The thickness of the finished fabric shall not exceed 0.30 millimeter (0.012 inch) or be less than 0.20 millimeter (0.008 inch). The tensile strength of the gray cloth shall be not less than 18 kilograms per centimeter (100 pounds per inch) of width in the warp, and 9 kilograms per centimeter (50 pounds per inch) in the weft. The average value for the warp and weft in the finished fabric must show an increase over the average value for the warp and weft in the gray cloth of at least 10 per cent of the strength of the gray cloth.
The finished fabric, when dry, shall weigh not more than 250 grams per square meter (0.5 pound per yard), or less than 200 grams per square meter (0.4 pound per yard).
ABSORPTION OF MOISTURE
At a normal relative humidity of 65 per cent it shall not absorb more than 5 per cent of moisture, or expand (an average of both directions) to exceed 0.45 per cent, and when subjected in a closed case to a saturated atmosphere for two hours at a temperature of 20° C. (68° F.) shall not absorb more than 10 per cent of moisture or expand (an average of both directions) to exceed 2 per cent. All increases to be computed on the basis of the dry weight and dimensions.
FOLDING ENDURANCE
The folding endurance, as determined by the Schopper folder, shall be not less than 65,000 double folds for the warp and not less than 10,000 for the weft.
COLOR
Before coating, the fabric must be dyed with a purely mineral or inorganic color (such as iron salts), so as to imitate the color of the finished cloth, but somewhat lighter in shade, so as to give the desired "linen" effect. All coal-tar, aniline, vegetable, or other organic dyes and lakes must be strictly excluded from both fabric and coating, even in traces; except where necessary to match color, lake may be used in red, blue, or brown. The following substances must be also excluded: Chrome yellow and other chromates, ultramarine, browns containing bituminous or other organic matter, compounds of lead and arsenic.
The color of the finished fabric should be slightly darker than the standard sample, which will be furnished on application, and it must not show the slightest change of color after exposure under prescribed conditions to direct sunlight, or to the action of ammonia gas, sulphuretted-hydrogen gas, sulphur-dioxide gas, or illuminating gas.