As the outer part dries it is prevented from shrinking by the wet interior, which is still at its greatest volume.

This outer portion must either check open or the fibres become strained in tension.

If this outer shell dries while the fibres are thus strained they become "set" in this condition, and are no longer in tension.

Later when the inner part dries, it tends to shrink away from the hardened outer shell, so that the inner fibres are now strained in tension and the outer fibres are in compression.

If the stress exceeds the cohesion, numerous cracks open up, producing a "honey-combed" condition, or "hollow-horning," as it is called.