The elastic modulus (Young's Modulus) of ceramics is usually higher than for metals, because ceramics are bonded either covalently or ionically. Glasses have a lower modulus than other ceramics due to their non-uniform (amorphous). Ferrule materials determine the mechanical precision, optical alignment, thermal stability, and long-term reliability of fiber optic connectors. A ferrule's job is to hold the fiber core in perfect concentric alignment while maintaining extremely tight tolerances according to IEC 61755, IEC 61300. Technical ceramics are characterised by a high level of hardness compared to typical metallic materials. They are designed to be inserted into or around another component to provide a protective sleeve, guide, or housing.