Take a look at the Wiki on the page Mystic Forge. It will tell you where the forge is, how it is used, and some predictable recipes. The forge lets you do two things. First you can create recipes to craft other items like consumables, weapons, or armor. Second, the forge can be used to create better crafting materials of one type higher.
The examples the wiki gives are as follows:
Common Crafting Material Promotion
Common crafting materials can be promoted to a small pile of higher
tier materials with the following formula. The tier of dust needed is equivalent to the tier of the wanted product. So, if you are hoping to create Orichalcum, a tier 6 common material, you need to use Crystalline Dust, a tier 6 dust. If common crafting material B is one tier higher than common crafting material A, then:
10-50 Crafting Material B = 250 Common Crafting Material A (i.e. 250 Mithril Ores) + 1 Common Crafting Material B (i.e. 1 Orichalcum Ores) + 2-5 Dust A (i.e. 5 Pile of Crystalline Dust) + 5 Philosopher's stone
In that example: That recipe will produce 10-50 Orichalcum Ores
Armor boxes and armor satchels
Recipes can be created using the Mystic Forge. They will allow you to
craft armor boxes and armor satchels, which will in turn give you
every piece of an armor set.
Armor set recipe
Ingredients
5 Bottle of Elonian Wine
20 Arcane Crystal
50 Mystic Coin
1 Armor item
Mystic coins seem to be a common item needed for Food Trays, Weapons and Armor. Also look at this page for current recipe research.
You can see this in action with this video on how to make a Giant Chocolate Cake. Details are in the video summary.
There is no direct way to convert the karma into coins as all the items you trade with karma are immediately soulbound on acquire. However there are some important uses for karma.
If you ever want to level your cooking you will need to save some to buy recipe items. Although the cooking items aren't worth much on the trading post, the magic find bonus will give you better drops and the stat bonuses could effectively keep you alive when otherwise you wouldn't have. In this way you indirectly make money by not having to fix your armor. A copper saved is a copper earned.
Another important use for karma is to obtain recipes for crafting. With the exception of cooking, you need to buy recipes from all the master crafters in order to reach level 400 crafting. The crafting only reveal insignia/inscriptions up to level 350. Once you reach about level 380, all the recipes are grayed out and you can't earn any experience unless you use the recipes from the master crafter that you need karma to buy.
You can use it to buy armor and weapons and accessories from karma traders around the world to supplement your gear if any of them fall behind.
Please note that you should not use your karma to buy cultural weapons. The cultural weapons do not go towards any title and is only rare not exotic so it is not max stat. Since they cost 63k each, it is not an effective way to spend karma.
However outside the Gate of Arah, you are able to buy maxed Exotic Armor pieces for 42k karma a piece. I find this the most effective way to spend your karma. As of now, a piece of Exotic armor is equivalent to about 2.5g gold coins. If you save up the karma you can buy a complete exotic armor set and save about 15g you would otherwise have to directly spend.
Best Answer
Short answer: As it seems for me only for the Runes of the Engineer.
These are the recipes I found who uses Iron Ore:
I haven't found othere recipes for it, but maybe another user may know more than me.
EDIT: Well, just look on the Iron Ore wiki page and look for Recipes -> Leatherworker. Seems as my mentioned runes are the only recipes that need Iron Ore.