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We have had some 1930's doors dipped in a caustic solution to remove the paint.

Before dipping, we found that a layer of yellow paint contained lead. It tested positive with: https://www.amazon.com/Testing-Results-Seconds-Suitable-Surfaces/dp/B07NBH7KJJ/

(The testing kit requires the swabs to be dipped in vinegar, where they change color from white to yellow. Rubbing for 30 seconds against safe paint results in no change from the yellow color, while lead paint causes the swab to turn red.)

What appears to be the bare wood of the dipped doors now tests positive when we use the paint-testing kit. (We were testing a small patch of unremoved paint, and initially tested the bare wood by accident.)

Not only does the bare wood test positive, but it turns a much brighter purple than the red color-change produced by the original lead paint.

Is the door now impregnated with lead as a result of dipping?

Or is this just a reaction between the caustic substance in which the door was dipped and the testing chemical?

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    $\begingroup$ You're using a sodium rhodizonate kit. The deep violet color likely comes from lead that was absorbed by the wood. There is at least one alternate source of the color you're seeing, which would be a reaction at high pH with the indicator, which would in turn form croconic acid which can form colored complexes. I'd let some more knowledgeable contributors chime in, as I'm not the best source for organic/inorganic chemistry. Also, you might find a definitive answer in the DIY StackExchange site. $\endgroup$
    – Todd Minehardt
    Aug 7, 2021 at 14:46
  • $\begingroup$ @ToddMinehardt Thanks, I've cross-posted to diy.stackexchange.com/questions/231577/… $\endgroup$
    – fadedbee
    Aug 8, 2021 at 4:07
  • $\begingroup$ @fadedbee have a read: chemistry.meta.stackexchange.com/questions/594/… $\endgroup$
    – Nilay Ghosh
    Aug 8, 2021 at 8:35

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