Producing a good x-ray relies on getting the x-rays to just pass through the material. You want them to pass through the slightly less dense areas and not to pass through the denser areas in order to pick up internal structure.
The x-rays generated by the machine can produce lower energy secondary x-rays when they hit other things (like the bottom of the x-ray machine). These low energy x-rays can affect the x-ray plate and fog the image slightly. You can put different filters in, in different places to remove secondary x-rays but you have to judge it so that you get the best results to suit the material.
 
Lynn  has doing a lot of ork on various materials with various filter locations.  You can view what she uncovered by reading the full article here:

http://www.wessexarch.co.uk/blogs/news/2014/11/10/x-ray-machine-low-and-high

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