Tarnish
Metals need to be protected against rust and tarnish because rust and tarnish eat metals.
yes, a camphor block can slow silver tarnish
yes
No.
darker soft drink will remove the tarnish form the pennies
No, although hot water and washing soda in an ultrasonic cleaner seems to remove the yellow lustre silver flatware gets, but not dark tarnish.
There are some wonderful tarnish remover products on the market that you can purchase. An at home remedy for removing tarnish off silver would be to use a soft cloth and rub the surface with toothpaste.
The US Mint never issued a gold Roosevelt dime. Any such coin as been gold plated at some time after it left the mint. Also, a dime can tarnish to a golden hue if in the right environment.
MENOR!
It is probably bronze colored due to tarnish or some other discoloration resulting from contact with a chemical.
18 carat gold should have no tarnish whatsoever, under normal circumstances. It may have some grime or dirt, which can be removed with soapy water.
There is a type of cleaner that you can get at places like Wal-Mart specifically for removing tarnish from metals. I'm not sure what its called but I know of people that have used it for that reason.
Dark sodas remove tarnish better than light colored sodas because of the citric acid and high sugar amounts in the soda. The darker the soda, the more acid is put into the soda.
Tarnish.
Tarnish
A brown dime could just be tarnished or discolored. Different chemicals can cause a coin to tarnish in different ways, especially if it is silver. If the dime was minted after 1965, it is possible the nickel layer on the outside of the dime is missing. This could be due to an error at the mint in which case the dime is collectable. It could also be due to a chemical treatment that removed the nickel from the dime which is considered damage to the coin and is not collectable. I would suggest you take the coin to a coin dealer since it is always easier to identifiy a coin if someone can actually see it.