All flat surfaces in a drawing should be lighter or darker, in accordance with their distance from the eye. In laying on flat-tints when the surface is not in shade, it must be borne in mind (1) that all surfaces which are parallel to the plane of the picture, and therefore equally distant from the eye, should receive a tint of uniform intensity; (2) that those surfaces which are farthest from the eye should receive the darkest tint; and (3) that surfaces which are inclined to the plane of the drawing should receive a tint of varying intensity, the depth of the tint increasing as the surface recedes from the eye. When the surfaces are in shade, the converse of these rules holds good.
_Conventional Colours._--In representing objects by means of colours, the natural colours of the objects are in some cases adhered to; and in others, for the sake of greater distinctness, a conventional colour is adopted. In engineering, architectural, and mechanical drawings, the latter mode is nearly always resorted to, while in plans of estates the former is very frequently employed. Unfortunately, practice is not uniform among draughtsmen in the conventional use of colours; but the following Table shows the colours mostly employed, and represents the general practice.
Carmine or crimson lake For brickwork in plan or section to be executed. Prussian blue Flintwork, lead, or parts of brickwork to be removed by alterations. Venetian red Brickwork in elevation. Violet carmine Granite. Raw sienna English timber, not oak. Burnt sienna Oak, teak. Indian yellow Fir timber. Indian red Mahogany. Sepia Concrete works, stone. Burnt umber Clay, earth. Payne’s grey Cast iron, rough wrought iron. Dark cadmium or orange Gun metal. Gamboge Brass. Indigo Wrought iron--bright. Indigo, with a little lake Steel--bright. Hooker’s green Meadow land. Cobalt blue Sky effects. And some few others occasionally for special purpose.
Sections are represented either by lines of the colour drawn with the pen or the point of the brush, or by a darker shade of the colour. In mechanical drawings, sections are frequently shown by ink lines drawn over the colour.
In plans and maps, as we have said, some attempt is made to give the true appearance of things. As this--which may be called the natural mode of representation--allows more scope for artistic skill than the conventional, a great deal must be left to the judgment and the taste of the draughtsman. But there are general principles and features that may be laid down and described, and such are the following:--