“I wanted ask you if the LAST preservative ever gets old and can’t be used? I found a couple of bottles that are still sealed in the package, but they are probably 20 years old or more. Does the preservative ever go bad? –Thanks again!!!” B.
LAST Preservative kept in bottles with finger tight closure, has a shelf life of many decades. One of the reasons that the preservative works as well as it does, is because of the inherent stability of the chemistry. Those old bottles are as effective and safe as when they were brand new.
BTW, you may notice a slightly different odor when using the old material. Some years ago we changed the carrier component to avoid using chlorinated compounds in the formula. The old carrier had a slightly “sweetish” smell.
Best regards, Walter