This information is excerpted from NBS Monograph 90
Mercury separations commonly occur as a result of exposure to shock and rough handling during shipment. This condition can generally be corrected by carefully following the procedure below, which was developed by the National Bureau of Standards, the predecessor of NIST: The National Institute of Standards and Technology.
1. Many inquiries are received concerning separated mercury columns, particularly after shipment. Because no means of avoiding such occurences has yet been found, the recourse of stating some directions for joining the mercury is adopted. The process of joining broken columns consists of one or a series of manipulations which may be effective, and these are briefly described here. The bulb of the thermometer may be cooled in a solution of common salt, ice, and water to bring the mercury down into the bulb. Moderate tapping of the bulb on a paper pad or equally firm object, or the application of centrifugal force usually serves to unite the mercury in the bulb. If the salt solution does not provide sufficient cooling, carbon dioxide snow (dry ice) may be used. Because the temperature of dry ice is about -78°C and mercury freezes at -40°C, it will cause the mercury to solidify. Care must be taken to warm the top of the bulb first so that pressures in the bulb due to the expanding mercury may be relieved. Care should also be taken in handling dry ice as it may cause severe burns.
2. If there is a contraction chamber above the bulb or an expansion chamber at the top of the thermometer tube, the mercury can sometimes be united by warming the bulb until the column reaches the separated portions in either enlargement. Great care is necessary to avoid filling the expansion chamber completely with mercury, which might produce pressures large enough to burst the bulb. Joining the mercury is more readily accomplished if the quantity in either cavity has been shattered into droplets by tapping the thermometer laterally against the hand.
3. As a last resort, especially for thermometers having no expansion chambers, small separated portions of the column can sometimes be dispersed by warming, into droplets tiny enough to leave space for the gas to by-pass, and these droplets can then be collected by a rising mercury column. The procedure for thermometers in which organic liquids are used is similar. Liquids in the stem can more readily be vaporized and may then be drained down the bore. The latter process is aided by cooling the bulb. All of these manipulations require patience, and experience is helpful, but they will yield results if care is used. A convenient method of ascertaining that all of the liquid has been joined is a check of the ice point, or some other point on the scale.