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Keep air and bacteria out as much as possible. A jar with a spring-top lid works well, as it is very airtight. Refrigerate this gel for a much longer shelf life. If you make a lot at once, you can store some in the freezer for up to 6 months. Put a few spoonfuls into a small squeeze bottle to use for a couple of days at a time. It's easier to apply to your hair, and you're not introducing bacteria to the main gel with your hands. Plus, if you forget and leave this jar out, you've only wasted a little gel. You should be able to tell easily if the gel isn't good anymore. It will have a bad, off-putting smell that will overpower the essential oils. If it smells rotten, it's time to toss the gel and make more.  If you find you can't use all the gel before it goes bad, try making it in smaller batches.  It can last as long as 2 months.
Pour the mixture into an airtight container and store it in the refrigerator. Portion out a little at a time into a small squeeze bottle. Smell the gel every time you open it to see if it's gone bad.