Primary belt scrapers are the first line of defence against material build-up, and they are designed to remove residual material after the belt has unloaded. Primary scrapers are especially suited to removing coarse particles, and gels and liquids. Secondary scrapers are used to remove any remaining residue after the primary scraper has passed along the belt. There are a number of different designs, to take into account different plant uses. For example, some belt scrapers are designed to work at lower or higher belt speeds, and with different types of belt surface, such as ridged or chevroned. Other factors which might affect the choice of scraper include the working environment and temperature. The belt scraper can also be selected so that it is the best fit for the type of materials transported. For example, the moisture content, acidity or alkalinity levels, the consistency, such as fine powder or heavy clays, and the particle size. They are typically made of plastic, rubber, or metal, and can consist of a multi-blade system, or a single blade. For material which is of a wet or sticky consistency, non-contact scraper air systems may be the best solution. These use an air suction method to remove materials from the belt surface. They are also suitable for removing fine, dusty or powdery materials from the belt surface, when used in conjunction with a spray bar. Non-contact scraper air systems have the advantage of causing no damage to the belt, so they are ideal for harsh and abrasive materials, and will work effectively on damaged belts.