The boiler (marked A in the diagram) is filled with water almost up to the safety release valve (some models have an etched water level sign) and the funnel-shaped metal filter (B) is inserted. Italians generally do not preheat the water used, though established baristas such as James Hoffmann and Tristan Stephenson recommend using preheated water in the base. Simulations run by physicist Warren King also indicate that using cold water results in the coffee extracting at too low of a temperature, but that boiling water may result in the temperature being too high. King recommends a 70°C preheat for optimal results. Finely-ground coffee is added to the filter as shown below. How tightly the coffee is packed impacts how quickly the coffee extracts. The pot is placed on a suitable heat source, so that the water is heated.