Iron cannons

Noora Hirvonen <noora.hirvonen [at] kolumbus__fi> writes

>I am seeking information to help me to determine the right
>conservation treatments for over hundred iron cannons that are
>located outdoors around Suomenlinna fortress
>
>    <URL:http://en.wikipedia.org/wiki/Suomenlinna>
>
>Most of them are from 18th century but there are also cannons from
>19th and 20th centuries. Some of them have been painted, but there
>is no record left how it has been done.
>
>The treatments should be planned taking into consideration the
>northern climate by the Baltic Sea. We have four seasons with sun,
>rain, wind, snow and wet snow. Temperature varies from up to +30
>deg. in summer to occasional low of -30 deg. in winter. The cannons
>can also be freely accessed by the visitors and many climb over them
>and unfortunately, they are also damaged on purpose. Also we don't
>have a laboratory at the Suomenlinna and therefore I do not have any
>equipment and chemicals at the moment.
>
>The two major problems are vandalism (graffiti made by spray can
>paint and scratches on the surface) and active corrosion. ...

In my experience, you have to start thinking of these cannon as a
special type of metal sculpture, and treat them accordingly.  Of
course, in the good old days when they were in active use, there
were squads of artillerists and enthusiastic staff sergeants keeping
them maintained.

Now, we have to come up with some form of system that affords
reasonable protection with low-cost long-term maintenance.

But before you go applying any protective systems, you have to
identify the base causes of your problems:

    1.  Active corrosion:  is this due just to weather and
        freeze-thaw cycles, or is there chloride or other salt
        contamination from the environment?  This can be tested by
        scrapings or samples of the corrosion products.  If you have
        salt-driven corrosion, this will have to be addressed before
        you can do anything else.  Unfortunately, there is still no
        good alternative to removing salt contamination than
        alkaline soaking, or electrolytic techniques.  Putting a
        coating over an untreated surface may be asking for trouble,
        see Pearson, 1987, Conservation of Marine Archaeological
        Artifacts, pp 219-221.

    2.  Weather protection:  If the corrosion is simply due to
        weather and the environment, then you should start thinking
        about coating systems. I would guess that most iron cannon
        on display around the world have some sort of paint coating
        on them.  Here your choices are huge, but most boil down to
        an anti-corrosive primer coat, with a pigmented and weather
        proofed surface coat.  The arguments are endless about
        enamels vs epoxy vs whatever.  It is worth consulting with a
        local surface coatings engineer who can recommend something
        appropriate for your local weather conditions, but will also
        meet your esthetic display objectives. We also usually then
        buff a wax layer on top of the paint system, which acts as a
        sacrificial layer to protect the paint from UV and weather.

    3.  Graffiti and vandalism:  the paint coatings will also help
        here, making it easier to remove graffiti (and there are
        some special graffiti removal cleaners on the market now).
        And if the vandals are going to scratch their initials on
        the cannon, better they scratch into the paint, than the
        iron surface.

    4.  Closing openings:  Cannon not in service traditionally had
        "tompions" inserted into the muzzles and touch-hole plugs to
        prevent moisture from getting into the bore and the charge.
        Fitting a modern tompion will also keep trash and loose
        material from getting in and creating an internal
        environment that encourages corrosion.

    5.  Maintenance:  Any protective system is only as good as the
        ongoing maintenance.  The wax layer must be reapplied as it
        weathers off (annually?  More often?  Depends on local
        conditions).  Any vandalism or other surface damage must be
        repaired by proper and prompt re-application of the coatings
        so that you don't get corrosion activating under the
        coatings.  And even the best of systems have a life-span.
        You should expect to have to recoat at some time in the
        future, so build that into your plans.

Howard Wellman
Lead Conservator
Maryland Archaeological Conservation Laboratory
410-586-8577


                                  ***
                  Conservation DistList Instance 21:7
                   Distributed: Monday, May 28, 2007
                        Message Id: cdl-21-7-009
                                  ***