Craig tube

What is a Craig tube?

The Craig tube is a small-scale apparatus utilized in preparative and analytical chemistry for recrystallization of quantities up to 100 mg (microscale). It comprises two parts โ€“ a test tube resembling a test tube with rough ground glass in the neck and a plunger typically made of Teflon or glass. Although the two parts connect, they allow liquids to flow between them even when closed, reducing the need for transfers between containers and minimizing the risk of solid loss during handling.

Craig tube - microscale
Craig tube โ€“ microscale

To recrystallize, the solid is dissolved in a solvent, and crystals are formed. The stopper protects the solution from atmospheric contamination, and the crystals are separated from the mother liquor by inverting the tube and stopper in a centrifuge tube and centrifuging. The stopper retains the crystals, while the mother liquor passes into the centrifuge tube.

The Craig tube offers several advantages over traditional methods, including the ability to produce relatively dry and contamination-free crystallized products that can be recovered more efficiently than from a sinter funnel. The apparatus was invented by Lyman C. Craig and Otto W. Post and can be made by competent glassblowers or purchased commercially.


Improved Apparatus for Solubility Determination or for Small-Scale Recrystallization
Lyman C. Craig and Otto W. Post
Industrial & Engineering Chemistry Analytical Edition 1944 16 (6), 413-414
DOI: 10.1021/i560130a031