Wikipedia says that cotton is nearly 100 percent cellulose. Paper made from wood pulp should also be nearly 100 percent cellulose, as the claim is that it has been processed to remove the lignin and acid that causes it to turn yellow. The main difference between the two types of paper is that the cellulose fibres in the rag paper are longer, therefore making the paper stronger, more resistant to water, and a better surface to paint on. (It is not obvious that the cost of making a pure cotton paper explains the significant difference in price to consumers, however.)
The "sizing" of paper also has a bearing on how it works as a painting surface. Sizing is a substance used to control the absorption of a paper: gelatine is frequently used in manufacture. The most expensive papers appear to be both internally and externally sized. There is hard sizing and soft sizing. Hard-sized papers are most resistant to water and are best for some wet-in-wet techniques, whereas a soft-sized paper may not stand up well to repeated working.
Labelling on watercolour papers will give dimensions, weight and surface texture as standard. Most seem to claim 100 percent acid-free. Some will say archival, but what does this actually mean? Details such as composition and type of sizing can be harder to discover, although the degree of cotton fibre content is usually a selling point. And the price compared to other papers is usually a good clue.
The reality is that there is no (international) standard for watercolour paper, so the painter has to trust that the paper is fit for purpose and is what the manufacturer claims it to be. Consumers have to go on trust that the artwork will look as good on it a century (or more) later. How many people have the necessary equipment and knowledge to test that a paper is made of 100 percent cotton fibres and will not lose its original brightness after a hundred years? Do watercolour paper manufacturers actually do this? From a price point of view, I would expect Arches Aquarelle paper to be a better long-term investment than St Cuthbert's Mill Bockingford paper, but do we know if this is true?
I am still learning and experimenting. It would certainly be foolish to use a paper that is not labelled for use with watercolour. When I find a paper that seems to suit the way I (try to) paint, I will probably go with that -- assuming I can afford it in quantity. The chances are that it will be 100 percent cotton, but I am not ruling out something like Bockingford either. When I'm gone, do you think that the people disposing of my effects will care whether the paper is derived from cotton or wood pulp? ;-)
