It is not known how Goya learned the complex technique of etching. A publication of 1778 records him as an engraver, and his earliest etchings are thought to have been copies of Velázquez paintings in the royal collection. In making prints, Goyas most commonly used etching or aquatint, or a combination of the two. Etching was first used by 15th century armorers to create designs on metal by means of acid mordant; the technique was adapted for making prints in the early 16th century. The process involves covering a copper plate with a waxy, acid-resistant ground, then drawing a design in the ground with an etching needle, thus exposing the surface of the plate. Goya is known to have drawn his images on paper first and then transferred them to the plates. Once the design is etched, the plate (its back varnished for protection) is immersed in a bath of acid, which bites the exposed areas, embedding the design in the surface. The depth of the etched line depends upon the length of time the plate is submerged; the artist can re-bite lines that are too shallow, but the design, and even the plate, can be destroyed if left too long in the acid. Because etching involves drawing into a soft ground (rather than metal, for example, as in the case of engraving), the etched line is typically clean and can be delicate, intricate, and spontaneous--much like drawing.