Patina is a natural surface tint which occurs on many objects of age. Most collectors consider patina desirable, and will pay more for an object with authentic patina.
Ivory items naturally turn slight yellow [Image #1 - Vintage whale teeth] to golden yellow [Image #2 - Antique whale tooth] through exposure to sunlight, handling (absorption of skin oil), room smoke [Image #3 - Tabua], etc. Mammoth & Mastodon tusk sections have a dark brown patina from being buried in the ground for thousands of years. Authentic patina can penetrate quite deeply into the ivory [Image #4 - Mammoth scrimshaw].
Copper, silver, bronze, tin, and other soft metal items can also gain patina with age. This is usually a type of surface oxidation caused by human handling [Image #5 - coins] & [Image #6 - silver service], exposure to air [Image #7 - outdoor bronze statue] & [Image #8 - outdoor copper statue], or submerged in the sea [Image #9 - bronze cannon].
Iron & steel can also display a decorative dark patina color from being buried in the ground [Image #10 -cannon ball]. Usually though, iron corrosion is the undesirable brown/orange rust that continues to eat-away, and may eventually destroy that item [Image #11 - rusty chain].
Stone items can also acquire patina through burial [Image #12 - arrowhead], and atmospheric exposure [Image #13 - Stonehenge].
Patina on glass items is a special case, as the amount & color can be directly related to the glass formula, length of burial, etc. [Image #18 - glass bottle]. Worthologist Bill Lindsey has written an inclusive WorthPoint article about this subject, entitled Patination and Historic Bottles.
ALL of these types of patina can be faked through the use of dyes & washes [Image #14 - faux patina tooth], or through intentional exposure of metal items in slightly acidic solutions [Image #15 - modern weather vane]. Sometimes this is obvious, and sometimes it takes a well-trained eye to reveal the forged age. For this reason, many collectors like to view items with patina intact. Patina can be a measure of age, as well as an indicator of authenticity, or NOT.
Grime, dirt, transferred oxidation, etc., are NOT patina, and are therefore not desirable. I have carefully cleaned many antique scrimshawed whale teeth, using Q-tips and denatured alcohol, to remove grime from a hundred years of handling. Natural patina is NOT effected, and the scrimshawed image can become more obvious against the naturally patina-colored background. Care must be taken to not disturb the antique ink used to accent the scribed design [Image #16 - sea turtle shell].
I have also used a clean cotton cloth with a touch of “Silvo” paste, to slightly surface-clean antique silver jewelry to reveal the natural color of the metal, and to enhance the engraved design. The residue is then buffed-off with another clean cloth. Immersion into a liquid cleaning solution will remove all of the oxidation, even in the engraving. This may be desirable for contemporary silverware & silver service in use, but usually not desirable for show pieces, like antique silver service, candlesticks, jewelry, picture frames, etc. [Image #17 - antique Sterling silver].
Remember, it is easy to clean “just a bit more”, but impossible to “undo” removed patina.