A Craig tube is an item of apparatus used in small-scale (up to about 100 mg) preparative and analytical chemistry, particularly for recrystallisation.
The second is a loosely-fitting generally-cylindrical stopper, possibly with a teardrop-shaped head, of glass or of another inert material such as PTFE, which seats on the constriction.
The stopper allows the mother liquor to pass into the centrifuge tube but retains the crystals, which can subsequently be recrystallised again or collected.
The apparatus has the advantages that the crystallised product is relatively dry, is free from contamination by fibres from filter paper, and can be recovered more efficiently than from a sinter funnel.
[2][3][4] Craig tubes can be made by competent glassblowers, and are also available commercially.