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Testing Test for Water Penetration (CobbTest)

Test for Water Penetration (Cobb test)

General

For carrying out a quick performance test for resistance to feathering, write on the surface of a test piece with a latem pattern nib and blue-black superior writing ink (conforming to IS: 222-1962). For testing papers which have to stand erasure, such as ledger papers, repeat this test on the same spot after ensuring the original writing. Examine the edges of the writing for definition. The presence or absence of feathering (the Irregular spread of ink on either side of the written line) shall be reported. It is indicative of the quality of the sizing.

Apparatus

Any type of apparatus which permits the following may be used:

a) An immediate and uniform contact of the water with the part of the test piece submitted to the test, and

b) A rapid withdrawal of the test piece without the risk of contact with the water outside the test area by means of a suitable fixing device which makes it possible to maintain the times envisaged in Table 3.

All the components constituting the apparatus which are in contact with the water or may have contact with it shall be made from materials not liable to be affected by the liquid.

The apparatus for this test consists of a short metal cylinder with a cross-section 100 cm2 (11. 29 cm internal diameter) and height about 5 cm, capable of being damped on to the surface of the test piece. It is necessary initially to check the internal diameter of the cylinder. A cylinder of a different diameter may be used provided a suitable correction is made for the difference In area. The thickness of the wall of the cylinder is not important, but may conveniently be about 6 mm; the lower edge shall be machined smooth. The paper to be tested is placed on a baseboard, backed by a piece of sheet rubber. The cylinder is then placed on top of the paper and damped down firmly.

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