Sometimes a postage stamp affixed to a cover doesn't appear to be worth very much.
If it is taken off the cover and the water mark is examined it may turn out to be more valuable.
My question is:
a. is it acceptable to take a stamp off the cover?
b. can the stamp be re-affixed after examination (if so what adhesive should one use)?

I have recently taken stamps off of covers, ensuring that I made good quality scans of the cover/s with the affixed stamp/s beforehand.

Hello J Bodan
I personally consider it unacceptable to remove any collectable stamp from its original cover. I assume that we are talking about stamps of some value or of historical interest.
I think that specialists and discerning collectors would frown upon this as it would mean that the stamp would no longer have its original gum and would have to be re attached
exactly as it was so as not to arouse any form of suspicion.
I do appreciate that in the case of a forensic examination of an expensive stamp when
the watermark type has to be determined this would be a different situation. I know that
when examining old master oil paintings they can see underlying paint layers using modern technology, so maybe there is no reason to remove the stamp even for this.
I would attempt to determine the watermark while still on its cover and only remove it as
a last resort. Some watermark detectors are reputed to be able to work on stamps still on their covers , i have never tried this maybe other members can help you on this.

Good morning JB,
When it comes to stamps on cover that possibly has some historical interest I would advise caution ! Especially a cover with various postal markings on it, by removing the stamp for inspection you will make the cover almost worthless, however, if it is just a common stamp with no particular markings the temptation is obvious 'your choice'.

I recently found a Queensland 1d cancelled in the 1900's, but the cancellation was of Marseille in France, unfortunately in the past it had been removed from its cover, although unique, how much better would it have been for postal historians to have discovered it on its original cover ?

I think enough said ! WM.

Gentlemen,
thank you for your advise.