Posted this in another thread but always good to refresh:
Watermarks are special distinguishing marks pressed into the paper before the stamps are printed on them, so they are part of the paper.
They are usually invisible to the untrained naked eye.
They show up by looking at the back of the stamp when the stamp is placed on a black surface or sometimes with a light behind it. Some are easy to see, some very hard, depends on the printers and the paper used.
You may try wetting the stamp with
Water (easiest but do not use on Mint stamps (still with gum on)
Watermark fluid, expensive but safe to use chemical that is OK to use on mint stamps and used, will evaporate quickly (put the cap back on the bottle quick!) and shows and defects or watermarks up very nicely. Used also to detect repairs done to older stamps (1895 and before roughly)
Lighter Fluids -(Ronsonol brand) dangerous to use (don't inhale! or breathe in!), use in a well ventilated (windows open) area). Works like watermark fluid but cheaper and more available. Just a lot more dangerous.
It is best to know what you are looking for and to read the catalogue or online to see a picture of the watermark and understand that some catalogues show the mark as if you were looking at the front of the stamp, which is confusing.
Some watermarks are stars or initials or crowns or multiple copies of these.
Modern (1920 +) usually do not have watermarks in the paper. Sometimes you see them in envelope paper when held up to a light. They are used to prevent forgers copying the stamps and so cause a loses in revenue to the post office. Now other security measures are taken or used.
Be wary that some people use Stanley Gibbons catalogues but others use Scott's catalogues (and still others other catalogues) and the stamps are usually numbered differently and the watermarks numbered differently in each catalogue.
Looking for watermarks is called watermark detection. There are expensive machines now that can help you find them also.
From another post still, continuing on . . .
Usually when the paper has a watermark in it and stamps are printed upon that paper the watermarked paper is fed into the printing press the same way, that is, in the same orientation.
This could result in stamps with watermarks upside down as a usual thing or most of the time, right side up is the usual thing. Depends on that issue and that stamp and that printing of that stamp (some stamps were used a lot and have many printings, some different than others).
Assuming that a stamp on watermarked paper is usually found with the watermark right side up then on the odd occasion, when a sheet of paper was fed into the printing press upside down or sideways then, then the stamp is printed right side up, the watermark looks as if it is in upside sown or sideways or both sometimes.
Stamps were usually printed in the millions. So a run of stamps (say 10,000 of them) that had the paper fed into the press in an unusual orientation, would be the rarity and command the extra premium over and above the regular price of that stamp.
It is the rarity factor that makes one worth more. And that rarity factor is not the same for every stamp or every issue from the same country. It changes.
To know the proper (usual) orientation of a watermark on a stamp the catalogues have assigned numbers (and letters) to each known watermark used by each country. They then, next to each stamp listing in the catalogue, put a notation or note telling you which watermarks (or watermarks) were used. Then, if one denomination of an issue is known to exist with a different orientation of that watermark, the catalogue will note this by making a sub-number for that stamp.
Say the regular stamp is number 333, then an upside down watermark on that stamp would be numbered 333a or 333i or similar, to differentiate it from a regular stamp and to give it an extra line and pricing.
The pricing depends on the rarity of the difference. If only one sheet was fed upside down then wow, very expensive. If half the run was fed that way then not so expensive.
Pricing is assigned similarly due to other varieties like colour shades or perf changes, types of paper used, etc, etc. Different for different countries and different eras also. Thus the use of specialized country catalogues rather than the general world catalogues.
If anyone has anything to add to my bumbling about with explanations of things I have no real experience with then please do jump right in.
A scanner of some sort (flatbed is best) is ideal to examine stamps in detail and to see the little re-entries and flaws and faults. Watermarks can be scanned but show up most mostly only when wet, preferably in watermark fluid in a petri dish or container using a black background. Otherwise very hard to scan.
A link to a page that has information on scanning in general and scanning watermarks (with pictures)
http://www.pgacon.com/tips_on_scann...20WatermarksA link that will search for 'watermark' in titles on Stamp Community:
https://www.stampcommunity.org/sear...sp?mode=DoIt