Although in workshop drawings, line-shading is rarely employed, yet where a design rather than the particular details of construction is to be shown, line-shading is a valuable accessory. Figure 295, for example, is intended to show an arrangement of idle pulleys to guide belts from one pulley to another; the principle being that so long as the belt passes to a pulley moving in line with the line of rotation of the pulley, the belt will run correctly, although it may leave the pulley at considerable angle. When a belt envelops two pulleys that are at a right angle to each other, two guide pulleys are needed in order that the belt may, in passing to each pulley, move in the same plane as the pulley rotates in, and the belt is in this case given what is termed a quarter twist.

It will be observed that by the line-shading even the twist of the belt is much more clearly shown than it would be if left unshaded.

An excellent example of shading is given in Figure 296, which is extracted from the American Machinist, and represents a cutting tool for a planing machine. The figure is from a wood engraving, but the effect may be produced by lines, the black parts being considered as simply broad black lines.

Fig. 295.

The drawings from which engravings are made are drawn to conform to the process by which the engraving is to be produced. Drawings that are shaded by plain lines may be engraved by three methods. First, the drawing may be photo-engraved, in which process the drawing is photographed on the metal, and every line appears in the engraving precisely as it appears in the drawing.

Fig. 296.

For this kind of engraving the drawing may be made of any convenient size that is larger than the size of engraving to be produced, the reduction of size being produced in the photographing process. Drawings for photo-engraving require to have the lines jet black, and it is to be remembered that if red centrelines are marked on the drawing, they will be produced as ordinary black lines in the engraving.

The shading on a drawing to be photo-engraved must be produced by lines, and not by tints, for tints, whether of black or of colors, will not photo-engrave properly.

It is generally preferred to make the drawing for a photo-engraving larger than the engraving that is to be made from it, a good proportion being to make the drawing twice the length the engraving is to be. This serves to reduce the magnitude of any roughness in the lines of the drawing, and, therefore, to make the engraving better than the drawing.

The thickness of the lines in the drawing should be made to suit the amount of reduction to be made, because the lines are reduced in thickness in the same proportion as the engraving is reduced from the drawing. Thus the lines on an engraving reduced to one-half the dimensions of the drawing would be one-half as thick as the lines on the drawing.

Drawings for photo-engraving should be made on smooth-faced paper; as, for example, on Bristol board; and to make the lines clean and clear, the drawing instruments should be in the best of condition, and the paper or Bristol board quite dry. The India rubber should be used as little as possible on drawings to be photo-engraved, because, if used before the lines are inked in, it roughens the surface of the paper, and the inking lines will be less smooth and even at their edges; and for this reason it is better not to rub out any lines until all the lines have been inked in. If used to excess after the lines have been inked in it serves to reduce the blackness of the lines, and may so pale them that they will not properly photo-engrave.

To make a drawing for an engraver in wood it would be drawn directly on the face of the box-wood block, on which it is to be engraved. The surface of the block is first whitened by a white water color, as Chinese white. If the drawing that is to be used as a copy is on sufficiently thin paper, its outline may be traced over by pencil lines, and the copy may then be laid face down on the wood block and its edges held to the block by wax, the pencilled lines being face to the block. The outline may then be again traced over with a pencil or pointed instrument, causing the imprint of the lead pencil lines to be left on the whitened surface of the block. If the copy is on paper too thick to be thus employed, a tracing may be made and used as above; it being borne in mind that the tracing must be laid with the pencilled lines on the block, because what is the right hand of the drawing on the block is the left hand in the print it gives. The shading on wood blocks is given by tints of India ink aided by pencilled lines, or of course pencilled lines only may for less artistic work be used. Another method is to photograph the drawing direct upon the surface of the wood block; it is unnecessary, however, to enter into this part of the subject.

The third method of producing an engraving from a drawing is by means of what is known as the wax process. Drawings for this process should be made on thin paper, for the following reasons: The process consists, briefly stated, in coating a copper plate with a layer of wax about 1/32 inch deep, and in drawing upon the wax the lines to compose the engraving, which lines are produced by means of tools that remove the wax down to the surface of the copper.

The plate and wax are then placed in a battery and a deposit of copper fills in the lines and surface of the wax, thus forming the engraving. Now if the drawing is made on thin paper, the engraver coats the surface of the drawing with a dry red pigment, and with a pointed instrument traces over the lines of the drawing, which causes them to leave a red imprint on the surface of the wax, and after the drawing is removed the engraver cuts these imprinted lines in the wax. If the drawing is on thick paper, this method of transferring the drawing to the wax cannot be used, and the engraver may take a tracing from the drawing and transfer from the tracing to the wax. It is obvious, also, that for wax engravings the drawing should be made of the same size that the engraving is required to be, or otherwise the tracing process described cannot be used. Figure 297 represents an engraving made by the wax process from a print from a wood engraving, and it is obvious that since all the lines drawn on the wax sink down to the surface of the copper plate, the shading is virtually composed of lines, the black surfaces being where the lines have been sufficiently close together and broad to remove all the wax enclosed within those surfaces.