Imagine a personal heating system that works indoors as well as outdoors, can be taken anywhere, requires little energy, and is independent of any infrastructure. It exists – and is hundreds of years old. The hot water bottle could save a great deal of energy and money without sacrificing thermal comfort.
Hot water bottles work both indoors and outdoors. Illustration: Marie Verdeil.
A hot water bottle is a sealable container filled with hot water, often enclosed in a textile cover, which is directly placed against a part of the body for thermal comfort. The hot water bottle is still a common household item in some places – such as the UK and Japan – but it is largely forgotten or disregarded in most of the industrialised world. If people know of it, they usually associate it with pain relief rather than thermal comfort, or they consider its use an outdated practice for the poor and the elderly.
Nevertheless, when I sent two dozen hot water bottles to friends and family as a Christmas present, the reactions were almost unanimously enthusiastic. People show themselves very much surprised that such a humble object can provide so much comfort. Because I don’t have the time nor the budget to send hot water bottles to everyone, I have written this article. It’s largely based on my personal experience – I have been using hot water bottles for many years and they are the only heat source in my apartment.
The history of the hot water bottle
Croation inventor Eduard Penkala patented the rubber hot water bottle – which he dubbed the “Termofor” – in 1903. However, it did not come out of nowhere. In fact, the history of the hot water bottle goes back thousands of years, albeit in different guises.
Rubber hot water bottle, made in Germany (1925-35). Source: Museum Rotterdam.
The first “hot water bottles” – quite literally – were other people and animals. Since time immemorial, people have warmed themselves by huddling together. For example, it was common for the whole family to sleep together in the same bed – and this included potential visitors.  People also took advantage of the heat from animals – “hot water bottles” with a standard fur cover.
They snuggled up against cows and pigs, which were either sharing the living space or lived in the stables below it. In the eighteenth century, wealthy women kept specially bred “hand dogs” – toy poodles – around to keep their lap and hands warm.  Personal heating devices also took the form of objects – stones, bricks, potatoes – that were heated in or near the fire, wrapped in cloth or paper, and kept in people’s laps, in pockets, or in the bed.
As early as the 1500s, people started to use all kinds of portable containers filled with hot coals from the fire. These were used as foot warmers, hand warmers, and bed warmers.  Most were made of metal, either brass or copper, and placed inside wooden or ceramic enclosures to prevent skin burns. Over time, hot coals were replaced by hot water, which is a cleaner and safer heat storage medium.
Initially, these first “real” hot water bottles were made from hard materials such as glass, metal, or stoneware. It was only with the invention of vulcanised rubber in the nineteenth century that more comfortable lightweight and flexible hot water bottles became an option. Spanish friends told me that hot water bottles used to be made from animal skins, but I could not verify this. It may well be true, because all over the world there’s a long tradition of using “water skins” for storing liquids.