This can bring out a lot of argufication at times, but the gist of it
a lot of smokeboxes were not lagged (insulated and covered with a thin
jacket). For modelers this had two effects: first, it exposes the
rivets. Second, without insulation the smokebox got hot, and this was
on paint, but the metal still had to be protected against rust. This
by covering it with a heat-resistant paint made of graphite and oil,
aluminum powder was sometimes added. (Google PRR "Front End Paint"
for an example). This paint was also used on other hot areas like
It should probably not be a bright shiny silver; more of a medium
to almost pencil-lead gray.
President, a box of track and some plans
Despite what the model companies produce, it's not actually silver,
it's graphite coated and therefore sort of a dirty silver color.
That front quarter is the smokebox, and the reason a graphite coating
is used is because of the heat present in that area. Normal paint would
burn off in short order.
You'll notice the firebox, at the bottom rear of the boiler, is also
quite often that "dirty silver" graphite color for the same reason.
As for the rest of the boiler area, the outer jacket is protected
enough from the heat by the insulation and the water surrounding the
flues that a "normal" paint can be used.
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