Over time a cast iron fireplace grate can get scratched and marred from the use of tools. For most people this isn't a big deal, but it can be unsightly. Removing these scratches isn't the simplest process, but with some time and patience you can do this on your own.
Step 1 - Wash the Grates
Remove the grates and wash them to get rid of any residue left from the fireplace itself. Using a nylon scrub brush will help speed this process along. Use an old towel to dry them completely.
Step 2 - Use the Shortening
Use a standard cooking shortening for this step. If your fireplace grate will fit in your oven, this will make it even easier. If not, you can use other heat sources if you are careful. Apply a thin amount of shortening to the scratches. You shouldn't see any shortening built up, rather just use enough to make the scratched area slightly glossy. Heat the grate to 400 degrees, and allow it to cool completely.
Step 3 - Repeat as Necessary
If after the cooling process you still see some gouges, you will need to repeat this process again. For deep scratches it may take a few rounds before the scratches can't be seen again. Clean the grate thoroughly before placing it back in the fireplace and using it again.