Chocolate – To thin chocolate candy coating, how much oil to use

You'll want to temper the melted chocolate. This means holding it at 88° F (31° C) while it cools until it is completely hardened, then finally allowing it to return to room temperature (or lower) afterward.

If you temper the chocolate this way, it will be much harder and last indefinitely at room temperature.

The easiest way to temper chocolate is the "seed method", which means adding a piece of solid (never-melted) chocolate to the fully-melted chocolate after removing it from the heat source and stirring thoroughly until the temperature reaches 88° F / 31° C, and then keeping it at that temperature until you're ready to use it.

(Note that this is for dark chocolate; for temperatures for other types of chocolate and for a more detailed guide to tempering, I suggest you check out the tempering page at Cooking for Engineers.)

_{The technical explanation for this is that the temperature at which melted chocolate hardens determines its final consistency as a solid. If chocolate hardens at room temperature, it ends up forming a very weak crystalline structure that begins to melt at even the slightest rise in temperature (i.e. in your hand). Allowing it to solidify at the highest possible temperature below the actual melting point ensures that it forms a denser crystalline structure and will not melt at room temperature.}

How do you make chocolate thin enough to dip strawberries?

Melt it. I'm not trying to be funny here -- you should have no problem dipping strawberries or pretty much anything else (that's solid) in melted chocolate. When candy makers coat things in chocolate, that's all they're using -- they don't add anything to make the chocolate thinner. If you're really having trouble in this respect, then perhaps your chocolate isn't fully melted, or perhaps it's something other than real chocolate.

If you want the chocolate to be "tempered" when it hardens/dries/crystalizes, you're going to have to read up on tempering and be pretty careful with the temperature (but you'll end up with a superior product if you do).

If I make it thin enough to dip (with butter) will that affect allowing it to set-up and harden?

Yes, that will definitely affect the way the chocolate sets up. When chocolate hardens, the cocoa butter contained in the chocolate forms crystals. The crystals can actually take several different forms, and only one of those forms makes for the best appearance and hardness. That's why you temper chocolate, btw. Adding other substances, particularly another fat, will affect or prevent that crystallization process.