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. ...

Conserving and preserving cast-iron objects which will be kept in
such harsh environments needs a different approach than when that
object is kept in a clean, dry and warm environment. You must weigh
up the various treatment methods in perhaps a slightly different
light to a situation where they may be kept in more favourable
conditions. There are very few treatments that are relatively quick
and provide the best possible surface treatment to prevent further
damage and corrosion.

The most important consideration is the surface treatment that the
guns will be given after treatment and of course the selection of
that treatment is dependent on the conditions in which the guns will
be kept, stored and displayed. From what you describe I would
suggest that you must look very carefully at the very best practice
in industry to protect iron structures. From my experience this
involves the use of a very high zinc content paint next to the iron
itself, followed by an epoxy sealing layer followed by a top sealing
coat of the finish you desire. If you want a surface layer that will
provide the best protection then this is the way to go. The high
content zinc layer gives protection even when the layer is damaged
and prevents further corrosion until it can be repaired. The epoxy
layer is very hard and will provide good overall protection while
the top coat can be removed and/or re-applied to cover graffiti and
other disfiguring marks.

Now for the down side. In industry this type of paint finish is
applied over a very heavily air braded, that is grit blasted,
surface to provide the necessary key for the first layer and this
heavy air abrasion can be very damaging to the surface. However this
is not absolutely necessary and the way round this is to either do
the grit blasting yourself or to have it done under very close
supervision by trained staff so that you can carefully regulate the
removal of old paint, corrosion etc and do as little damage as
possible to the surface.

Now this sounds rather drastic but it must be weighed against the
other options. Carefully and skilfully carried out, grit blasting
can be very fast, it can be controlled by the operator, given proper
training and guidance, and still provide a suitable surface for the
paint layers. I have had this done to many cannon over the years and
the results are very good indeed--often original tool and casting
marks are revealed and preserved for example. Careful grit blasting
has proved to be a good treatment--provided it is done well--and the
paint layers have stood the test of time--some of the cannon have
shown no further corrosion after 20 years.

Finally there is the problem of reversibility especially for the
epoxy layer. This of course can only be removed by grit blasting
again but again this can be done by a professional and do the least
possible damage.

So I would advise seeking advice and information from industry--ask
how they protect oil rigs, bridges and the like. I would investigate
very high quality paint systems--those used in industry. I would
strongly recommend looking at a system like that outlined above and
carefully weigh up the various options.

Robert Smith


                                  ***
                  Conservation DistList Instance 21:7
                   Distributed: Monday, May 28, 2007
                        Message Id: cdl-21-7-008
                                  ***