19th century wooden water pipe rings

Melissa Heaver <mmheaver [at] firemuseummd__org> writes

>My museum is working on an exhibit on 19th century Baltimore wooden
>water pipes ...
>... Both rings
>have some white excretions on them ...
>... We would like to put the rings on display with a piece of wooden
>pipe that is a "repro". However, I am concerned about whether we
>should try and remove the rust (ie. wire brushing it), clean the
>iron ... and then coat the iron
>with something to better preserve it.
>
>We aren't looking for something that involves lots of money and/or
>time, these will just be for display. ...

My first question: are these considered museum artifacts intended
for long term preservation? (I would argue that they are). If they
are considered temporary props for display, then preservation is not
an issue, and treatment is not necessary. If they are museum
artifacts, then preservation needs to be taken into consideration.

My second question: are they actively corroding?

If you are not sure what signs indicate active corrosion, CCI
(Canadian Conservation Institute) has a small but very good
publication showing pictures of different kinds of corrosion, and I
believe the pictures are also on their website.

If the metal is actively corroding, then treatment is necessary. If
it is not, then treatment is aesthetic. However, dirt tends to
promote active corrosion, so a minimal level of cleaning is probably
necessary.

Mechanical cleaning with scalpels, picks, glass bristle brushes,
Dremel tools, and air abrasives (or wire brushes, hammers, chisels,
and sandblasters depending on how much force is
appropriate/necessary) is the best. Do not clean down to bare metal,
stop at the original surface of the object, especially if the object
is more archaeological in the nature of its corrosion. The
extraneous corrosion above the surface tends to be full of dirt and
concreted material. The iron oxides that are part of the original
object are free of concreted dirt and tend to be more uniform.
Developing an eye for where to stop takes a fair bit of experience,
so if this is new to you, consider leaving the stable corrosion in
place. It won't do any harm sitting there.

If the object has heavy active corrosion, then you may want try some
salt removal, although this is usually a lengthy process. It is most
simply done by washing in daily changes of hot distilled or
deionised water, until the salts are washed out (usually takes about
a month of daily changes, but check the conductivity levels, and it
can take significantly longer). Dry the object in air, with heat, or
in solvents (a week under heat lamps does most objects, solvents are
faster but more expensive and more dangerous). Clean off any flash
rusting afterwards. If the object has any paint, original surface
coatings, or composite components then you can't wash it.

Tannic acid treatment may be desirable as well as (or instead of)
washing. Keep in mind that tannic acid will change the surface
appearance from a rust-brown colour to a shiny blue-black colour. If
the iron is not actively corroding then it is probably preferable to
not treat it with tannic acid, just keep an eye on what's happening
with the corrosion.

If the surface is actively corroding, and treatment is necessary,
brush on a 2%-5% solution of tannic acid and keep brushing it in
until dry. You can't treat just part of the object, you have to coat
all the metal surfaces. Many coats of thinner tannic acid, brushed
in, create a better, more adherent coating than a few thick coats
left to dry without being worked in. The more corroded areas will
react with the tannic acid first, so keep applying coats until the
surface is uniform.

Valerie Tomlinson
Conservator
Auckland Museum
The Domain
Private Bag 92018
Auckland 1142
New Zealand
+64 9 306 7068


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                  Conservation DistList Instance 24:8
                   Distributed: Sunday, July 11, 2010
                        Message Id: cdl-24-8-004
                                  ***