Freshly cleaved surfaces of sodium fluoride have been thermally etched in the range of temperature between 860°C and 930°C for the required time. The formation of circularly terraced pits has been discussed. The topography of thermal etch patterns is studied by multiple beam interferometry. Interaction of circular etch fronts from the point of nucleation of two or more dislocations is illustrated. Under controlled conditions of thermal etching, in addition to the thermal etch pits, spiral structures have been observed. It is conjectured that these spirals might have been formed as explained by Lang (1957). It is conjectured that the etch patterns appearing similar to the pattern observed at the Frank-Read sources might have been formed due to a pair of dislocations of opposite sign.

The domain structures inside the thick crystals of KNbO3 have been revealed using a liquid polishing agent to remove layers of crystals by dissolution without introducing any stress. It is found that entirely new structures unrelated to those on the original surface exist inside. A method has been suggested to analyse complicated domain patterns simply by viewing under a metallurgical microscope. The peculiarities of domain formations observed have been discussed.

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Cleavage faces of potassium chloride single crystals have been thermally etched. The thermal etch patterns consist of irregular spiral patterns and closed loops. Inter-ferometric study has revealed that they are depressions. The centres of the spirals are not distinct. It is therefore conjectured that the spirals without distinct centres might have been formed as explained by Lang (1957). While the patterns similar to that developing at Frank-Read sources and the complicated pattern originating from number of sources concentrated in a small region may be due to screw dislocations.

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