Thank You
My understanding is that the smokebox was not insulated like the boiler, so paint would burn off. Instead they used a graphite based coating. Lower parts of the fireboxes were also sometimes thus coated, as seems to be the case in your photo.
MidlandMikeLower parts of the fireboxes were also sometimes thus coated, as seems to be the case in your photo.
Keep in mind that most of the 'silvering' on the firebox legs was on the outer wrapper, which does not see heating above saturation temperature for the engine's rated pressure. Paint is not nearly as likely to burn off these as it is from a smokebox. I suspect there are other factors involved in using a graphite composition there, perhaps little more than visual appeal. Many railroads used paint even over flexible staybolt caps on lower fireboxes.
On that pictured Clinchfield locomotive the smokebox certainly would have been the standard graphite n' linseed oil finish.
Toward the end of the steam era heat resistant paints had been developed, so the Lackawanna used a silver paint on their smokeboxes, which weathered to a grey look anyway. The Magma Arizona used a very classy copper pigmented paint on their steamers which bore up very well. Some 'roads used black right up to the smokebox.
Yes, what I've always heard / read was that paint wouldn't hold on the hot parts of the engine, so the smokebox and firebox (as shown on the model picture in the OP) would be treated with a mix of graphite and oil instead.
Note that some railroads extended the boiler jacketing all the way to the front of the boiler. Often that's why the entire boiler is black.
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