PENCIL HARDNESS TEST -

Pencil hardness is a popular test in the paint industry. It is used to determine scratch (mar) and/or gouge hardness of coatings. Its popularity is due to two of its basic strengths; it is inexpensive and it is quick.  The amount of pressure used and the angle of the pencil to the substrate affect the results.

The simplicity of the pencil hardness test is its great advantage. The special pencils used for doing the test are made of various combinations of graphite and clays, baked in ovens to harden their compositions.  The ambient temperature where the tests are done may have an effect on the hardness of the coatings, as this can be a factor in the drying times. Some coatings may get harder after a week's time and, if retested, would show a higher pencil hardness rating after more time elapses.

This test method covers a procedure for rapid, inexpensive determination of the film hardness of an organic coating on a substrate in terms of drawing leads or pencil leads of known hardness.  The test is especially useful in developmental work and in production control testing in a single laboratory.  Obviously, the results may be inconsistent from one operator to another because the pencils are pushed at different pressures.

Most coatings are formulated for specific types of finishes, various conditions or different substrates. So the pencil hardness test is to be used as one criterion for selection. However, we should not judge any coating by pencil hardness alone, as there are many other significant characteristics to consider.

A coated panel, such as that in many ASTM Standard Tests, is placed on a firm horizontal surface like a laboratory bench.  The pencil is held at a 45^o angle and pushed away from the operator in a ¹/₄-inch (6.5 mm) stroke.  The process is repeated from the hardest lead to the softest lead to reveal which is the hardness of the coating. 

Although, some people may consider it to be low tech, using pencils to test hardness is still a method used by many coatings manufacturers and some finishing shops. The test uses special pencils with different degrees of hardness to scratch the coating, which then determines its hardness.