Prior to use, the PC0 , PC-7 PC-16 and PC-21 must first be frozen solid. PC0 and PC-7 freeze easily in a typical household freezer (around -19 Celsius). We recommend freezing the PCM for 24 hours before use (See Freezing Times for TCP PCMs Chart below). The units should be rock hard and turn from clear liquid inside to a cloudy color. PC-16 is more difficult to freeze and requires that the freezer be colder than -16 Celsius. Some household freezers may only reach -15° Celsius and therefore the PC -16° will not freeze. The freezer must be at least -19° Celsius, otherwise, the PCM may take several days to freeze. In household freezers that reach a measured temperature of -20° Celsius, the PC-16 will freeze in 24 hours. The PC-16 must become cloudy in color and rock hard, otherwise, it will not perform correctly. Please see photographs below showing frozen and non-frozen PCMS. PC-21 requires an ultra-cold freezer to prepare. In a -45° Celsius freezer, it is rock hard in 34 hours or less. Of note, PCMs stacked together will take longer to freeze.