The most famous and often-quoted passage is from the autobiography of William Holman Hunt, one of the founders of the Pre-Raphaelite Brotherhood. In this, Hunt describes the use of transparent glazes over a wet ground and the addition of the resin copal to the paint medium.
"Select a prepared ground originally for its brightness, and renovate if necessary with fresh white when first it comes into the studio; white to be mixed with a very little amber or copal varnish. Let this last coat become of thoroughly stone-like hardness. Upon this surface complete with exactness the outline of the part in hand."
"On the morning for the painting, with fresh white from which all superfluous oil has been extracted by means of absorbent paper, and to which again a small drop of varnish has been added, spread a further coat very evenly with a palette-knife over the part for the day's work, of such consistency that the drawing should faintly shine through. In some cases the thickened white may be applied to the forms needing brilliancy with a brush, by the aid of rectified spirits."
"Over this wet ground, the colour (transparent and semi-transparent) should be laid with light sable brushes, and the touches must be made so tenderly that the ground below shall not be worked up, yet so far enticed to blend with the superimposed tints as to correct the qualities of thinness and staininess which over a dry ground transparent colours used would inevitably exhibit. Painting of this kind cannot be retouched except with an entire loss of luminosity."
Hunt, WH 1905-1906 'Pre-Raphaelitism and the Pre-Raphaelite Brotherhood'. London: Macmillan & Co Ltd., Vol 1 p276
Claudio and Isabella. (1850) by William Holman Hunt. Oil on mahogany. support: 758 × 426 x 10 mm frame: 997 × 668 × 80 mm. [Source: Wikimedia ]
This reminds me of the fresco technique where the ground is applied on a small surface that the artist can work during his session. I am sure Hunt read Leonardo da Vinci’s advice on colurs: “For those colours which you wish to be beautiful, always first prepare a pure white ground. I say this in regard to colours which are transparent, because those which are not transparent do not benefit from a light ground. An example of this is shown by coloured pieces of glass which, when interposed between the eye and the luminous air, reveal themselves as exceedingly beautiful and this they cannot do when they have behind them a shadowiy air or other darkness.”
Hunt’s technique seems very constrained and dictated the very detailed and delicate approach of some of the Pre-Raphaelite paintings. It worked very well to get luminous colours that stood the test of time. Pre-Raphaelite paintings in museums look as fresh today as when they were painted. The drawback was less spontaneity in the execution.