Railways Foot Warmer Boiler System

Trains transformed travel in many ways. For example:- trains (unlike horse drawn coaches) could travel all night allowing travellers from many country towns to leave work in the afternoon and be in Sydney the next morning. But, besides being tired from a night on a moving train, they had also suffered a cold night. The answer was to give the passengers durable hot water bottles that could sit on the floor of the carriage. The problem was how to make the hot water bottles say warm for a long time.

Railway Foot warmers

Passengers who travelled on an overnight train from Tumut to Sydney were given sealed metal tubes called foot warmers. Passengers from other places were given foot warmers at some stage in their overnight journey. For example, those travelling on the train from Armidale to Sydney were given foot warmers at Tamworth, while those travelling from Coonabarabran received their foot warmers at Mudgee.

The foot warmers were delivered hot (straight out of a tank of boiling water). Much later in the journey (when they were shaken) there was a rattling sound and this produced more heat (the act of shaking seemed to produce a miracle). But, any later shaking did not produce the rattling sound and did not produce any more heat. It was important for passengers judge when to shake the heaters, as shaking them too early in the trip meant the heaters were useless later in the journey (in the early hours of the next day).

So what was going on?

The chemistry of the process involves the crystallisation of sodium acetate trihydrate.

The sodium acetate trihydrate crystals had been sealed into the metal foot warmers years before in a factory.

On heating the sealed foot warmer in boiling water (1000C), the temperature obviously goes above 58.4oC. At this temperature the sodium acetate trihydrate (NaO2CCH3.3H2O) begins to lose its water of hydration (the 3H2O bit) and begins to dissolve in that (now hot) water.

The solution, once the crystals are completely dissolved, will gradually cool to room temperature without reverting to crystals. Instead of crystallising, the solution becomes a supersaturated solution of sodium acetate.

This means the first part of the train journey, the heat coming from the foot warmer is the result the solution losing heat to the air in the carriage (like a standard hot water bottle).

By later shaking the foot warmer, the passenger causes metal balls crashed together forming nucleation points which caused the solution to crystallize into solid sodium acetate trihydrate again. In the presence of these nucleation points the solution “freezes” (i.e. it forms a crystalline solid).

The bond-forming process of crystallization is exothermic (i.e. heat producing) so the solidifying solution will reach its freezing point (58.4oC).

At this point the contents of the foot warmer became a hot mass of solid crystals which will gradually cool until cold.

At the end of the journey (say Sydney or Tumut) the foot warmer would be put back into boiling water ready for a train heading out overnight. The Tumut foot warmer heater bath (being at the end of the line) was needed to heat the foot warmers for the next overnight train to Sydney.

Do we still use sodium acetate to warm our bodies?

Yes, is still used today in consumer heating pads or hand warmers. Instead of metal balls in a metal container, today we have a metal disc inside a clear plastic bag. By clicking on a metal disc in the clear plastic pad a nucleation centre is formed causing crystallization and heat. Sodium acetate heat packs can be easily recharged by placing in boiling water (on say a stove) for a few minutes until all crystals are dissolved. The pad can then be used as a hot water bottle and when it gets cold it can be reheated by clicking the metal disc. They can be reused many times.

The Land Newspaper, 1930

 “Young (sic) asks how the foot warmers used by the Railway Department are prepared, and how they might be renewed.

The Railway Commissioners have kindly given the following information:—

(1) Foot warmers are charged in a vacuum with sodium acetate and hermetically sealed, and do not require, refilling. They are heated by being completely submerged for 1¼ hour in water maintained at boiling point for the whole time. After heating, foot warmers should retain heat for 10 to 12 hours undisturbed, or 12 to 14 hours if turned over endways 7 or 8 hours after boiling.

(2) Sodium acetate is not dangerous to flesh or clothing.

(3) The heating of foot warmers charged with sodium acetate is brought about by boiling as described above. After having become partially cooled, the heat can be regenerated by shaking or turning the foot warmer over endways.”

 The Country Man's Inquiry Department! Conducted By "The Man on The Land” The Land (Sydney, NSW), Friday 12 September 1930

World War I

An interesting footnote is that train travellers in Victoria were supplied with less foot warmers during the latter part of WWI because (at the time) Germany was the only source of sodium acetate.

Diagram by P. Prewett. - 2013