Hahnemuhle Photo Rag paper

Peter Geraty <pgeraty [at] praxisbindery__com>

>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. ...
>
>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. ...
> ...
>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

The phenomenon you describe, called "dark fading" in the industry,
is well-known.  There is not a good scientific explanation for it in
the ISO's work that I have seen so far--but I have been barred from
receiving ISO minutes since I am a gadfly and not a member of
industry!  So much for international cooperation.  The phenomenon
has to do with confined storage so probably has to do with
off-gassing, as you describe.

There is also no known remedy for the problem, but returning the
stock to light seems to at least mitigate it.  My recommendations to
artists and printers has been, and remains:  Buy only what you need
immediately and do not store the product.  Unfortunately, people who
like to keep a stock of products for use don't like that advice, but
"just in time" inventory is well-used by commercial suppliers.

Mark D. Gottsegen
Chair, ASTM D01.57, Artists' Materials
Materials Research Director, ICA
Co-Director, AMIEN
  336-707-3647
Fax: 336-334-5270


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                  Conservation DistList Instance 21:9
                   Distributed: Friday, June 8, 2007
                        Message Id: cdl-21-9-008
                                  ***