We replaced an old fence and can't tell if this wood has been treated or not. Pressure washed a couple of pieces to get the old grime off. https://imgur.com/a/SYsmZ

  • Where (country, state, etc) have you found this old fence? – Maxime Morin Oct 21 '17 at 23:01
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    You can't tell visually if wood has been treated because some preservatives aren't coloured, and over a long timeframe a coloured preservative may wash out or weather away from the surface of boards. But your fence boards are a softwood species and if the fence is quite old then you can virtually guarantee it has been treated (in some way) because otherwise it would have begun to rot by now. – Graphus Oct 22 '17 at 7:07
  • Have to agree. It's very likely to have been treated, or it would be significantly more degraded than it is. Also anything that is in contact with the ground basically has to be treated, even hardwoods. There may be some species that are durable enough not to need it but they're not the kind of thing your fence is going to be made of. – WhatEvil Oct 22 '17 at 22:43
  • Welcome to the Woodworking Stack Exchange! Unfortunately, wood identification questions are off-topic for this site, but please feel welcome to post other woodworking questions. See woodworking.stackexchange.com/help/on-topic for more info. – rob Nov 2 '17 at 4:39

Other than chemical analysis, or direct knowledge of someone associated with installing the fence or otherwise in the know, there can be no definitive answer to your question as stated in combination with the picture.

However, there are ways to make an educated guess and this picture gives only a small slice of information. Possibly the most useful datum that is needed to adequately answer this question is the age of the fence. That coupled with the species and location would be sufficient to triangulate a reasonably accurate, educated guess.

When in doubt, and the situation indicates the possibility i.e. fencing, assume it is treated. So if you wanted to know whether it would be safe to burn, which is likely all it is good for at this point, and unless you have some of the aforementioned data points, eschew incineration.

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