the paper must not be used where a permanent colour is required. Some tints will bleach completely with the treatment, and should be avoided as stock lines. To test water-fastness a piece of paper is left in warm water, or placed in cold water and heated slowly. If the colour is soluble it will very soon tint the water.
Coloured papers for pasting to book covers or boxes should be tested by pasting down to the boards intended for use. Some boards have a curious effect on certain tinted papers, owing to the presence of chemicals in the finished boards, and acidity or alkalinity may render change of covering paper necessary.
Absorbent Papers.—Blotting paper may be tested by the mounting, test. To carry this out, cut strips from each direction of the paper—length and width—6 inches long by 1 inch in width. Make a pencil mark half an inch from the end, and immerse the strip as far as the pencil mark in water or ink. The fluid immediately commences to mount the strip, and the speed at which this takes place is an indication of the relative initial absorbency of papers tested by this method. In practice, blotting paper must absorb immediately, as the pressure usually applied will, if the paper is not sufficiently absorbent, spread the ink. For this reason the rising of the fluid should be marked and checked in the first ten to sixty seconds, and when several tests in each direction have been made, the figures tabulated and the mean rate calculated. It is convenient to measure in millimetres rather than in fractions of an inch. Ink makes the better testing fluid, as the way in which the blotting paper carries up the colouring matter can be seen at once, and a paper which is superior in this respect will usually be the
