Internal carbon particles generate internal stress sites that are ripe for breakage. However, cast iron can be welded, because the heat from the welding process softens the metal enough to allow for its formation back into shape. The carbon content of cast iron must be high enough to give it hardness and durability but not so high as to make it brittle. The amount of carbon required depends on how the cast iron is to be used. For example, if you need a pan that will not bend even under heavy use, then you should use iron with a higher carbon content than if you were making a skillet that would only be used to cook light meals. The best way to tell how much carbon is in cast iron is with a carbon content test. There are several reputable companies that offer this service, usually for about $10-20. As long as you use proper care of your cast iron, it will last a lifetime. However, like any other tool, it does require maintenance to keep it in good working order. The most common problems with cast iron include: rust, mold, and holes. Rust is easy to remove with a wire brush, dish soap, and water. Mold also requires regular cleaning, but not as harsh chemicals as for steel.