Lectures on Landscape by John Ruskin

61. The distinctions between schools of art which I have so often asked you to observe are, you must be aware, founded only on the excess of certain qualities in one group of painters over another, or the difference in their tendencies; and not in the absolute possession by one group, and absence in the rest, of any given skill. But this impossibility of drawing trenchant lines of parting need never interfere with the distinctness of our conception of the opponent principles which balance each other in great minds, or paralyze each other in weak ones; and I cannot too often urge you to keep clearly separate in your thoughts the school which I have called[11] "of Crystal," because its distinctive virtue is seen unaided in the sharp separations and prismatic harmonies of painted glass, and the other, the "School of Clay," because its distinctive virtue is seen in the qualities of any fine work in uncolored terra cotta, and in every drawing which represents them.

[Footnote 11: "Lectures on Art, 1870," Sec. 185.]