I was an art major, and did intaglio prints. As I remember, a zinc plate was coated with a tarry-like substance that dries somewhat. Then the design is scribed through the coating. The result looks kinda like an old film negative, with the areas to appear dark shiny silver, and the areas to apprear white dark. The plate is then put in an acid bath where it "bites" into the areas that had been scribed. The plate was then cleaned with a solvent to remove the asphaltum. Ink was brayed on the surface and then polished with a cloth that removed the ink from the "unbitten" areas. A strong paper is laid on top of the plate, and the whole mess goes through a high pressure wringer kinda press, and voilà !