These mixtures, which are more like liquid shoe blacking than writing fluid, were used, with immaterial modifications, by the scribes of the dark ages. Useful as they may have been for their methods of writing, they could not have been applied to the inking of a metal surface engraved in relief. If the brass stamps described on a previous page had been brushed over never so carefully with these watery inks, the metal surface would not be covered with a smooth film of color. The ink would collect in spots and blotches. When stamped on paper or vellum, the ink thereupon impressed would be of irregular blackness, illegible in spots, and easily effaced. Writing ink, thickened with gum, has but a feeble encaustic property. It will not be absorbed, unless it is laid on in little pools, and unless the writing surface is scratched by a pen to aid the desired absorption. The flat impression of a smooth metal stamp could not make a fluid or a gummy ink penetrate below the writing surface. It was, no doubt, by reason of the inferior appearance of impressions of this nature that the brass stamps described on a previous page found so limited a use. An unsuitable ink may seem but a trifling impediment to the development of printing, but if there had been no other, this would have been an insurmountable obstacle. The modern printer, who sees that the chief ingredients of printing ink are the well-known materials smoke-black and oil, may think that an ignorance of this mixture, or an inability to discover it, is ridiculous and inexcusable. Modern printing ink is but one of many inventions which could be named as illustrating the real simplicity of a long delayed improvement. Simple as it may seem, the mixing of color with oil was a great invention which wrought a revolution in the art of painting. This invention, attributed by some authors to unknown Italian painters of the fourteenth century, and by others to [p041] Hubert Van Eyck of Holland, at or about the beginning of the fifteenth century, immediately preceded the invention of types. The early typographic printers, who could not use the ink of the copyists, succeeded only when they mixed their black with oil. After four centuries of experience in the use of printing ink made with oil, and after repeated experimentation with impracticable substitutes, it may be confidently asserted that an invention of typography would have failed, if this use of oil had not been understood. The invention of types had to wait for the invention of ink. Typography had to wait for the invention of paper, the only material that is mechanically adapted for printing, the only material that supplies the wants of the reader in his requirements for strength, cheapness, compactness and durability. Paper was known in civilized Europe for at least two centuries before typography was invented, but it was not produced in sufficient quantity nor of a proper quality until the beginning of the fifteenth century.