Help needed with sodium acetate trihydrate (hot ice)

Aug 2015
Hello chemistry people of the internet!

I am in need of some advice. I am an artist working on a sculpture installation where as part of the work, I am using the chemical sodium acetate trihydrate C2H9NaO5 (SAT from here on), commonly known as "hot ice". Please forgive that I am not very well versed in chemistry, so I might use the wrong terms for things, but I am eager to learn and would really appreciate any help.

I am making large PVC bags of SAT, and I need it to stay supercooled in liquid form for several weeks. In my smaller scale (about 2 litres) tests I have been able to make this work no problem, but moving up to larger scale (about 20 litres), I have had trouble with getting the SAT to stay in clear liquid form. Even when I did get it to stay as clear liquid after melting down the SAT, filling the bag with it and cooling it down, after about three days it started forming white crystal formations inside the liquid. It still hasn't reacted and turned solid after more than a week, and actually seems surprisingly stable in this state (hasn't reacted even when the bag was moved), but still what I am aiming for is a clear bag of liquid.

So there are a few things I have suspected of having an impact on this, but I'd really appreciate your input if you have any more ideas or can expand on mine:

- Adding more water (as long as there is enough) does not solve the problem – it doesn't make it any less prone to crystallising, it just makes the SAT be softer when it is in its solid form.

- However, how the bag of SAT is cooled down has a clear effect on it staying liquid or not. As the several instructions for making "hot ice" I've read suggest, it should be cooled down faster than in just room temperature. I have filled the bags with the SAT when it still has been hot after melting it down, since it is virtually impossible to fill the bags with room-temperature SAT without it reacting and turning hot and solid. After filling the bag with hot SAT and sealing the bag shut, I have either put the bag in a cold water bath or taken it outside to cool in the nighttime when it is colder. Now if I, for example leave it outside to cool for too long, it seems that the corners of the bag cool down too fast compared to the centre mass of the bag, and it starts forming white crystals at the corners. If, however I just leave it in room temperature to cool, it just turns solid as it cools slowly. So it's clear I do need to cool it down faster, but somehow I should manage to make it cool down in a uniform manner. Any tips on this? Is it better the faster I can get it to cool, or is there a sweet spot where it happens fast enough but not too fast?

Here's the material I am using:
I am also filtering it when melting it down, as I noticed it does have some visible impurities in it.

- Now another thing I have considered could be the issue is the type of plastic. I made most of the smaller scale tests using large "Minigrip" ziploc bags, and with those it worked well. Those are made of PE polyethylene plastic. However, the problem with PE plastic for my use is that in the thickness I've been able to buy it in my area, it is too weak – especially at the seams when I heat seal them. In general it tends to not take well to heat, and I do need to fill the bags when the SAT is hot, and in large quantities/weight, the test bag I made with PE plastic wouldn't hold it, but broke at the seams. So I moved to PVC plastic, with which I have been able to make very strong seams, and which can sustain high temperatures without blistering or breaking. But so, I have been able to fill PVC bags of SAT and have it stay clear liquid for several days, but in this large quantity of 20 litres, so far it always starts to crystallize at some point, even if it doesn't actually turn solid.

Any ideas on what I could still try, or how I could improve my process? I am not working in laboratory conditions, but have tried to make my working process and environment as clean as possible. If you have any questions, please ask.

I would very much appreciate any notes or suggestions. The exhibition is coming up soon and I'm getting a bit anxious thinking if I should somehow change my plans, but I'd really prefer not to.
Thanks in advance!