Let Me Tell You About My Sacks

1 comment

Today was mainly spent shopping. I got a late start. I woke up at one and didn’t feel like leaving my hotel room till something like 3pm, laying around in my own filth, watching CNN. I had some trepidation about the upcoming shopping experience, since at the market on Friday when I’d haggled too hard the salesladly had abruptly grabbed me by the shirt and started punching me in the head. “You waste my time!” she shouted, drawing a crowd. “You big waste of time!”

Today’s experience was smoother, though, and in fact so smooth that toward the end of the day so grotesquely laden with goods was I that I had to go luggage shopping just to have a way to carry the bounty back to the hotel. In my defense, I was only buying presents for people I know, nothing for myself. As my friends know, I exist only in the world of ideas and have no material needs.

But at one booth in the massive shopping complex, I saw something so strange and cool I was compelled by an external force to get it. The vendor there was selling plastic sacks filled with a transparent red fluid. I have taken pictures so that you can share in my amazement and wonder:

Swimming in the fluid is a tiny metal disc with four perforations around the center.

The disc is slightly dented, and when you squeeze the dent, the perforations make a snapping sound and the dent pops to the other side of the disc. Then, something magical happens.

The fluid immediately around the disc starts to form into opaque crystals and become hard. And it gets hot.

The heat crystals quickly spread. Like ice-9.

In five or ten seconds, most of the bag is opaque, with the consistency of a cold slurpee. And it’s warm, almost hot.

These things are advertised as heat packs to soothe tense muscles. The flyer explained that to restore the liquid to its original transparent state and prepare the heat pack for another go I need to boil it in water. The flyer says that it works through “frictional reaction” and that it has “some efficacy on invigoating the blood circulation.”

Does anyone know how this works?

Posted on 26 September 2005

1 comment

  1. James Scott-Brown’s avatar

    James Scott-Brown on 31 December 2009 at 6:18 pm

    These products are often marketed as hand-warmers. They are essentially just a supersatured solution of sodium acetate in water. When you click the disc, the sodium acetate crystallises; this is an exothermic process, so gives out heat.

    When you heat the pad in boiling water, the crystals dissolve again, and the pad returns to its previous liquid state. When it has cooled to room temperature, more sodium acetate is dissolved than is soluble at room temperature (the solution is supersatured) – but it cannot recrystallise until triggered by clicking the disc.

    Reply