For special purposes of illustration drawings are made which must be
tinted. In such cases the paper must be expanded and stretched evenly
all over its surface; otherwise when the moist tint is applied the paper
will wrinkle and get out of shape; to do this cut the paper at least
half an inch less in size than the drawing board; lay the paper face
down, turn up a margin or edge of about three-fourths of an inch all
round, then dampen the paper with a sponge and clean water; allow it to
soak for a few minutes until it is evenly dampened or moistened all
over, turn the paper upside down (face up).
Apply strong paste to the under side of the margin all round; rub down,
on the drawing-board, working from the center of the board outwards so
as to exclude the air and prevent creases or furrows. The board is then
inclined and left to dry slowly; make sure that the paper is all well
pasted and every part of the edges attached to the board.
If tracings are required to be tinted or shaded, the color may be
applied before the tracing is cut off, or what is more usual, the color
may be applied on the back of the tracing; then there is no liability to
wash out the lines.
Mechanical drawings are seldom tinted, but are mainly produced in India
ink. Where, however, a fine effect is desired, working drawings are
colored, so as to show at a glance the material of which the different
parts are to be made.
The colors required are few but should be of the best quality. Besides
India ink the following water-colors are generally used:
1, Neutral-tint. 2, Prussian Blue. 3, Chrome Yellow. 4, Gamboge. 5, Raw
Sienna. 6, Carmine. 7, Vermillion. 8, Venetian Red. 9, Sepia. 10,
Indigo. These come in hard cakes.
Certain colors and tints represent different metals and materials as
follows:
Wrought Iron--Prussian Blue.
Steel--Carmine and Prussian Blue, mixed to give a purple shade.
Steel Casting--Same as the above darkened by Venetian Red.
Cast-Iron--Neutral Tint made of India Ink, indigo, mixed with a little
carmine.
Brass--Gamboge or Chrome Yellow.
Babbitt--Emerald Green; sometimes light mixture of India Ink.
Copper--Purple Lake.
It is sometimes found necessary to prepare a highly finished and shaded
drawing of the work in hand. Such elaborations, in fact, are much
admired by the uninitiated, although the complete shading of the drawing
is no criterion as to the scientific value of the machine. An
illustration of this is told in the note.
NOTE.--A consulting engineer had to lay before a board of directors
plans of horizontal engines for their consideration. One of these
drawings was of a very superior machine, but being only depicted
lineally was at once rejected by them, for a highly finished
representation of a very inferior apparatus. The engineer, wishing to
induce the board to decide for the best, suggested that the matter
should be postponed to a future day, and in the meantime had the
drawing of the superior machine highly colored and finished. At the
next meeting the directors unanimously decided that this was the very
one which they preferred and had chosen.
Reproducing Drawings.
When once finished, one or more copies of drawings are frequently
required; these are produced, 1, by blue printing, as described before;
2, by tracing. A tracing is a mechanical copy of a design or drawing,
made by reproducing its lines as seen through a transparent medium--as
tracing-cloth or tracing-paper.
Tracing-cloth is a thin linen fabric, coated with size; this is called
_tracing-lines_; _tracing-paper_ is so prepared as to be transparent, so
that it will receive marks either in pencil or with pen and ink.
Tracing-cloth must be fastened to the board, over the drawing, by pins
or other tacks; moisture or dampness should be carefully avoided and the
drawing done on the smooth side of the cloth.
When tracing cloth will not take ink readily a small quantity of pounce
may be applied to the surface of the cloth and distributed evenly with a
piece of cotton waste, chamois, or similar material, but the pounce
should be thoroughly removed--by washing--before applying the ink.
In making tracings the same order is followed as described under the
section “Inking”--to repeat: 1, ink in the small circles and curves; 2,
ink in the larger circles and curves; 3, then all the horizontal lines,
beginning at the top of the drawing and working downward; 4, next ink in
all the vertical lines, commencing at the left and moving back to the
right; 5, draw in the oblique lines; 6, all the center lines red
(carmine), and dimension and reference lines in blue (Prussian blue) or
_vice versa_. The figuring and lettering should always be done with
India ink, thoroughly black.