Polychromed iron bed

Andreia Cristina Ribeiro <andreia [at] genhex__org> writes

>I am studying a polychromed canopy iron bed from the XVII century, in
>Portugal at IPCR (a Portuguese institute for conservation and
>restoration). This polychromy is probably composed by pigments with
>oils--right now it's being studied in a lab. The conservation
>problem I have is how to fix the polychromy and stabilize the iron
>corrosion which is contaminated with chloride ions. I would like to
>know if someone has any suggestion to solve this question. The
>biggest problem is that the bed can't be immersed in a solution of
>inhibitor because it would affect the color of the pigments. The
>other problem is to fix the polychromy which one is cracking and
>detaching.

If the iron corrosion tested positive for chlorides, it makes you
wonder where they came from? Was the bed buried in the ground for
some time, or are the chlorides coming from the paint layer? And are
the chlorides everywhere or just on one spot?

If the chlorides are everywhere and the object can be compared with
an archeological object, probably the best option is to keep the
object within very low relative humidity ( <30% ); all the treatment
to loose the chlorides will damage the object (especially the
paint).

Maybe you can try and test if the chlorides are indeed omnipresent
by putting a small fragment (if possible!) from the object in a high
relative humidity environment and see if the corrosion really starts
to bloom.

When there is no real aggressive bloom of the corrosion by high
relative humidity, and the corrosion is more behaving like 'normal'
corrosion on iron (put a blanco in), then you can just keep it in
normal low relative humidity for metals ( <40% ), and it will not be
necessary to inhibit the (not too much present)chlorides.

A possible treatment then can maybe be based on the following
research, done by a colleague of mine, Rebecca Timmermans, modern
objects conservator at the Stedelijk Museum Amsterdam, (together
with Bart Ankersmit, researcher at the Institute for Cultural
Heritage Netherlands). Her research was published recently in the
ICC-publication of the Conference in Bilbao (2004), with the title:
"Conservation of a work by Soto: treatment of iron corrosion on
paint" The work presented in this paper describes the research for
the conservation of the object 'Espace Virtuels: Jaune et Blanc' by
J.R. Soto. White en yellow paint layers on iron were damaged by
corrosion. It was decided to focus on a treatment that would leave
the historic paint intact. Aged painted iron plates were chemically
treated with different chelating reagents such as
ethylenediaminetetra-acetic acid (EDTA),
diethylenetriaminepenta-acetic acid (DTPA), diammonium citrate
(DAC), triammonium citrate (TAC) and phytic acid (PA). It was found
that extracting ions from the binding media produced considerable
paint disruption. However, blocking an excess of iron ions inside
the polymer by phytate did not disrupt the paint, and the
transformation of the brown spots into a white iron-phytate complex
makes them less visible in the white and yellow paint matrix.

You could try and see if the phytate works in your case. The
white/grey color of the newly formed complex could be more agreeable
then the brown color of the corrosion. This product can probably
only be used when the iron corrosion has only colored the paint, not
when there are large voluminous corrosion pustules under loose paint
layers. These pustules often need to be removed, they disintegrate
easily by touch, and are disfiguring. The pustules you can
mechanically remove, and the remaining paint layers and the iron
surface from under the cleaned pustules can for example be
consolidated with 5% Paraloid B72 solution. The areas where the
pustules are removed can be retouched. The consolidating method also
coats the iron, and so works also a bit as a barrier against the
forming of new corrosion.

Joosje van Bennekom
Metal Conservator, Sculpture Department
Stedelijk Museum Amsterdam


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                  Conservation DistList Instance 18:28
                 Distributed: Tuesday, January 4, 2005
                       Message Id: cdl-18-28-001
                                  ***