In Hotchkiss v. Greenwood (1850) 52 U.S. 248, the patentee claimed a patentable invention in his discovery of the utility of potter's clay or porcelain as materials for the manufacture of door handles instead of the customary woods. The Supreme Court sustained a lower courtinstruction that if the patent merely involved the substitution of the superior materials disclosed in a known process, then it was invalid for want of invention. The Supreme Court laid down the general rule: "The improvement consists in the superiority of the material, and which is not new, over that previously employed in making the knob. But this, of itself, can never be the subject of a patent. The difference is formal, and destitute of ingenuity or invention. It may afford evidence of judgment and skill in the selection and adaptation of the materials in the manufacture of the instrument for the purposes intended, but nothing more. Unless more ingenuity and skill . . . were required . . . than were possessed by an ordinary mechanic acquainted with the business, there was an absence of that degree of skill and ingenuity which constitute essential elements of every invention. In other words, the improvement is the work of the skillful mechanic, not that of the inventor." (52 U.S. at 266-267.) The rule in Hotchkiss, in essence, is that a patentable invention must evidence more ingenuity and skill than that possessed by an ordinary mechanic acquainted with the business. The Supreme Court held that the mere substitution of materials was not patentable. Hotchkiss et al. v. Greenwood, frequently referred to as the "Porcelain door knob case," the court said: "for unless more ingenuity and skill in applying the old method of fastening the shank and the knob were required in the application of it to the clay or porcelain knob than were possessed by an ordinary mechanic acquainted with the business, there was an absence of that degree of skill and ingenuity which constitute essential elements of every invention. In other words, the improvement is the work of the skilful mechanic, not that of the inventor." Unlike novelty and utility, which were prerequisites to patentability mentioned specifically in the Patent Act of 1793, the requirement of nonobviousness was of judicial origin, first appearing in the case of Hotchkiss v. Greenwood. In 1952, Congress codified the Hotchkiss doctrine in section 103 of the patent revision act of that year.