Brazing (sometimes known as silver soldering or hard soldering) requires a higher temperature than soft soldering (> 450 °C). As well as removing existing oxides, rapid oxidation of the metal at the elevated temperatures has to be avoided. This means that fluxes need to be more aggressive and to provide a physical barrier. Traditionally borax was used as a flux for brazing, but there are now many different fluxes available, often using active chemicals such as fluorides as well as wetting agents. Many of these chemicals are toxic and due care should be taken during their use.