Ask Lemmy

It's doubtless an artifact of history. Rubber water bottles like that go way back. Before the days of electric blankets, space heaters, boiler heating, gas furnaces, etc, heating was often provided by wood- or coal-burning stoves. With a rubber bladder like that, you could boil some water on the stove and take it to bed with you. If all you have is a fire to keep you warm, it's hard to use that fire to directly heat your bed. For someone sleeping in a cold bedroom in an old drafty house, a hot water bottle and a pile of blankets was how you often got through the cold winter nights. And stoneware versions of the same concept go back at least half a millennium.

But ice available in the home? Some homes in the late 19th century and earlier sometimes had ice boxes - literally just insulated boxes that you could put ice in to keep food cold. The ice had to be cut off of frozen lakes in the winter and stored in big insulated ice houses for the rest of the year. But such ice would be too expensive and precious to fill a water bottle with. Maybe someone really wealthy could afford to do that. Maybe you could do it if someone was severely ill and needed a fever cooled. But pre-WW2, even if you had access to ice, it was too precious for most people to be able to justify using it just as a sleep aid.

To make something like this practical, you really need a modern freezer. Even in the days of ice boxes, you wouldn't be able to pull something like this off unless you were willing to use up two liters of expensive bought ice every night. That's just not something most people could afford.

The first domestic freezers as we know them now didn't appear until the 1940s. And it took decades for them to become ubiquitous in the homes of people in wealthy countries. It's only in the last 50 years or so that you could just assume a random person in a developed country has access to a freezer. And there are certainly still people who don't have such access.

So yeah, we've had hot water bottles for many centuries, but the concept of a cold bottle or cold pack is only something that's been feasible for less than a single human lifetime. We were doubtlessly calling these things "hot water bottles" generations before the freezer was invented. It turns out they can also be used as ice packs, but the name was already established.

Feel free to reply to this after you give it a try. It worked for me, but I'm curious if it works for anyone else.

I think the bottles are sealed.

It doesn't leave wet patches. If you used the bottle without the cover, it would. But the cover makes it so that heat energy only slowly leaches into it. In other words, the surface of the covered bottle is probably around 60F/16C. And the surface is fluffy, not smooth.