Copper gives green in most fritted glazes, the colour being darker and richer in lead bearing glazes. In alkaline type glazes, the colour tends to blue, as is also the case in emulsion opacified high boron glazes, sometimes called boron blues. Large amounts of copper in glazes, especially lead glazes, give metallic effects and even graphite-type matts. Copper is an active flux and it may increase glaze melt fluidity; it may also cause crazing, due to its thermal expansion. In certain glaze compositions, under controlled reducing conditions, copper compounds can produce the reduced reds of rouge flamb and sang-de-boeuf; however these are difficult to obtain reliably. Since beautiful reds can now be reliably made with cadmium-selenium glazes, there is no longer any necessity to produce the reduced copper reds. Copper carbonate ensures a more evenly distributed colour.