Digital BW, The Print

Moisture content in the paper may be playing a significant role. Basic open air/natural light exposures generally allow the paper to desiccate during the light cycle.  Longer term tests with more controlled RH (like picture frame environments not exposed to direct sunlight) generally keep the moisture content in the paper at a higher level. The wooden beam in your environment could have easily created a microclimate effect for the paper area pressed against it (even loosely).

Of course, there may be batch-to-batch differences as well. The manufacturers try to hold fairly tight tolerances, but we can measure even tighter with our spectrophotometers.

cheers,
Mark
http://www.aardenburg-imaging.com

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Paul" <roark.paul@...> wrote:
>
> In my fade tests as well as the more sophisticated ones Mark does at
> http://www.aardenburg-imaging.com/ <http://www.aardenburg-imaging.com/> 
> it is clear that natural (no optical brightening agents) papers bleach
> with extended light exposure.  This is not a major problem, but I was
> surprised at how much one of my favorite papers changed.  Looking at the
> Lab B, the main characteristic that changes, the Epson HP Natural paper
> white changed from a Lab B = 3.3 to 1.6 in 10 MLux Hrs.  This is a
> change of 1.7 Lab B units.  After 10 MLux Hrs, the Lab B was stable.
> The initial warmth of the Epson HP surprised me as did the extent of the
> change.  So, I decided to test samples I had.
> So, taking 10 measures of the paper before light exposure, and keeping
> one piece in the box as a control, I put a sheet of the paper outside,
> facing south.  Since my spectro reads slightly differently after each
> calibration, the change in the Lab B from the control seemed like the
> most accurate way to see what would happen.  These are the results:
> After 4 hours the Lab B changed 0.63 units (average of 10 reads) from
> 1.89 to 1.26.
> After 9 hours the Lab B changed (from the original control) 0.86 units. 
> (2.1 to 1.237)
> After 15 hours the Lab B was still 0.86 units lower than the control.
> So, it looks like, from this test, if one has a full 8 hour exposure to
> sun, the paper is bleached and probably stable.  This sounds reasonable
> and is within what I would have expected.
> I don't know why the paper I have started cooler and changed less that
> the samples Mark has tested.
> Out of curiosity, I measure the back of the paper to see what happened. 
> It turns out it had bleached 0.64 Lab B units.  OK, so either the paper
> is rather transparent to the light and/or the reflecting light probably
> bleached the back.
> Now for the weird result.  The paper had been pinned to a beam that had
> old white paint on it.  I had measured the area below the beam.  When I
> measured the back of the paper that was against the beam, it had
> bleached 1.18 Lab B units!  More than the apparently stable point of the
> rest of the paper.  What could cause this?  Perhaps the light hit the
> off-white beam and came back through the paper, causing more exposure. 
> Perhaps the beam did not allow the paper to dissipate as much heat as
> the rest of the paper, indicating that heat may be one of the factors
> including is what the final stable tone of the paper is.   Any guesses?
> Paulwww.PaulRoark.com
> 
> 
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