Code 4. “Color with discontinuous oil” for thicker oil layers 50-200 m thick. For oil films more than 50 m thick, the light is reflected by the oil and not by the surface of the underlying sea, allowing to see the true oil color of the spilled oil. Only gradually does the true oil color become the dominant color, with brown and black oil appearing black. However, code 4 is a transition code because thinner layers alternate with thicker layers due to the action of the waves. This code is also often described as “the real oil color on a metallic background.” In particular, codes 4 and 5 (along with real oil paintings) contain considerable amounts of oil on a small area and are therefore, in most cases, considered slicks that can be fought at sea. The other codes for thinner oil layers, which often cover the largest areas of an oil spill, contain too little oil to be a reasonable prospect, even if dispersants are used. Minimum and maximum volumes derive from part of the minimum thickness of the layer in which a coloration is visible and from the maximum thickness of the layer in which the coloration moves to an “upper” code. The volume of oil is then calculated from a simple mathematical formula, in which the code-polluted surface is multiplied by the percentage of that code, present in a slick, and by the minimum and maximum thicknesses. The minimum volume estimate is generally used in criminal matters (to give the quantities specified as a minimum), while the maximum volume estimate is generally used in cases of accidental marine pollution (the worst typical cases in crisis management). Code 3. `Metallic` for 5-50 m thick oil layers: typical of this appearance is matte metallic shine.
The color of this metallic shine is largely determined by the color of the underlying water, because the oil layer is not thick enough to block the light reflected from the surface of the water, and partly also by the “color” of the sky (. B for example blue sky or grey clouds) generated by the reflection of light. An approximate estimate of the magnitude of an oil spill. To some extent, this is possible from a ship, but the margin of error will be high, especially in the case of larger slicks that cannot be considered entirely from a ship.