I have some old Procion MX dyes. They are just short of three years old, but for about one of those years they were stored (by mistake) in an unheated, unairconditioned storage barn. I've tried them on bleached muslin and they seem to be okay, although the colors may be a little less intense than we would have achieved when they were new. Is there anything else I should be looking for regarding washfastness or lightfastness? I would hate to send my grandson a tie-dyed onesie that will bleed forever! Thanks.

The issue in your case is that some, but not all, of the powdered dye has probably hydrolyzed, which means it can no longer react with cellulose. However, it can stain cellulose by its substantivity (the property of loosely associating with the fiber). Fresh dye can also do the same thing.

Substantivity is the property that binds 'direct' dye (such as that found in all-purpose dues such as Rit) to cellulose. It works best if dyed in hot water, with plenty of salt, and afterwards washed only in cold water. By reversing this, and making sure not to use hot water until all salt has been washed out, you can avoid the problem of loosely-associated dye that bleeds forever. Note that salts can come from the reaction of soda ash with acid, or from their use as a diluent in dye powder (usually as sodium sulfate). Sodium chloride is not the only salt that matters, here.

You should be fine if you dye at room temperature (or not hot, anyway), then wash quite thoroughly in cold water (with or without Synthrapol, but certainly without other detergent, which usually contains soda ash) to get rid of all of the soda ash and any salts that may be present, then wash two or three times in HOT water with detergent (preferably Synthrapol). Only the dye that reacted properly with the fiber should remain, when you are finished.

To test your dyed items after completing washing out, dampen them and iron them onto a white cloth. If no dye transfers, all of the unreacted dye is gone. If dye does transfer, do another wash cycle in hot water.

I believe that lightfastness will not be affected by the age of the dye.