There are several ways to prepare the metal to enamel
The purpose is to clean the metal from grease and dirt.
You can preheat the metal or put it in a degrease solution.
Copper has the property to sweat grease.
So you have to degrease it again if you do not use it after 1 day

Sanding	CLEAN THE COPPER This is the first step. Use sandpaper, a sanding block or whatever you like to use to get the copper bright and shining. All the dirt has to be removed. Do not touch the copper with your bare fingers, to prevent the copper to get grease from your fingers.  After you are done, you can pick any of the ways described below to degrease the metal.
Degrease with a kiln	DEGREASE INTO THE KILN Burn the copper approximately 1 minute in the kiln and you can see to get it a light blue haze.  Get the copper out of the kiln en throw it into a bowl of cold water and the blue haze will loosen from the copper.  Do not touch it after this with your bare fingers.  Sand the copper clean again, use a sanding block or sandpaper.  When the copper is clean again I put it again in a degrease solution (see later in this chapter).  After this, the copper is ready to be enameled
Degrease copper	DEGREASE SOLUTION Sand the copper on both sides till the copper is bright shining and dirt spots are gone.  Take 1 tablespoon salt and put it into 1 cup of household vinegar. Place the copper in this solution and stir. Let it stand for 15 minutes and occasionally stir.  Rinse the copper with water and check if the water forms a film over the copper.  If the water forms drops it is not decreased proper. Dry the pieces with a paper towel. The copper is ready to enamel now.

CILLIT BANG

This is a household degrease solution in a spray bottle. You can buy it at the supermarket.

Clean and sand the copper piece.
Put the copper in a plastic bowl.
Spray the degrease solution over the copper and stir for 1 minute.
Rinse throughout with water and dry wit a paper towel.
Handle now the copper only with the tweezers.
The copper is ready to enamel now.