X-ray radiography

The question of what kind of x-ray machine to get for general museum
use is rather well answered by Tom Dixon.  The Andrex 10-190 kV unit
has the range necessary for everything from paintings up to light
metals.  Heavier metals may be a problem.  Normally for bronzes we
use 220-320 kV and in some cases Ir-192 isotopes, where the bronzes
are thick and we want to reduce the contrast between thick and thin
sections.  You have to balance the work you plan to do against the
cost of the x-ray machine and the shielding involved. I should also
point out that I have only used Ir-192 when working with certified
industrial radiologists--I have no desire to handle radioisotopes
myself.

The Andrex may be a little high on the low end for paper, and I
don't know what kind of window material it has--beryllium windows
are best.

You also want a fine focal spot--it determines the quality of your
final radiographs.  Ideal size is 1 mm or smaller.

In terms of films and processing, I believe that industrial
radiographers usually go for higher density and view films with
brighter or special high-intensity light boxes.  You see more and
clearer detail.  These recommendations were brought up in a training
class on radiography taught at the National Gallery, Washington,
some years ago, and may be repeated in the Kodak Industrial
Radiography handbook.  I think that this means that mammography film
may not give the requisite density.  We have been using Kodak AA, M
(and sometimes SR) and Agfa Structurix D2, D4, and D5 films, either
hand processed or done in a Kodak X-omat.

If I were setting up an x-ray facility, I would strongly consider
setting it up with a fluorescent screen and a TV or digital camera
that could be run from outside the shielding, so that you could see
a reasonable representation of the radiograph before you make the
final shot.  The camera could be shielded, off to the side, and view
the fluorescent screen with a mirror.  It would save time and film,
as you could be sure of the correct view before you shoot it.

One last remark--try out any machine you are thinking about buying
with samples that represent the kind of thing you want to
radiograph.  It's worth some traveling to do this--you'll learn a
lot and probably end up much more satisfied with your final
purchase.




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                  Conservation DistList Instance 15:6
                  Distributed: Tuesday, June 26, 2001
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