The relationship between thickness and snapping strength in egg shells

The relationship between snapping strength and thickness was studied by filing away part of the egg shell in order to produce varying thicknesses. Thick shells of the ostrich, rhea and emu were used, because it was necessary to establish whether the new approach was worth while, before studying the thinner egg shells of the domestic hen which technically present a much more difficult problem.

It was found that when strips of shell of standard length and width were filed on the inside and snapped inwards, or filed on the outside and snapped outwards, then there was a rectilinear relationship between strength and the square of the thickness. When shells were filed on the inside and snapped outwards, the shell strength remained constant until quite a large fraction had been filed away, then the typical rectilinear relationship appeared. When shells were filed on the outside and snapped inwards, the removal of a little of the shell resulted in an increase in strength before the rectilinear relationship appeared. These results are discussed in relation to shell structure.

The use of concentrated hydrochloric acid to thin the shell was unsuccessful because it weakened the remaining shell far more than filing did. Treatment of the shell with boiling sodium hydroxide solution (10 per cent) either before or after filing, showed that the alkali is able to penetrate a complete shell as readily as it can penetrate into one which has been filed, and that the shell is weakened.