Like watercolour paints, fountain pen inks fade too. But don’t let this put you off. This is what I do to enhance and preserve my fountain pen ink artworks:
Paper: Use heavy paper stocks of good quality (Bockingford, Arches, Waterford Saunders). The thicker the better. The paper will absorb the inks deeper into the fibres. Thin papers fade faster.
Varnish: When an artwork is finished I spray with a waterbased varnish. Don’t go mad with it! Very gently apply the varnish with the artwork a good 50cms from the can. Allow a small fine spray to settle and let dry. Repeat this process 5-7 times. Build up the varnish slowly and never saturate the art! Otherwise the ink can go back into solution and start to run. Matt and gloss varnishes are available. The big picture shows a test sample created in 2012. The area on the left has been varnished. The area on the right has been left unvarnished and shows signs of fading. (Pic taken today). Look how the varnish preserves that gorgeous gold effect of the bleach!
A good friend of mine who lives in tropical Queensland, uses a 2 pack epoxy varnish on her watercolours, as the heat and humidity of the tropical environment can destroy a decent artwork in a matter of months!
Light: Don’t ever leave artworks in direct sunlight. Hang your artwork sensibly on walls where direct light will never fall on the picture.
I have tried all sorts of things over the years but the the 3 tips mentioned work best and serve me well. Hope this helps.