These prints are made frome an engraving technique called "Eau forte". A varnished layer is applyed on a metal block (here, a copper sheet) and the drawing is made with a blunt tool. Then, the block is immersed in an acid bath and is engraved wherever the varnished has been removed. 

 

By using some more varnish and multiplying the acid bath, you can produce a gradation when you later apply the ink. The more engraved the block is, the darker the part of the drawing will be. 

 

The first print was printed with black ink on a thick moisten sheet of paper. 

For the second print I used sepia ink. It is darker than it should be because I didn't clean my copper block in-between prints.

And here is the copper block after its first acid bath, when the drawing strokes have been engraved. It needed three more acid bath to create the shading on the cats' faces. 

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