Electron radiography of paper

I recently presented a paper at the Toronto conference 'Looking at
Paper, Evidence and Interpretation' where I discussed the use of
electron radiography for recording watermarks in the prints by James
McNeill Whistler at the Freer Gallery of Art.  Many individuals
spoke to me afterwards asking for the details of this procedure,
which will eventually be published in the conference postprints.  In
response to the many individual requests I would like to post the
following information, excerpted from my paper.

For practical purposes, electron radiography using a high kilo
voltage was the easiest technique to use for this project.  It also
has the advantage of taking a radiograph of the whole sheet of paper
up to the size of the film and the vacuum envelope--14 x 17 inches
(35.5 x 43 cm); and it will not record the media on the surface of
the paper, thus there will be no interference in the image of the
watermark.  With the whole print radiographed you can also see the
orientation and location of the watermark within the print.

In the process of electron radiography, high kilo voltage, filtered
x-rays irradiate a lead foil and produce electrons.  These electrons
pass through a specimen of low atomic weight (such as paper) and are
absorbed differently according to the density or structure of the
paper.  The differential absorption is then recorded on a sheet of
film placed beneath the paper.

For an electron radiograph, the film, paper artifact and lead foil
are placed together into a vacuum envelope to promote good contact
between the components. When I was radiographing the Whistler prints
for this project, to make handling of the prints safer and easier in
the darkroom, I made a Mylar(TM) polyester folder to place around
the prints.  In the darkroom with a safe light I lifted up one side
of the polyester and placed the lead foil in contact with the recto
side of the prints.  Then I flipped the package and lifted the other
side of the polyester to put the film next to the verso of the
prints.  With the prints aligned and enclosed in the polyester
sleeve I could safely slip the whole package into the vacuum
envelope.  I had originally hoped that I could put the lead foil and
the film outside of the polyester sleeve, but experimentation with
the x-ray technique proved that the print had to be in direct
contact with both the lead foil and the film in order to get an
image.

I made all of the exposures using a 50-inch focal distance, 250 kV,
10 mA, wide focus, for five minutes, while a vacuum was pulled on
the envelope package. The following guidelines are recommended in
order to record watermarks and structure of paper: The x-rays used
should be at least 250 kV, with a filter at the tube port of about
6-8 mm. Copper is recommended for the tube filter, but for this
project a 6 mm lead filter was used with good results. The paper to
be radiographed was placed face up on top of the film with the
emulsion side of the film face up (Kodak SR film was used), and a
lead foil (0.005 inches thick) is placed on top of the paper.

With the above procedure I was able to expose and develop forty
watermarks in less than four days.   Because of the ease, quickness
and thoroughness of the electron radiography technique for recording
watermarks, I urge anyone who has access to a high kilo voltage
x-ray machine to use this technique for recording watermarks on
medium to small works on paper which can be safely placed in a
vacuum envelope without damage to the support or media.

Supplies:

    Gemini 320 Industrial X-ray Unit by Diano Corporation, New
        Haven, CT

    Raydex Super Smooth 0.005 inches lead screen, by Roentgen
        Industrial Corp., Highland Park, IL

    E-Z EM's VAC-U-PAK (TM)

    Kodak SR 5 14 x 17 inch film (Industrex(TM) R)

    Kodak GBX developer and fixer

    Glacial acetic acid and water stop bath

    Heico Archival Speed Perma Wash

References:

    Bridgeman, Charles F. 1965
    Radiography of Paper
    Studies in Conservation 10 (1, February): 8-17.

    Radiography in Modern Industry 4th ed. 1980
    Rochester: Eastman Kodak Company

Victoria Bunting
Paper Conservator
Boston, MA

                                  ***
                  Conservation DistList Instance 13:3
                  Distributed: Thursday, June 24, 1999
                        Message Id: cdl-13-3-001
                                  ***