Lead

Alan J. Hawk <hawk [at] afip__osd__mil> writes

>I have a model of a ship made in 1875 for the Centennial Exposition.
>Some of the parts (such as the steering wheel and pullies) were cast
>from lead. However, the lead has developed a heavy white powdery
>coating (which I assume is oxidation) that is falling to the floor
>of the case.  What is the best treatment to stabilize the lead?  I
>was considering gently brushing off the powder and then painting the
>lead with Arcoloid B-72 liquid.  Would that be a good approach? What
>are your suggestions?

According to your description, which is only a partial view, and
let's assume that in the close proximity of those parts you have an
active source of acidic vapours coming from the wood, the paint or
fabric or a combination of all. You may first think if the ship
model is still correctly exposed under the latest knowledge in this
field. Then, you have two options.

Either the powder is coming from a degraded coating, or is in fact
lead corrosion. You should be able to smell a bad odour from those
areas which could help the diagnosis. But remember; lead is toxic
and must be kept out of reach of respiratory and skin contact! Keep
the necessary precautions. Beside the fact of toxicity, brushing as
mentioned will only help to loose the surface and could conduct to
the loss of the wheels and pullies.

My experience is suggesting a local reduction applied by the
following steps:

    *   Local reduction could be done by a 10% solution of Na2CO3aq.
        {+ on a pencil with cotton wool}, {- on the Pb steering
        wheel and pullies}.

    *   Generous rinsing must follow and before drying occurs, the
        following should be done:

        *   Passivation of the surface into PbSO4 is achieved by a
            2% solution of H2SO4aq.

        *   Again a generous rinsing with tap hot water must follow.

        *   Drying by ethanol and hot air before coating, as you
            suggest in your question, a Paraloid B-72.

The application of a generous hot tap water on a 1875 ship model, or
the eventual projections from the reduction itself represent good
reasons not to apply those series of treatments. As, certainly,
dismantling the corroded areas is totally out of question for
obvious historical reasons, the approach of the difficult protocol
process is the fact that strongly suggests the use of a qualified
conservator. Remember finally that the above treatment implies an
elevated RH% surrounding, so beside the toxicity involved, proper
conditioned ventilation's working area must be assured.

David Cottier-Angeli
Jeweller & Metals Conservator
Geneva


                                  ***
                  Conservation DistList Instance 14:34
               Distributed: Wednesday, December 20, 2000
                       Message Id: cdl-14-34-005
                                  ***