The designation given to a sheet of paper in terms of weight of 500 sheets (one ream) in the standard size for that grade:
Offset - 25" x 38"; Reprographic - 17" x 22"; Tag - 24" x 36"; Vellum Bristol - 22 1/2" x 281/2".
The basis weight of a typical grade of reprographic paper would be 20 lb. Bond (indicating 500 sheets, 17" x 22" weighing 20 pounds).
The caliper (or thickness) of paper at a given basis weight determines its bulk and is affected by processing done to get smoothness and porosity. Variations in caliper across the sheet will cause problems such as loose edges, baggy areas and misregister.
Caliper is measured by a micrometer having a foot diameter of 5/8" which is clamped against the sheet as 7 p.s.i. pressure. Results are reported in ten-thousandths of an inch.
A caliper reading is usually the average of several tests across the sheet.
The smoothness is a measure of paper surface irregularities. The property affects many end uses, particularly the appearance of printing. The test is conducted by clamping the paper against a flat surface and measuring rate of air flow passing between the two surfaces.
The Sheffield Instrument is the most commonly used, a higher number indicating a rougher sheet. Some typical smoothness values are: 10-30 for very smooth coated paper; 100-150 for smooth offset and 200-250 for Vellum offset.
The porosity is an indication of the openness of paper, as measured by resistance to the passage of air through the sheet.
Two types of instruments are generally used to measure porosity - Gurley and Sheffield. The Gurley Instrument measures the seconds required for given volume of air to pas through a single sheet of and is generally used for porous papers. A high reading indicates a less porous (or more dense) paper. Sheffield porosity measures the flow rate of air through a single sheet and is generally used for non-porous or dense sheet. A high Sheffield reading indicates a more open paper. A typical Gurley porosity test for 50 lb. smooth offset would be 10-20 seconds. Sheffield readings of 60 lb. coated paper would be 10-20 units of air flow.
The moisture content of paper is affected by both the drying conditions when made and the relative humidity environment in which it is used. Ideally, paper should be made with a moisture content in equilibrium with the relative humidity where it will be used. Of course, environmental conditions vary at different places and times of the year. Ideal conditions seldom occur. If paper is drier than its environment, it will pick up moisture; and if lower it will lose moisture. Under extreme differences, these changes in moisture level will cause dimensional changes in the paper and cause problems such as curl, warp and waviness. Ideally, paper is in balance with an environment of 740F and 50% relative humidity.
To achieve accurate and reproducible results, exacting test procedures are followed to determine paper moisture. The sample is weighed, dried in an oven at 1050C and then weighed dry. The difference in weight is the amount of water in the sheet removed by drying. Paper is generally made to contain between 4% and 7% moisture.
(Dennison) is a measurement of paper surface strength. This test evaluates surface bonding strength and relates to the tendency for tacky inks to pick fibers or particles from paper surface.
To test for these properties, a series of waxes of varying hardness are melted, placed on the sample, and, after cooling pulled off. The softest wax (lowest number) that removes surface fibers designates wax pick number. A typical wax pick range for uncoated printing paper is 10-12.
Aside from its aesthetic importance, paper brightness affects the legibility and contrast of printing. The brightness test measures the reflectance of paper under strict optical conditions and relates it to a white standard (Magnesium Oxide). This test is mainly applicable to white paper grades. Basic offset grades are usually in the 80% brightness range.
Opacity is the lack of transparency that allows a sheet to conceal print on its reverse side. Opacity is greatly influenced by basis weight, brightness, type of fiber and filler. In testing, reflectance of paper is measured when backed successfully by a white body and a black body. The ratio of these two measurements determines the opacity reading. Typical opacity of 50 lb. white offset is 88-90; 20 lb. business paper is 84-88.