Cox, David: A series of progressive lessons intended to elucidate the art of painting: with introductory illustrations on perspective and drawing with pencil (London, 1845)

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sketch possessing the greatest freedom may be proceeded with, and if intended
to be finished in pencil, such as are of soft lead are most proper for the fore-
ground, whilst those of hard texture are to be used for the distances. The
lights must be preserved clear, and the shading worked up to the effect with
a free pencil. Endeavours must be made to acquire correctness, particularly
in the earlier efforts, as freedom of hand and facility can only be obtained by
practice. A minute attention to the accompanying progressive examples is
particularly recommended.
When the pencil drawing is finished, it may be fixed or secured by a thin
wash of diluted isinglass, lightly given with a flat camel hair brush ; or the
drawing may be pressed against a linen cloth, or folded blotting paper, soaked
with the solution of isinglass.