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Re: SFRD

Bruce Smith
 

Tony's comments are spot on. Paint pigments are not UV-stable long term and so the fading seen in steam era freight cars is in large part due to the action of sunlight on the pigments (yes, there is also oxidation of the carriers, which, btw, are not always a "clear matrix", but can in fact contribute their own color to the paint. In addition, pigment molecules exposed to the oxygen in air will oxidize, particularly as any protective coating from the paint begins to break down

Since fading due to UV is due to sunlight, and sunlight is part of the weather, this fading is, imho "weathering". Now, just like the saying "all newts are salamanders, but not all salamanders are newts", fading is weathering, but not all weathering is fading.

Taking into account the fading of paint when considering the overall weathering of a model or prototype care is an important component to understanding why the car appears as it does.

Regards,
Bruce
Bruce Smith
Auburn, AL

From: main@RealSTMFC.groups.io <main@RealSTMFC.groups.io> on behalf of Tony Thompson <tony@...>
Sent: Friday, September 16, 2022 10:52 AM
To: main@realstmfc.groups.io <main@realstmfc.groups.io>
Subject: [EXT] Re: [RealSTMFC] SFRD
 
CAUTION: Email Originated Outside of Auburn.
I think Randy overstated the stability of pigments. The UV in sunlight DOES change pigments.
Tony Thompson 


On Sep 16, 2022, at 8:29 AM, Randy Hees <randyhees@...> wrote:



I think that Nelson has described the difference between weathering and fading pretty well, with one cravat… If “fading” is the loss of color, this really isn’t what happens to the paints used by railroads.  Paint is at its simplest, pigment held in place by a clear matrix.  The pigments don’t loose color, but as the matrix fails, it (the matrix, independent of any pigment) forms a cloudy film, which makes the paint look less colorful, and more grey.  In some cases some of the pigment is released by the matrix.  This is particularly true of white lead paints used for lettering on cars, and that white lead will wash down the side of the car below the lettering.

If you simply wipe weathered paint with a wet cloth you will while it is still wet, the original color.  Historically, passenger cars were regularly varnished, to keep them looking bright, shiny and fresh.

Randy Hees
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