The Hen's egg: Factors affecting the shearing strength of shell material

Experiments are described from which it is concluded that

1. the force required to shear egg shell material can be measured with high precision by pressing a narrow, flat‐ended plunger on to the shell;

2. shearing force falls and the precision of its measurement rises as the radius of the plunger is reduced;

3. shearing force is related linearly to shell thickness ;

4. the regression line of shearing force on shell thickness does not pass through the origin but has a substantial positive intercept on the thickness axis, at about a third of the mean thickness ;

5. there are differences between hens and between strains of hen in the magnitude of the intercept, but not in the slope of the regression;

6. shearing force is affected also by the glossiness of the shell;

7. shearing force was not significantly affected by stripped membrane thickness, number of mammillae per unit area of shell, shell organic content (standardised for shell thickness), shell curvature, shell colour, mottling score, translucent patch score, speckling score, age of the egg or age of the hen that laid it;

8. measured with a plunger of radius 165 μm, the mean shearing strength of incremental shell was 57 kg wt/mm² (3–7 ton/in²).

The findings suggest that the shearing strength of egg shells might be increased by direct selection and by selection for shell glossiness.