Refrigerators of Antiquity
Even some 1500 years ago, people in the Far East knew ways to cool their food and drinks. To do so, ice is crushed and mixed with salt. The “refrigerant” that is created can drop the temperature down to as low as -21° Celsius!
This method was brought to Europe by mariners, though not until centuries later. The method they introduced involved cutting blocks out of the ice that covered lakes and rivers, which would then be stored in deep cellars under a layer of straw. This would then be used to cool drinks in summertime. If they needed particularly low temperatures, they would crush the ice and mix it with salt, thus creating the “refrigerant” we mentioned.
You will need
- ca. 1/2 kg ice or snow
- 150 g salt
- pickle jar
- wooden spoon
- dish towel
- Wrap the ice cubes in the dish towel and place on an impact-resistant surface. Now smash the ice cubes up as much as possible with the hammer (make sure you don't hit your fingers!)
- Pour the crushed ice into the pickle jar until you have about 2-3 cm of ice in the bottom, then add about a 1 cm layer of salt, followed by more ice, etc. Stir frequently with the wooden spoon.
- Now, before the mixture you have created becomes too hard, stick a thermometer into it and observe as the temperature drops.
If you mix salt and water, the salt always wants to dissolve in the water. But in this experiment, you first have to melt the ice before the salt has a chance to melt.
The ice begins to melt on the surface, and the salt immediately dissolves in the water. This reaction also requires heat, however, in order for it to happen. This heat is drawn from the environment, which in turn cools down as a consequence. The result is temperatures as low as -21° C!
This technique was known by the Romans. At that time, they used salts which had formed on the walls of horse stables. This wasn’t kitchen salt, rather saltpeter, which consists mainly of ammonium nitrate. Ammonium nitrate is formed when slurry breaks down due to bacteria. Napoleon’s soldiers used this same technique, though in their case it was a byproduct of manufacturing gunpowder. Gunpowder used in weapons of those days was made from saltpeter, charcoal and sulfur.