I also tried systematicly to find out which exposure time suits best which material. In the follwing sequence of pictures I exposed an iron ring (1mm thick) and a wooden chock (max. height about 1cm) for 5, 10 and 20 minutes. I tried to make the pictures look about as white or black as the real film, and did not artificially increase the contrast.
Even after only 5 minutes both ring and chock are both well visible, whereas after 20 minutes the chock is only barely visible. The ring does not change its shade, which means that it absorbs just about all X-ray, whereas the wood absorbs nearly nothing even at its thickest point (towards the ring).
To give you a better impression of the very high resolution of the original, here are two larger scans of details of the alarm clock and torch.
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