While ink can go bad, it is quite unusual. In principle, you could put some distilled water back into your ink bottle, but rather than risk it, you could simply throw it away. Some inks are prone to the growth of mold or slime. The fountain pen community has even coined an abbreviation for it: SitB. This means that you should never store a pen in its ink bottle. In general, though, if you follow the instructions that come with your pen, it should last forever. If you forget to take it out of its bottle, then its ink will begin to dry out and become brittle. That's why most fine writing instruments have some sort of mechanism for removing the cap from the top of the bottle. It also doesn't help that many people who sell pens on eBay or at craft fairs will just dump any old ink into a bottle and call it day. These pens were usually not meant to be used with other pens so they do not share a cartridge. They use black ink exclusively. Thus, they require constant refilling. If you want to use old ink bottles, there are two things to keep in mind. First, make sure that they are not filled with vinegar or another acid. Second, make sure that you wash out all the residue from the bottle before filling it with new ink.