What is the best way to store collectible coins so they will retain their value?
There are special cases made especially for storing coins. The fancier version has a clear top and a soft lining inside with an impression for the coin to fit into. Sometimes it's foam, sometimes it's a velveteen type of lining. The cases are very thin, maybe 1/2 an inch thick. A lot of coin dealers sell these on the internet and you might even be able to find a place where you can buy them wholesale, if you have a lot of coins. There are also little envelopes made if you wanted to go a cheaper route. They are manilla type, sometimes white, sometimes ecru or another color and they have a clear plastic window so you can view the coin. Then there are larger versions: like if you have a lot of old pennies for example...these cases are folder type cases that usually have more than two sides, some have three. They have a strong cardboard or other type of strong material for the outside cover and will open up into sections; two, three or sometimes more. Often the dates are printed in on them so you can place the corresponding coin in the correct place so they are in order by year. There are other types of storage for coins but the above are usually the most common. After all, I think you know this already: you want to store them so they do not rub together and cause more wear by friction (like storing them in coffee cans, etc.) Whatever you do DON'T clean your coins. Coins are one of the collectible items that lose value when they are cleaned. And if you have a lot of uncirculated coins, use extra care in handling. Body oils can cause damage and often, when a coin is uncirculated and you wish to sell it later collectors will be looking for mint condition, no damage, scratches, etc. If they are not already in a case (which is usually the way the special coins are stored and sold to consumers), it's probably best to use latex gloves (or the alternate version if you are allergic to latex. That way you won't have to worry about any body oil contaminating your pristine coins.) Hope this helps!! There are many products available from coin dealers to preserve your coins. There are the paper envelopes, 2x2 cardboard and mylar holders, and a myriad of special plastic cases. The main considerations are to use only acid-free products and minimize exposure to sunlight, heat, and air (and of course to minimize any handling). One thing to remember - when the coin is sold, you will not get any extra for the holder, so it is not economical to preserve a coin worth 25 cents in a holder that costs four or five dollars unless you expect its value to increase dramatically.
Assuming their dates are 1960 or later, your coins would be ordinary circulation coins that were in use up to early 2002 when they were replaced by euros. In early 2012, French banks definitely stopped to accept the old currency. They can only retain some value as collectible items. In that regard, coins issued after WWI have a value close to nothing, especially if the coin had been circulated.
To a collector of dimes, yes. To be collectible merely means that there is someone out there who would collect it. If you mean valuable that is different from collectible as not all collectible coins have much value. 1960 dimes are common and worth their melt value in silver about $2 right now. Exceptionally well preserved, or mint state coins will have an added premium worth in the $10 range.
There are really several answers to this question, dealt with below. Legal tender refers to lawful money issued by the central bank or mint of the country. In the US all coins ever issued by the Mint for commerce are still considered legal tender, worth their face value. In numismatic circles the word "coin" is limited to legal tender items, excluding many, but not all, bullion coins, especially those that, in place of a denomination…
Yes, but it depends on the coin, the year, the condition and the country of origin. Modern foreign coins are unlikely to bring a premium over the equivalent face value of them unless they are collector coins (such as gold maple leafs, gold pandas, Britannias, Kuggrands, etc.) but older foreign coins are collectible and can be worth quite a bit of money.
The first rule of coin collecting is "NEVER CLEAN A COIN" because if the coins have any collectible value, cleaning them kills it. I have seen many coins that have been reduced to face value from cleaning. Please show them to a collector or dealer BEFORE you clean them. You may be very glad you did.
In general the 1 Euro coins have no additional collectible value; they trade based on the current exchange rate with the US dollar. Some countries' Euros may have a slight additional collectible value, depending on the design, but the availability of the coins and their recent introduction hasn't produced much of a collectible/resale market yet, except for complete sets, especially first-year sets, of all the coins. The special-issue 10 Euro coins are a different story…
Don't clean any collectible coins. However, vinegar can clean outward corrosion off of a penny and strip off the outer layer of copper to make it seem shiny again. But please, if you have any coins that are even remotely collectible, don't clean them, it completely ruins the value of them, especially coins with little intrinsic values such as pennies.