is higher in tin? Any clues?
Thank you and I look forward to your input.
Roberto
Serpentino Stained GlassAttached Image (viewed 62 times):
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Roberto
AGG member
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Sat Jan 13th, 2007 04:33 am |
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| Light brown oxide 2 Attached Image (viewed 60 times):
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Roberto
AGG member
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Posted: Sat Jan 13th, 2007 04:34 am |
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| Black oxide 1 Attached Image (viewed 58 times):
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Roberto
AGG member
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Sat Jan 13th, 2007 04:36 am |
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| Black oxide 2 Attached Image (viewed 57 times):
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Roberto
AGG member
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Posted: Sat Jan 13th, 2007 04:36 am |
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| Black oxide 3 Attached Image (viewed 56 times):
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Ardbeg
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Sat Jan 13th, 2007 10:37 am |
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Roberto, I think you're spot on with your analysis of the problem. If you compare the solder joints, they too are black. It could be that in some areas (because the black lead looks quite random), the lead was floated / tinned between solder joints.
Pure, 100% lead is the most susceptible to lead corrosion of the type shown and discussed, the white, powdery basic lead carbonate. By adding, even just 1% tin, the lead becomes much more resistant, and does not corrode so easily. 1.5%, plus other tiny %ages of impurities, such as silver, antimony, copper and bismuth makes a real difference, and the lead is really very stable. More than 3% impurities, and the lead becomes very stiff, and too difficult to work with from our point of view, so the ingredients are really very critical to get the mix right.
Most glass workers, until relatively recently, made their own lead calmes, so obviously there are big variations that came out of their lead-pot-mix.
Anyway, the blackened lead looks very stable. I will add one more post about the basic lead carbonate .....
Linda
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Ardbeg
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Posted: Sat Jan 13th, 2007 11:10 am |
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Basic lead carbonate...
I just wish to clarify an earlier comment...
basic lead corbonate is, in itself, NOT corrosive.
For instance, sulphuric acid is corrosive. If you spilt some suphuric acid on your hand, a sore hole would soon appear. Hydrolfuoric acid is corrosive. It burns its way into your bones. Coca cola is corrosive to your teeth, and so on, and so on
Basic lead carbonate is inert. It sits as a white powder, on the surface of the lead, as by-product of decomposition.
Acetic acid is the catalyst which eventually causes the lead to turn into this white powder. By its nature, as a catalyst, it is finaly released, unchanged, back into the environment, to attack more lead until there is no lead left. That is why the environmental conditions are vital to get right. Basic lead carbonate does not form in normal atmospheric conditions, but only when acetic acid is also present, even in small quantities, along with carbon dioxide and water, which are found, of course in the air.
So, where does this acetic acid come from?
Usually the main culprit is wood, laminates, glues, paints, silicon sealants etc. Does anyone know of any window that has any of these components next to it - of course you do. Some woods are worse than others. Some wooden composits are worse than others. But, as a catalyst, you only need a tiny amount of acetic acid to do the damage.
Why then, are some leads white, and others black, or uncorroded. That depends on many factors, primarily, as discussed, metalurgical analysis and environmental conditions.
The basic lead carbonate is NOT corrosive in itself, but its presence does indicate that the lead is in an environment which is harmful to the lead and THAT needs to be addressed. Simply removing the white powder and puting the window back in its original situatiion unchanged, will just encourage the still-present acetic acid to do its catalyic damage.
Two things can be done to reduce the damage - either increase the air flow, to let the acetic acid escape into the atmosphere, or seal the window up in a tight vacuum. If the latter is chosen as the preferred option, then continual monitoring of the lead has to be regularly done. Vacuum seals have a habit of breaking down, and in that case the lead would be much more vulnerable to acetic acid attack as there would be very little air flow in the less-than-perfect sealed environment.
Enough already, before I bore for the planet, but I thought I should clear up that slightly misleading statement - basic lead carbonate is not corrosive - but it is an indicator that the lead is in a corrosive environment and that (the environment) has to be improved to prevent any further deterioration.
As an aside...
when I worked for the museum, we had to deal with this problem and we tried, over many years, much research and many experiments, to find the cause / answer to basic lead carbonate on leaded glass. Two things which did not work to prevent further damage were: coating the lead with a varnish / patina, as the acetic acid fumes still found their incidious way into the lead: and placing the (stored) panels on acid free card. The card soaked up the environmental acetic acid like a sponge, and the lead then sat on an acid soaked sponge - not good! Eventually we opened up the stores, go rid of the "brand new" wooden storage, invested in metal mesh cage storage and encouraged as much air flow as possible in the stores.
Some leads were completely resistant to the attack. When analysed, their composition was ideal, as described above, and ranged from 13th century to 19th century. The modern, purer leads suffered worst.
Last edited on Sat Jan 13th, 2007 04:05 pm by Ardbeg
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mmezalick
AGG member
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Sat Jan 13th, 2007 11:12 am |
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Linda,
Do you know what effect hydrofluoric acid (HF) has on lead?
Michael
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Ardbeg
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Posted: Sat Jan 13th, 2007 11:15 am |
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No.
Why?
Why not try?
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mmezalick
AGG member
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Sat Jan 13th, 2007 11:17 am |
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| Just curious. I'll try today.
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Ardbeg
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Posted: Sat Jan 13th, 2007 11:22 am |
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| Just curious too - why would you want to use hydufluoric acid on lead which, even in tiny quantities is so harmful to glass? How can you be sure that all residual Hfl is removed once you're finished your experiment? What what why why?
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