Toning On French Stamps

I'm relatively new to this hobby but have been collecting mint French stamps over the last few months, particularly from the 1930s to 1950s. I have noticed a much higher incidence of gum toning than with the other countries I collect. (I am not talking here about foxing or rusting, I'm talking about an even tan of greater or lesser degree). I've seen a couple of vague references to this elsewhere so assume I've not just been unlucky. Does anyone know why this is? Is it to do with the paper used by the French Postal authorities at the time? Are there other countries where this is typical?

Hi Sandgrounder, and welcome.

This from the Stamp Collector Blog.......

Thanks, Terence, that's very helpful. Would you (or indeed anyone else) know if you should expect the price of a French or Italian stamp to be marked down for some gum discolouration to the same extent as stamps of countries less prone to toning, or would collectors be more forgiving, recognising that it is considerably harder to get clean white gum from those countries? And does anyone know what would be a reasonable markdown for an otherwise perfect stamp that looks post office fresh from the front but has some 'tanning' (but not foxing) on the back?

Are there not some older Foreign stamps that they recommend storing new mint stamps by removing the gum first?

Some older German stamps e.g. German Ostropa souvenir sheet, Scott B68, and the Hindenburg airmail stamps, Scott C57-58 had sulphuric acid in the gum.