Transformer cores are constructed predominantly of ferromagnetic material. The most common material used is iron, with the addition of small amounts of silicon and other elements that help improve the magnetic properties and/or lower losses. Cores made of silicon steel are constructed of multiple layers of the material in sheet form. The material is fabricated in rolling mills from hot slabs or ingots. Ferrite cores are made of sintered powder. They generally have isotropic magnetic properties. They can be cast directly into the desired shape or machined after casting. They have extremely high resistivities, which permits their use in high-frequency applications. Thus, in a transformer core with joints, the effective permeability is reduced relative to that of an ideal core without joints. This requires a higher exciting current to drive a given flux through the core. The core losses will also increase mainly due to flux distortion near the joint region.