The Pencil Hardness Gage is practical for use in the laboratory, or the production line, or in the field to assess quantitatively the rigidity or firmness (elastic modulus) of organic coatings applied to rigid substrates such as metal. Hardness values may define requirements for particular coatings applications, or may be used to evaluate state-of-cure or aging of a coating.

In this test, pencil leads of increasing hardness values are forced against a coated surface in a precisely defined manner until one lead mars the surface. Surface hardness is defined by the hardest pencil grade which just fails to mar the painted surface.

The gage is composed of a set of eight mechanical drawing lead holders (pencils) permanently mounted in a circular array on a plastic cylinder. A small metal tube through the center of the cylinder provides storage for spare leads and is a guide for positioning pencils for a test. Alternate pencils in the assembly point in opposite directions, so that in either direction four pencil tips define a square around a guide tube, and each pencil may be precisely positioned instantly to perform a test. Pencil positions are identified by numbers 1 to 8 on the plastic holder.

The Pencil Hardness Gage is unique only by the fact that it provides a compact single unit for performing this test rather than a set of easily lost individual pencils. The importance of this advantage should not be discounted, especially when several individuals may be working with a single instrument. Leads supplied with the unit, softest to hardest, are as follows: 6B, 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H. The leads mounted in the holders, as supplied, are:

Remove the gage and sanding disc from the carrying case. Using the disc face as a guide, adjust the exposed lengths of lead uniformly to approximately 1/8". Square the ends of the leads, four at a time by gently rotating an abrasive paper supported by the plastic disc until all squared edges of the leads are sharply defined. This completes the preparation for testing.

Begin testing using the hardest pencil (normally mounted in position number 8). Grasp the holder firmly and bring the metal tube end down onto the test surface. Rotate until the selected pencil is nearest the operator, and then incline the assembly downward until the lead point and the tube end are simultaneously in contact with the surface. This defines the correct lead angle of 45° to the surface. Push the gage forward (away) about one-half inch. Observe the pencil track. Sufficient pressure must have been applied either to cut or mar the film or to crush the sharp corner of the lead. If neither marring nor crushing is observed, repeat the test with greater pressure applied until a definite observation is made. If crushing of the hardest lead should occur, the film is extremely hard and is beyond the measuring range of the test. If scratching or marring of the film occurs, proceed with the next softer pencil grade and repeat the testing process until a test lead is found which crushes and does not mar the film.

Confirm the result with duplicate observations of the last (crushing) lead and the next harder (marring) lead.
In addition to the mar or scratch hardness described above, ASTM Specifications D3363 defines a “Gouge Hardness” as “the hardest pencil that will not cut through the film to the substrate for a distance of at least 1/8 inch.” This severe test is more applicable as a service simulation of coatings expected to receive heavy mechanical abuse.

1. Smith, W. T., “Standardization of the Pencil Hardness Test, “Official Digest”, 28, p. 232, ff (1956).
   
2. ASTM D3363, Method of Test for Film Hardness by Pencil Test.
   
3. Pencil Hardness Tests. Sherwin Williams Industrial Test.
   
4. NACE, TPC Publication No. 2 “Coatings and Linings for Immersion Service”, p. 22, (1982).