physical irregularities or damage to the shell occurring within the mould during the initial seconds of solidification. Excessive turbulence within the mold may cause an irregular shell pattern that grows abnormally or it may entrap slag droplets within the shell which reduces the wall strength. A common occurrence is for the shell to stick to the mould's surface and tear; modern instrumented molds and computer control systems typically detect this and slow the caster down temporarily to let the wall refreeze and heal while it is still supported in the mould. Should the tear occur near the exit of the mould or be of unexpected severity, the shell may still fail in a breakout once it emerges from the mould wall. If the incoming metal is severely overheated, it may be preferable to stop the caster than to risk a breakout. Additionally, lead contamination of the metal (caused by counterweights or lead-acid batteries in the initial steel charge) can form a thin film between the mould wall and the steel, inhibiting heat removal and shell growth and increasing the risk of breakouts. Other considerations Another problem that may occur is a carbon boil – oxygen dissolved in the steel reacts with also-present carbon to generate bubbles of carbon monoxide. As the term boil suggests, this reaction is extremely fast and violent, generating large amounts of hot gas, and is especially dangerous if it occurs in the confined spaces of a casting machine. Oxygen can be removed by "killing" it through the addition of silicon or aluminium to the steel, which reacts to form silicon oxide (silica) or aluminium oxide (alumina). However, too much alumina in the steel will clog the casting nozzles and cause the steel to 'choke off'. Computational fluid dynamics and other fluid flow techniques are being used extensively in the design of new continuous casting operations, especially in the tundish, to ensure that inclusions and turbulence are removed from the hot metal, yet ensure that all the metal reaches the mould before it cools too much. Slight adjustments to the flow conditions within the tundish or the mould can mean the difference between high and low rejection rates of the product. Starter bar The starter bar, also called a dummy bar, has a free end portion which is flexible for storage and a substantially rigid portion at the end which plugs the mold. The starter bar is