People are always asking me about freezing henna and making it last longer but I don't ever freeze it.  I do a lot of henna work and so I make it and use it.  I interviewed my friend Antoinette because she freezes it often and this is how she does it.  Thanks Antoinette!

How have you mixed henna when you freeze it?
I mix the henna the same way I mix it any time...I usually make a large batch for whatever event I have scheduled, and then freeze whatever left over I have.  Then I can re-use the smaller amounts for private appointments, or small parties.  If I were to freeze it without using it first, I would make sure the dye had already released, and the terps had been added.  (So, mix it up, let it sit at least 24 hours depending on the henna, and THEN freeze it.) 

How much and how little have you froze at a time?
I've frozen a medium bowlful, a single cone, and everything between.  Probably max 200gm so far.

What's the longest you froze it for?
I'm going to guess it was in there for 4 months or so.
 
How long does it take to thaw?
Surprisingly a lot less time then you'd expect!  A cone left on the counter will thaw in less then an hour...stick it in your pocket or bra and it's ready to use in 1/2 hour.  It's usually better to let it warm up to at least room temp before using though.

Do you notice any consistency change with henna that has been previously frozen?
Nope...the henna acts exactly the same after freezing. It's like you've put it in stasis in the freezer.

Any staining issues with henna that has been previously frozen?
None that I've noticed, although I haven't left it in the freezer for more then a few months.  I've heard, however, that eventually (perhaps around a year) it will "go bad" but I haven't tried it yet.

Anything else you want to tell me?
Make sure you either push all the air out of the cone, or if freezing in a bowl or tupperware, "seal" it with plastic wrap skin (again, pushing all the air out).  You don't want water crystals to form, nor for the paste to dry out on the edges of your container.

I've frozen henna with and without terps.  I henna alzheimers patients at an assisted living home, so for them I use henna without any oils, in deference to their often sensitive skin.  I make up a small yoghurt container of henna once every 3 months or so for them, and the day before my monthly visit, just scoop out a spoonful for the cone, and then stick it back into the freezer.  It still works just as well the last time I use it as the first time.  If any of the caregivers want a henna, I just brush the terp directly onto their skin prior to application.  The un-terped henna works really well for the elders, sometimes they still have a little bit of the previous design when I return one month later!  It's all to do with the slowness of rejuvenation as we age.

Here's a good web page: http://www.hennapage.com/henna/how/keepit.html