What does "expire" mean?
Dictionary definition - "To come to an end; terminate" (source)
In regards to cosmetics?
FDA definition - In summary, they say that there are no actual requirements in the US for cosmetic expiration's, however the concern is bacterial growth. (source)
European Commission definition - They have something called "Period of Time After Opening". You can find this symbol on the packaging of the cosmetic. Here are some on different polishes:
Top Left - China Glaze Crackle Glaze in Lightning Bolt
Top Right - OPI Italian Love Affair
Bottom Left - Orly Royal Navy
Bottom Right - Zoya Veruschka
"The period of time after opening informs consumers of the authorised period of time a product may be used after opening without any harm to the consumer." This became mandatory in March of 2005, so you might have older polishes that do not have a PAO. You can read more about PAOs here.
Well that explains that cosmetics in general expire, but what about nail polish?
I contacted OPI about this because OPI is the most widely known nail polish brand. I got an absolutely AMAZING response:
The Period-After-Opening number is required for most cosmetics by the European Union (EU), but it is rather meaningless for nail lacquer. Nail lacquer does not go "bad" with bacteria after opening (or ever), because the solvents are chemically hostile to microbes. Indeed, research shows clearly that microbes don't survive in nail lacquer, whether in a salon environment or even if deliberately contaminated in a laboratory test.
For most other cosmetic products, such as skin lotions or hand creams, the preservatives eventually get used up, especially with repeated opening and closing , and bacteria can then colonize them and start to grow. So the PAO makes sense for these products. I don't think they had nail polish in mind when they wrote the rule.
So.....Why 24 months PAO for nail lacquer? Because the EU is distrustful of larger numbers. In reality, nail lacquer should stay safe forever. It might not be any good after many years -- due to slow color changes or if it evaporates to a solid, useless block -- but it won't be unsafe. And as you know, the PAO number is about how long, after opening, is the product SAFE ("no harm to the consumer"), not about whether it will work properly!
Is this what you wanted to know?
Paul Bryson, Ph.D.
Director of Research & Development, OPI Products Inc., www.opi.com"
Well that should clear a bunch of stuff up.
But what if the polish does "evaporate to a solid, useless block" (this is my favorite thing he wrote)? Or what if it gets thick and goopey? What should you do? Can it be saved?
Absolutely it can be saved!! Polish starts to harden or become thicker because of volatile ingredients in the polish that may eventually evaporate. All you have to do is replace to ingredients.
What should you use?
Definitely NOT nail polish remover!! Remover is too harsh to add directly to your polish. It may last a little while, but you're potentially doing significant, irreversible damage to your polish. Don't believe me? I highly suggest reading this blog post by Lacquer Laine. I'm not going to experiment on my own polish with remover, but she, unfortunately, did.
What you actually want to use is nail polish THINNER!! Yes. Thinner. They sell nail polish thinner specifically for this purpose. It's super cheap, lasts an awesome amount of time. I still haven't come close to going through an entire bottle. I bought mine at Sally's and it's by Beauty Secrets. You can buy it online here, or find it in the store.
Here's a post that shows how amazing thinner really is.
What your polish is separated?
Just shake it. It will be fine, it hasn't "gone bad". And if it's thick, add thinner. And yes, it is absolutely FINE to shake your polish. Yes, it creates bubbles, but those bubbles will go away. If you plan on applying polish immediately, then just roll it between your hands to avoid the bubbles. But for the purpose of adding thinner, shake the crap outta the polish.
So there you have it. No more throwing out old polishes. No more ruining polishes by adding remover. No more rebuying of your favorite polish because it's been 36 months since you opened it. And if you still don't believe me, I would be glad to take that "expired" polish off your hands. ;)