Technique For Lifting Stamps To Check Watermark

Don't worry, I have no plans to do this to any of my covers, so no yelling.

I am just curious: When a cover is sent in to be expertized, and the stamp needs to be lifted to check for watermark, is there a particular technique or chemical that is used? I feel like water would universally damage or stain the cover.

My approach is to submit the cover to a paper conservator to have it professionally "lifted." Unfortunately the technique they use is proprietary. But I have examples in which the stamp is off and the cover is completely intact, sans the stamp, of course. I can show a picture, if you like.

However, it may be possible to lightly brush water onto the surface of the stamp repeatedly until it is completely saturated and then the stamp can be removed. When the stamp is off, the residual gum on the envelope is also alternately moistened and blotted until the area is free of residue. Once free of the cover the stamp may then be soaked as usual. Both stamp and cover are blotted and pressed under pressure until dry.

This is painstaking work and definitely not for the impatient.