The Stone Implements of the Maori
The most marked feature in the faces and backs of Maori stone adzes is convexity. Longitudinal convexity of the face is practically universal in all forms, and transverse convexity the rule in the same feature. The back is often of a similar shape, but is sometimes flat or even concave longitudinally. A truly plane surface was, however, apparently abhorred by the Maori adze-maker, and presumably for two very good reasons—it entailed much extra work in grinding the tool, and also impaired its utility. Even in cases where the back is flat transversely, the habit of rounding off the longitudinal edges imparts an appearance of convexity to the back of the tool. In such specimens as have the cutting-edge in the centre—that is, in a line with the axial centre of the tool—both face and back are markedly convex longitudinally.
As a rule, the width of an adze increases gradually from the poll to the cutting-edge. In some specimens the widest part is at the shoulder, page 223in others across the middle. Other aspects are occasionally seen, as will be shown when describing triangular and abnormal forms.
The longitudinal and transverse convexity of the face has also its utility, and, in regard at least to the blade part of the tool, the old-time Maori had evolved the correct and most useful form for an adze-blade, one that is seen in the best steel adzes of the present time. In this wise: The convexity of the face imparts to the cutting-edge a slight curve downward-that is, it leaves a cut like that of a very shallow hollow gouge, thus . This means that the corners of the cutting-edge would not keep digging into the surface of the wood when the tool was being used, thus impeding the work and marring the appearance of the dressed surface. The result would be a chip with sharp edges, and a clean cut. In like manner, if the face of the adze was straight and flat longitudinally, the tool would be almost useless as an adze, for the cutting-edge would tend to stick in the timber, and would not perform the scooping lift so desirable in an adze when the operator is dressing the face of a baulk of timber.