Gilding is the art of applying a thin layer of pure metal leaf finishes gold, silver or copper leaf as such to any surface. The techniques have changed and with great evolution but now we can obtain variation of gold colours when it is mix with a small quantity of copper or silver and gold leaf thickness doesn't exceed 0.2 to 0.3 mm. In gilding we can use  materials such 23,24 carats pure gold and white gold leaf, aluminium leaf, copper leaf, oxide metal leaf, silver leaf, platinum ,palladium can be use as well.

Gold reflects the brightness of the light and doesn't tarnish with time. Gold is storage usually in the notebooks, between two leaves of paper of silk; on which ones was applied powder of gypsum.

Gilding is painstaking and a very skilled job, people have naturally tried to take shortcut. Over the years the magic recipe had been to use a gold paint bronzine. 

This substance applied to the frame give a very dark and green finish as it ages, it's not gold but it's just a bad paint looking dark & dirty because the oxidation with time.