The dendritic structure is very typical of cast metals. A lower melting point mixture of pure copper and cupprous oxide, called a eutectic, forms in the open spaces between the dendrites. The eutectic particles are usually dark, globular bodies dispersed in a copper background. The cuprous oxide particles form a network, outlining the dendritic cells. Pores, seen as dark spots in the microstructure, are also present in the as-cast material.
Copper melted under non oxidizing conditions is called oxygen free copper.
The structure of the as-cast material is similar to that described above. When the as-cast ETP copper is hot rolled the eutectic structure is completely destroyed. The microstructure of the hot rolled copper contains many small grains. Parallel straight lines extending across many of the grains are called annealing twins. They appear after a metal has been mechanically worked at a high temperature, called annealing, and deformed.