For those looking to skip the history lesson: > Saltpetre was most often used to cool wines during the Regency, by cooling the water in which the wine bottles

For those looking to skip the history lesson: > Saltpetre was most often used to cool wines during the Regency, by cooling the water in which the wine bottles were immersed. A large wooden tub, preferably of a cylindrical shape or, better still, wider at the top than at the bottom, was the ideal shape of a vessel to be used for cooling with saltpetre. This cooling tub should be lined with sheet lead or zinc and should also have a close fitting lid which would exclude as much of the warmer ambient air as possible. The thicker the surrounding wood, the better the cooling mixture would be insulated. A cooling tub with a capacity of ten to twelve gallons should be filled with four or five gallons of water. The cooler the water, the better, so water just pumped or drawn from a well would be most effective, since the water temperature would be about 75º Fahrenheit. Five to seven pounds of saltpetre should be pulverized to the finest powder possible. This finely-powdered saltpetre should be slowly sprinkled into the water and allowed to dissolve. Within about fifteen minutes the temperature of the water would drop twenty-five to thirty degrees, within a half hour the temperature would drop another four or five degrees. At that point, the temperature of the water would remain steady for over two hours, so long as the lid was kept on the tub as much as possible. After that, the water would begin to warm at a rate of about three or four degrees per hour, unless more powdered saltpetre was added to the water. And for the why it works: > Saltpetre cools water by producing an endothermic reaction. This is a chemical reaction whereby, as it dissolves, the saltpetre literally pulls the heat out of the water as part of that process, thus lowering the temperature of the water. For this reason, there is a limit to how cool the water can become. Once it has become fully saturated with saltpetre, the water is not able to absorb any more.