{"id":1443,"date":"2020-01-08T14:19:43","date_gmt":"2020-01-08T14:19:43","guid":{"rendered":"http:\/\/blogs.cardiff.ac.uk\/conservation\/?p=1443"},"modified":"2020-01-08T14:33:31","modified_gmt":"2020-01-08T14:33:31","slug":"mask-on-mask-off-masking-films-from-x-radiation","status":"publish","type":"post","link":"https:\/\/blogs.cardiff.ac.uk\/conservation\/mask-on-mask-off-masking-films-from-x-radiation\/","title":{"rendered":"Mask on, Mask off: Masking Films from X-radiation"},"content":{"rendered":"\n
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Photo by author<\/figcaption><\/figure><\/div>\n\n\n\n

So, I spent an\nhour and forty minutes X-raying my objects and developing the films. When they\nwere finally dry, I laid them on the lightbox to see them for the first time; I\nhad never X-rayed or developed films before so at first glance I was quite\npleased with the results until\u2026 \u201cwait, what is that empty space in the corner\nof my film?!\u201d *sigh* Just as I thought I had completed the process without any\nblunders, I had clearly made a mistake. Just to be sure though, I counted the\nobjects and sure enough, one of them definitely has NOT shown up. I referred\nback to the sheet of paper that I had used to trace my objects on before they\nwent into the X-ray machine, and there it was! I HAD traced around the object! So\nthen why wasn\u2019t it showing up? <\/p>\n\n\n\n

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X-\nRadiography- a quick overview<\/strong><\/h5>\n\n\n\n

X-rays are a\ntype of radiation; they are invisible, they penetrate matter, and like light,\nthey darken photographic film. Simply put, the object is placed on top of the\nfilm and the X-rays are fired at it; if the object is thick or high in density,\nit absorbs the rays and doesn\u2019t let them hit the film and so it shows up white\non the X-rays. If you\u2019ve ever had an X-ray at the hospital, this is the same as\nyour bone showing up white on the film; your bone is dense, so it appears\nwhite. If the object is thin or low in density, then the X-rays can pass\nthrough it and cause a chemical reaction in the film behind it which darkens it\nto the shape of the object. <\/p>\n\n\n\n

I won\u2019t be\ngoing into detail on the X-radiography and developing processes, but if you\nwant to read more about this, check out Stephanie Whitehead\u2019s blog at: https:\/\/blogs.cardiff.ac.uk\/conservation\/2016\/11\/23\/looking-beneath-the-layers\/<\/a>.<\/p>\n\n\n\n

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Back to the missing\nobject!<\/strong><\/h5>\n\n\n\n

For a bit of an\nintroduction, the objects that I had been given to treat were a group of\nMeroitic copper model tools from the Egypt Centre in Swansea. My \u201cmissing\nobject\u201d was sent to us in this group; unlike the others though, it is a thin\nsheet of metal. After realising that this was the missing object, I took a\ncloser look at it and concluded that this was a result of overexposure. <\/p>\n\n\n\n

All those dials\nand buttons on the X-ray machine can seem scary at first, but we only have to\nworry about the kilovoltage (kV) and time. I had done two X-rays of my objects,\none at 90 kV and the other at 100 kV, both set for 3 minutes. This kV was\nsuitable for the thick model tools, which absorbed some of the X-rays, but for\nthe fine sheet of metal it was far too high, the X-rays had passed straight\nthrough it and darkened the film, thus overexposing it.<\/p>\n\n\n\n

Although I was\ninitially bothered by the situation, I started to see it as an opportunity to\nlearn about \u201cmasking off\u201d (more on that in a moment). I was now curious about\nthis object and wanted to know exactly what kV was needed for it to show up. I\ndecided to take multiple exposures of the object at 30, 40, 50, 60, 70, and 80\nkV\u2019s, each for 1 and a half minutes. My object was so small, it would have been\nwasteful to use one film per exposure and so I used sheets of lead to mask off areas\nof film that I didn\u2019t want exposed, meaning that I could have all 6 exposures\non a single radiographic film, AKA, \u201cmasking\u201d.<\/p>\n\n\n\n

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Masking off<\/strong><\/h5>\n\n\n\n

\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\nAs the X-ray machine\nwas warming up, I went to the dark room to place the film into the cassette. We\nhave a choice of hard box cassettes or soft plastic ones; I prefer the soft ones\nas once the film is inside you can vaguely feel where the film lies inside it.\nThis is helpful to know because you don\u2019t want to place your objects past the\nedge of the film where they will be cut off in the image. After closing the\ncassette, I took it back up to the X-ray room and started with the logging\nprocess which entailed:\n\n\n\n<\/p>\n\n\n\n