Final Analysis: Porous Platinum Morphologies: Platinised, Sponge and Black

First paragraph: Platinum (as well as many other metals) may be prepared in finely divided forms that are porous at all levels of magnification. Cracks and crevices in the sponge-like opaque matrix then allow visible light (of wavelength around 0.5 μm) to enter, whereupon it undergoes multiple reflections and is unlikely to be reflected back out. This effectively produces a high value for optical absorption, resulting in a velvety black appearance to the eye, even though the platinum is in the metallic state. Other porous structures (such as snow and plaster) consist of inherently transparent or translucent crystals, so light can be refracted as well as reflected out and they appear matt white.