Welding wires can be identified by their chemical composition and also their section morphology, which may either consist of a single metal (solid wires) or have an internal core containing granules (tubular wire). Particular attention should be paid to the presence of grease or moisture on the surface of the wire electrode, because they could cause cracks, porosity or blowholes; in addition, if the wire electrode reel is not wound uniformly this could cause uneven wire feed resulting in unstable welding. Solid wires usually have the same composition as the base material, with added elements able to help clean the base material. The most widely used diameters are 0.6 – 0.8 – 0.9 - 1 – 1.2 – 1.6 mm. Flux Cored Wires , with gas shield, do not consist of solid metal but have an internal core filled with granular powder (flux); this has the same functions as the coating on coated electrodes. The granular powder or flux can be of rutile, basic or special type. Compared to solid wires, tubular wires have better arc stability and deeper penetration, ensure a better-looking seam, often eliminating the need for further finishing (e.g. spray grinding) and reducing the risk of defect formation, such as porosity; of course the use of tubular electrodes requires slag removal, as for welding with M.M.A. electrodes. The most widely used diameters are 0.6 – 0.8 – 0.9 – 1.2 – 1.6 mm.