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  QUESTION: Could silver layer on ancient iron bed be based on lead naphthenate?, 28 Apr 2005, Setubal Portugal

Im a Portuguese conservator and Im studying an ancient iron bed from the XVII century. It was restored before 1940 with a silver layer.

This silver coloured layer was identified at the lab as being a lead naphthenate. But in fact lead naphthenate as I saw is a brown compound. In your webpage I saw a reference to: metallic lead pigment and metallic lead pigment in www.lead.org.au/lasn/lasn006.html

Do you think that this silver layer could be based on lead naphthenate?

What is the chemical composition of the "metallic lead pigment" you mentioned at your webpage?

This information is very important, because this object is quite rare.

Regards
Andreia

ANSWER: 28 Apr 2005

Dear Andreia,

Thank you for your enquiry and patience. We apologise for the late reply.

Due to the immense number of emails we get from our website, it takes us a while to get through all of them. To answer your question regarding lead naphthenate, we have consulted Dr. Van Alphen, who wrote the article referring to the metallic lead pigment on our website. I have forwarded his answer to your question below. Your original email is also forwarded below.

Hope this helps.
Regards,
Elisa Idris
Volunteer Information Officer,

From: Mike Van Alphen To: The LEAD Group Wednesday, April 13, 2005

Subject: Re: Enquiry regarding lead naphthenate

Elisa,

I do not know the 'colour' of lead napthenate but it is unlikely to be silver in its own right from my experience. However a lead napthenate drier could be lightly pigmented to a light or dark colour for example for the material to be added to dark or light paints or stains. That said its not beyond possibility for a lead naphthenate containing paint to be pigmented silver. It seems unusual for an entire paint layer to be described as lead napthenate. Usually driers form a minor component of a paint layer by weight - eg maybe 0.5-2% but not 5-10% for example. Paint layers are usually 15 to 30 microns thick and could have lead contents from 20-60% by weight for older lead formulations. It would be likely that the lead from lead naphthenate may only form 0.5 to 2% in a Pb paint and an unpigmented lead naphthenate solution applied direct on a surface would be like applying a very watered down clear finish with little thickness upon drying eg 1-3microns.

Lead naphthenate is of itself not an enduring durable pigmented material although I guess it has some colour. Of the lead driers that I have seen (in old cans) they have been colourless or brown liquids but these have been LMC driers (lead/manganese/cobalt) based driers.

I would recommend analysing paint layers by X-ray diffraction plus elemental analysis to determine the original layer by layer compositions. Electron microscopy is also very useful for layer by layer analysis. I am not immediately aware of direct tests of the naphthenate and suspect that as this material dried and over time it could well be somewhat modified. Some more details may help in resolving the question.

cheers Mike

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