NOVELTY - Making porous silicon involves: contacting elemental particulate silicon with an aqueous chemical etchant, where the chemical etchant comprises hydrofluoric acid and a dissolved metal salt.    USE - For making porous silicon, particularly mesoporous silicon useful in foodstuffs, consumer care products, electronic devices, in lithium battery, as a hydrogen reservoir, as an explosive material, or as a catalyst or a medicament (claimed). Also useful in applications including cosmetics, personal care, fuel cells, chocolate, chewing gum, toothpaste, oral hygiene, drug delivery including oral drug delivery, in-vivo imaging, and in diagnostics.    ADVANTAGE - The method produces fully biodegradable highly porous silicon of 50-70 vol.% porosity. The porous silicon produced has high pore volumes of up to 0.5 ml/g, batch sizes of up to at least 100 g, and throughput of at least 100 g/hour. The method produces porous silicon in good yields i.e. up to 60 wt.% of the feedstock is converted to porous silicon, and the porous silicon can be loaded with high payloads of at least one active material, i.e. in amounts of greater than or equal to 40 wt.%. The stain etched porous silicon has high surface area, high pore volume, lack of silicon regions of largest width 10 nm, fully mesoporous regions at greater than 85 wt.%, preferably greater than 95 wt.%, presence of at least one metal selected from iron, silver, gold, aluminum, platinum, vanadium, or copper on the pore wall surface at a concentration of 0.0001-1 wt.%; and a high solubility in aqueous liquids such as body fluids. Moreover, the porous silicon obtained by the method can be used for porosifying less expensive feedstocks such as metallurgical grade silicon. Existing methods of porosifying silicon, particularly anodization, make use of high purity electronic grade silicon wafers which are significantly more expensive. Also, the present method provides a significantly higher throughput compared to existing methods.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) porous silicon obtained by the method; and    (2) stain-etched porous silicon having a surface area of greater than or equal to 200 m2/g.