Because paper is composed of a randomly felted layer of fiber, it's structure has a varying degree of porosity. Thus, the ability of fluids, both liquid and gaseous, to penetrate the structure of paper becomes a property that is both highly significant to the use of paper. The need to limit the spreading of ink resulted in "sizing" the paper with gelatinous vegetable materials which had the effect of sealing or filling the surface pores. Later, the term "sizing" was applied to the treatment of paper stock prior to the formation of the sheet, with water-repellent materials such as rosin or wax. Resistance towards the penetration of aqueous solution / water is measured by Sizing or Cobb values.
The Cobb size test measures the amount of water a specified area of paper will absorb in a given length of time. In this test, a circular metal ring is clamped on a weighed sample of paper backed by a rubber-covered metal plate. A measured volume of test liquid is poured into the ring, allowed to stand for a preset time, and then quickly poured off. The metal ring is removed, and the sample is blotted to remove all free liquid. The wet sheet is then weighed. The results are reported as the weight of liquid absorbed per are of paper surface, usually grams per 100 cm². The test is very satisfactory for paperboard and for many types of paper.
- Scott, William & Stanley Tosset, "Properties of Paper: An Introduction", p. 105-106, 1989.
- TAPPI T 441 om-13, "Water absorptiveness of sized (non-bibulous) paper, paperboard, and corrugated fiberboard (Cobb test)"