New vessels are rubbed on the surface with an abrasive powder. This makes the surface almost uniformly leveled.
Almost all the light falling on the buffed surface gets reflected uniformly along certain directions depending on the shape of the vessel. This makes them shine.
Upon continuous use, scratches are formed and so the surface becomes rough. Hence, the light falling on the surface is scattered randomly all around. Hence the reflectively of the surface is not as good as it was before. Dirt accumulating on the scratches, corrosion and oxidation of the metal surface dull the vessels further.
The surface of any metal when scrubbed and polished would have a shinning appearance. This is a characteristic property of metal and is due to reflection of light.
When exposed to air and moisture, most metals react with oxygen, carbon dioxide and moisture. Iron vessels rust and others such as copper and aluminum tarnish. When such vessels are used for cooking, this process gets quickened; aluminum gets coated with its oxide and becomes dull. Copper gets covered with its basic carbonate resulting in a green colour.
We are familiar with an age old practice of cleaning such vessels with tamarind.
Mild organic acids present in such fruits and vegetables dissolve the coating and restore the sheen. Washing powders which are alkaline also aid in dissolving the dull coat.
Metallic tin is resistant to many of these reactions. That is it used to ‘tin’ copper vessels. Stainless steel does not get tarnished due to the presence of chromium in it.
Even silver vessels get tarnished due to the trace amounts of hydrogen sulphide present in the atmosphere. Such vessels also require polishing in this case with the more poisonous cyanides!!!