At atmospheric pressure, He (the more abundant isotope of helium) liquefies at 4.2 K. By employing evaporative cooling, temperatures down to 1 K can be easily produced. While this technique is fairly simple to operate, it is inefficient for large helium baths because about 50% of the liquid helium must evaporate to attain to the lowest temperatures. If only a small volume needs to be cooled to 1 K, the 1-K pot is used. Only the surface of the small 1-K pot is pumped, leaving the rest of the liquid helium bath at atmospheric pressure.