I'm going to tell you the making process. It goes like this: you have a piece of metal (it can be copper, aluminium, brass, etc.) that piece of metal must be sandpaperd until you can see your reflection. It must be like a mirror. Meanwhile you have to make primer. It is made from bitum and thinner, you have to let the bitum melt in the thinner for one hour and after that, it is ready to go. (Once the primer it's applied on the metal, it has to dry in less than a minute, if not... well, it's not ready to go :D )
So, the primer is applied (front and back), it dries out and you begin to draw on it with a needle. After you finish drawing, the piece of metal must be put in acid (do not laugh, speak or smile above the acid or, your teeth can fall :)) ). For very smooth lines it can stay in acid between 7-10 minutes.
We are done with the lines, let's begin with the shadows. With a brush (one that worths sacrificed...) you apply the primer on the areas that you want to protect. For an extreme black, let the metal in acid 10-15 minutes, for a fade grey just a few seconds (3-5-10).
The engraving is ready. Now everything you have to do, is to print it on paper.