Mostly we can clean metals and non-metals, also, organic and inorganic ( stones) can be cleaned to a certain degree. To have a good cleaning result, we need an as dark as possible contamination and highly reflective light-coloured substrate to avoid damage. Black smoke deposit on a sandstone is easy to clean. Yellow paint on sandstone is more difficult because this yellow colour is not absorbing well the laser radiation. Laser will remove nearly all oxides in a very big range of metals and non-metals. A shrinking-off effect on the surface removes oxides; the oxide layer has a different energy-heating speed or flux than the substrate. Oxides also absorb more energy than the highly reflective aluminium surface. Example: You can remove ink on a white paper without burning the paper. Lasers are also used to remove hair or tattoos; this is based on the higher absorption of those black hairs and the darker-than-skin tattoo ink. The human skin is not absorbing very well the infrared laser light.