The following information is an excerpt from "Reproducibility of the temperature of the ice point in routine measurements" published by the Commerce Department, National Institute of Standards and Technology (NIST), 1995. The complete document is available on their website.
A properly prepared ice bath is one that consists of a cylindrical Dewar flask (a typical size is one that is approximately 8 cm in diameter and 36 cm deep), a siphon tube, shaved ice prepared from distilled water, and distilled water that just fills the voids between the ice particles but does not float the ice.
A Dewar flask is the preferred type of container for the ice bath since its insulating properties retard the melting of the ice.
The siphon tube should extend to the bottom of the Dewar for ease in removing the excess water produced by the melting of the ice. The shaved ice should have the consistency of snow. If a shaver is not available, the ice may be crushed such that the ice particles are no larger than 2mm to 3mm in size.
In preparing the ice bath, the Dewar flask should be filled to approximately one-third of its capacity with air-saturated distilled water and then the shaved or crushed ice should be added until the flask is full. Then the mixture of water and ice should be compressed until it is tightly packed. Any excess water is removed by the siphon. The bath should stand for approximately 30 minutes in order to permit the ice particles and the water to equilibrate before the bath is used. With use, the ice of the bath will melt, producing excess water. That water should be removed and the ice replenished so that the Dewar flask is maintained full.
Since contamination of the ice and/or the water will affect the melting-point temperature, it is essential that the necessary care be taken to prevent contamination. The flask and all other components used with the bath must be washed thoroughly with distilled water before being placed in the bath. Gloves should be worn when handling the ice and other items that will be placed in the bath. An adequate support must be provided above the Dewar flask for thermometers that will be measured in the ice bath.
For photographs of working ice baths and their use, please see the complete NIST document "Reproducibility of the temperature of the ice point in routine measurements" published by the Commerce Department, National Institute of Standards and Technology (NIST), 1995. The complete document is available on their website.