5×7″ watercolor, ink
Water colors are fragile. Even with high quality pigments and all cotton paper, they require special care: store away from direct sun, florescent lights, or too bright light of any kind. They need to be matted with non-wood pulp, acid free board and kept behind glass, with mats of sufficient thickness to leave air space between the paper and glass. There are other treatments–covering with resin, but I doubt if conservators would recommend them.
But then, we are at the end of human habitation on this planet (or any other), so who needs to consider ‘posterity?’ Let fragility be a virtue. Let our art perish with us–they’ll be no one to enjoy it when we’re gone. Let it feed the rats and roaches that survive us!