Digital BW, The Print

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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