Water normally freezes at 273.15 K (0.0 °C; 32 °F), but it can be "supercooled" at standard pressure down to its crystal homogeneous nucleation at almost 224.8 K (−48.3 °C; −55.0 °F). The process of supercooling requires water to be pure and free of nucleation sites, which can be achieved by processes like reverse osmosis or chemical demineralization, but the cooling itself does not require any specialised technique. If water is cooled at a rate on the order of 10 K/s, the crystal nucleation can be avoided and water becomes a glass —that is, an amorphous (non-crystalline) solid. Its glass transition temperature is much colder and harder to determine, but studies estimate it at about 136 K (−137 °C; −215 °F). Glassy water can be heated up to approximately 150 K (−123 °C; −190 °F) without nucleation occurring. In the range of temperatures between 150 and 231 K (−123 and −42.2 °C; −190 and −43.9 °F), experiments find only crystal ice.