The paper commonly used becomes brown and brittle within five or ten years, so that records on such paper are likely to be useless in a comparatively short time. The high-grade paper used for record cards as are shown in Fig. 205 and Fig. 206, contains only the finest selected linen stock and is guaranteed absolutely against any deterioration for twenty years. The chief advantage of this paper, however, is that it can be used in card form, yet be transparent enough to allow of blueprinting.

The time required to blue-print this paper may best be compared with the time necessary when ordinary tracing cloth is used. Blue prints are also frequently made from bond paper. Bond paper requires an exposure of, roughly, three times as long as tracing cloth. This special card requires an exposure of about six times as long as tracing cloth, or twice as long as bond paper.

When sunlight is used for blue-printing, there is no difficulty in getting a sufficiently long exposure to make good blue prints from the card forms shown in Fig. 205 and Fig. 206, if the forms are printed on special paper. Should electric blue-print machines be used, however, it may be found that certain of the older types of continuous printing

262 GRAPHIC METHODS

machines cannot be run slowly enough to give the required length of exposure. Some of the older machines can be changed at rather slight expense so as to have an extra belt-pulley reduction between the motor and the blue-printing machine itself. The newer types of machine can usually be run slowly enough to give the exposure necessary for this special paper. If a new blue-printing machine is to be ordered, however, it is well to make some preliminary tests with the cards.