Iron gall ink with white residue

I am preparing to treat a collection of mid-18th century indentures
in iron gall ink on paper, but some of the documents have a strange
white residue that my colleagues and I have never seen before.  It's
almost clay-like, like a bole or ground, and it is consistent
through the whole document, appearing around the perimeter of each
letter.  Only a few areas of writing in different hands (signatures,
dates) lack the residue, as they seem to be written in a different
batch of ink.  The residue appears to absorb UV light the same as
the iron gall ink.  Under magnification, sometimes striations are
visible.  It doesn't seem to be crystals or salts, and it's not
oily, waxy, powdery, chalky or any more friable than the degraded
ink itself.  It hasn't noticeably affected the way the ink has
degraded the paper, for better or for worse.  Does anyone have a
suggestion as to what it might be? Images can be seen at these
links, and more are available on request.

    <URL:http://farm4.static.flickr.com/
        3577/3546050189_59d2fa0bb2_b.jpg>

    <URL:http://farm4.static.flickr.com/
        3342/3546032311_f112fd3f3a_b.jpg>

    <URL:http://farm4.static.flickr.com/
        3330/3546839110_85a329042e.jpg>

    <URL:http://farm4.static.flickr.com/
        3330/3546857244_945abb90bf.jpg>

    **** Moderator's comments: The above URLs have been wrapped for
    email. There should be no newlines.

Grace White
Associate Paper Conservator
Etherington Conservation Services
6204 Corporate Park Drive
Browns Summit, NC 27214
USA


                                  ***
                  Conservation DistList Instance 23:2
                  Distributed: Thursday, May 28, 2009
                        Message Id: cdl-23-2-023
                                  ***