Introduction

It is necessary for the study of the grain structure of ice to record both the size distribution and the orientation of crystal axes of individual grains. However, no method has been proposed for the simultaneous observation of these two features. Methods for recording the size distribution were devised by SeligmanReference Seligman ¹ , Ahlmann and DroesslerReference Ahlmann and Droessler ² and SchaeferReference Schaefer ³ . The orientation of the c-axis has been determined by the use of crossed polaroids. Recently, NakayaReference Nakaya ⁴ has developed a simple method which made the determination of a-axis possible, by making Tyndall figuresFootnote * or vapour figures inside single crystals of ice. In recent experiments, the authorReference Higuchi ⁵ succeeded in producing the separated etch pits by an operation using the plastic replica film. Combining this etching technique with Schaefer’s replica method, it is possible to record the size distribution and the orientation of crystal axes by simple operations.

Determination of Crystal Axes by Etch Pits

Etch pits provide a reliable indication of the orientation of crystal axes of ice, as well as of many other crystals. The geometrical shapes of the etch pits belong to the hexagonal system, as expected from the crystal structure of ice. As shown schematically in Fig. 1 (above) each shape agrees with a section cut from a hexagonal column with a plane. The figure of a regular hexagon was obtained on the surface perpendicular to the c-axis, and the sides of the hexagon were found to be parallel to those of vapour figures, that is, parallel to the a-axesReference Nakaya ⁴ . These etch figures can be used as a simple method of determining the orientation of the a-axes as well as the c-axis. The angles between the ice surface and crystal axes can be calculated from the measurement of the microphotographs of etch pits. Figs. 2 and 3 (p. 122) are microphotographs of the replicas of etch pits, obtained by the following technique.

Fig. 1. Diagram to show the kinds of etch pits formed on ice

Figs. 2 and 3 Microphotographs of shadowed replicas of etched ice crystals. The crystal in Fig. 2 and the grain on the left of Fig. 3 have faces perpendicular to the c-axis. The right hand grain in Fig. 3 has a face parallel to to the c-axis. Magnification ×13

Procedure for Etching

In the author’s experiments, the ice samples were sawed from a large block of commercial ice, which comprised many columnar grains as large as several millimeters in diameter. The polished ice surface was coated with a 1-5% solution of polyvinyl formal dissolved in ethylene dichloride. The coating must be applied immediately after the polishing, because irregular etching starts quickly over the whole surface. After a time, the solvent evaporates and the plastic film covers the surface of the samples, adhering closely to it. If this covered sample is left in a cold chamber, numerous separated etch pits are produced on the surface of the ice. It is desirable to carry out these operations at a temperature of about −20° C. When the temperature of the chamber was −24° C. and the solution was 1%, the etch pits attained a mature stage of development within a few minutes after formation of the plastic film. In the case of the more concentrated solutions, half a day was needed. The etch pits were examined under a microscope and photographed.

Procedure for Making the Replica

The plastic film covering the ice surface was removed after confirming the production of etch pits under a microscope. Then the exposed surface was coated with the replica solution again. In this case, a 3% solution is suitable for making a replica of the etched surface. After evaporation of the solvent, the replica sheet was stripped off carefully, as described by SchaeferReference Schaefer ³ . In some cases, the surface of the replica was shadowed by the method used in electron microscope technique, in order to examine the details of the surface structure. Fig. 2 and 3 are microphotographs of shadowed replicas of the etched surface. Fig. 2 shows the etch pits produced on the face perpendicular to the c-axis, and the directions of the sides of the hexagons indicate the a-axes. Fig. 3 shows the grain boundary between two grains, in which the left is a face perpendicular to the c-axis and the right is parallel to the c-axis. Fig. 4 (p. 122) shows the replica of a stepped structure observed in the basal plane of the crystal ; that is, the base of a hexagonal etch pit. As reported elsewhereReference Higuchi ⁵ , the height of steps was found to vary between 3μ, and 16μ, the mean having been about 5μ.

Fig. 4 Replica of the stepped structure observed at the base of a hexagonal etch pit. Magnification 135

Acknowledgement

The writer expresses his grateful thanks to Prof. U. Nakaya for guidance and encouragement throughout this work.

MS. received 30 October, 1956