The Pencil Hardness test is used in the film manufacturing, printing and converting industries to determine how well a hard coating will last. This test has been used for many years due to the immediacy of results and how inexpensive it is. Using leads ranging from 6B, the softest, to 9H, the hardest, the tester performs five strokes on the surface of the film. If the surface is not scratched, the tester will move on to a harder pencil until the coating is scratched. In order to be considered a hard coating, the sample must be able to withstand the scratching by a 3H pencil or better.
MacDermid Autotype conducted a study to determine the usefulness of the Pencil Hardness test and found that this protocol is not reliable. The results find that the test is not always repeatable and is not an accurate measure of a film’s properties. Even when the test is performed by trained individuals, following a strict guideline and using the correct grade of pencil, variations still occur. Those variations are more pronounced when the tests are performed by multiple people; don’t have a control, and when the testing is done on different types of surfaces such as glass or rubber. MacDermid Autotype also found that pencils can vary greatly between manufacturers. Another inherent issue with pencils is that they are not designed to be used in scientific testing, but rather for drawing or writing.
It is conclusive that the Pencil Hardness test is not the most effective way of basing development or purchasing decisions. It is recommended that manufacturers agree on a common standard for testing hardness and ensure that clients are aware of these issues. Clients are also encouraged to work with manufacturers to guarantee that the product meets their specifications. MacDermid Autotype also recommends using alternative testis such as the Taber test, as it has been found to be more relevant in the final application of film.