Hahnemuhle Photo Rag paper

I am involved with a publisher who has printed a job on uncoated
Hahnemuhle Photo Rag paper.  The paper contains Optical Brightening
Agents (OBA).  It is the base paper which may later be coated to
enable digital printing.  In this case the paper is uncoated and has
been printed on a letterpress using a rubber based ink.  During the
sewing of the books, we have noticed a yellowing effect occurring. I
have seen the same effect on digital papers before, but they were
coated papers.  On the Hahnemuhle paper, the yellowing is a pale
acid-yellow.  On the coated digital papers I have seen, the
yellowing has been the same acid-yellow but much stronger.  The
yellowing is observable when looking at the surface of the paper,
but is more pronounced when viewing the paper backlit by natural
light.

The paper has been folded and stacked prior to sewing and then
returned to the stacks when sewn.  The effect was observed when
sewing some of the books.  Not all of the paper is affected.  The
paper which is affected shows the yellowing across the whole sheet
but it is more prominent toward the outer edge.  There is a band,
several centimeters in width, where the yellowing occurs on all four
edges of the stacked pages.  At the very edge of the paper, there is
a band 2-3 millimeters in width where the pages are white and there
is no yellowing.

Through research I have obtained the following information:

    *   The OBA's are causing this effect

    *   The OBA's alter the reflection of ultraviolet light in such
        a way as to make the paper appear cooler white

    *   The yellowing occurs in paper which is stacked or contained
        (apparently it has to do with out-gassing)

    *   exposure to light and air will get rid of the yellowing

What I am looking for is a better understanding of the cause and
possible treatment.  There is a lot of money tied up in the project
so reprinting is the least attractive option.  We would like to
light bleach the pages and then proceed with the binding.  I have
experimented with a UV light (I don't know if it is UVA or UVB) and
it has gotten rid of some of the yellowing.  It is very slow and
possibly I don't have the right UV bulb.  After exposing some of the
pages to natural light the yellow has dimensioned more evenly and
perhaps a bit better than with the UV bulb, but the edition is large
and natural light would be difficult.   I have thought of grow
lights for plants and sun-tanning lights as possibilities.  We can
rig up florescent lights and put the right kind of bulb in, if we
know which one to use.

The last query is whether this will come back after we have bleached
the paper.  Is it a one time phenomenon, or something intrinsic to
the paper which will re-appear?

Peter Geraty
Praxis Bindery
1 Cottage Street, Unit 18
Easthampton, MA 01027-1667
413-527-7275


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                  Conservation DistList Instance 21:4
                   Distributed: Friday, May 11, 2007
                        Message Id: cdl-21-4-022
                                  ***