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There's a murder trial ongoing in Ireland at the moment and one of the pieces of evidence is a spade found at the scene along with the skeletal remains. It was contended by the prosecution that this spade was belonging to the defendant but while they were similar

"forensic evidence found the paint splatters on the spade were not the same as on the [defendant's] garden fence"

My question is: can something left out in the open be changed either chemically or structurally by the environment such that it appears to be similar but not the same? If so why would this not have been called out as a caveat by the expert forensics witness?

The specifics of the chemical tests weren't gone into in the media (sample article here) so I don't know how accurate it was.

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    $\begingroup$ Well, you leave iron bars in the open and receive them rusted from mother nature a while after. ;) Paint is also something that could change, but the test on it apparently was real accurate. BTW, welcome to chemistry.SE! $\endgroup$ – M.A.R. ಠ_ಠ Mar 25 '15 at 12:22
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    $\begingroup$ Sunlight is a serious issue with paints due to UV incidence, and careful formulation is required to ensure a long usable lifetime. But who am I kidding, sunny days in the British Isles? Hah! $\endgroup$ – Nicolau Saker Neto Mar 25 '15 at 14:42
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Yes, weather can and probably will change the composition of materials and make them different.

However, forensic analysis usually takes that into account. There are some properties that might change because of weathering (you shouldn't compare water content, for example), but others won't. According to the sample article, the difference was found in the chemical composition of the paints. Since details were not disclosed, one can only guess what these differences are, but some possibilites are:

  • Presence or absence of metals in the paints
  • Substances that wouldn't appear or disappear because of wheatering
  • Different isotopic ratios of elements
  • Traces of the solvent indicating they were not the same
  • Probably others

Assuming the analysis were performed correctly, it is usually not hard to say two samples are different. On the other side, when the results are similar, you can hardly say they are equal, but only that they match and that no differences were found.

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