To prevent your thermometers from separating, make sure that you are properly using and storing your thermometers. While storing thermometers, store them at an angle with the bulb lower than the expansion chamber.
There are 2 methods for reuniting that work equally well depending upon the temperature range of the thermometer. Always wear your safety glasses and gloves when reuniting a thermometer column.
Prepare a solution of shaved ice and salt or C02 (dry ice) and alcohol. Place the thermometer bulb partially in the solution, keeping the thermometer upright. Allow the liquid column to retreat into the bulb. Swing the thermometer (bulb down) in an arc, forcing the trapped gas to rise above the column. Allow the thermometer to warm slowly in an upright position.
Heat the thermometer bulb in an upright position in warm liquid, warm air, or over a flame. Allow the liquid column to rise slowly until the separated portion of the column enters the expansion chamber (NOTE: Overfilling the expansion chamber will break the thermometer). With the thermometer in an upright position, gently tap it allowing the gas separating the column to rise above the column. Allow the thermometer to cool slowly in an upright position.