What is the Pencil Scale

The pencil scale is a test of hardness that gives an impression on how hard a certain coating is. The test is done by pressing a pencil with a certain hardness firmly on the surface on a 45 degrees angle. The highest grade that does not make permanently mark the surface is the score for the pencil scale.

Different types of determining the hardness

In the world of detailing it is very common to see a coating with a hardness being mentioned. Some claim their product has a hardness of 10H, 9H or 8H. The higher the number, the harder the coating is. A harder/tougher coating would withstand more wear and tear and offers better scratch resistance. However, it is not always clear what scale is used to determining the hardness. The most common scales used for a test like this is either the pencil scale, of the Mohs scale. The Mohs scale is used to determine the toughness of a certain material, and goes from 1H to 9H. A diamond (the hardest material known) has a score of 10. This is often written as “10H”. Unfortunately, the pencil scale also has scores that as written in the same way. “9H” is the highest scale in the pencil scale, but is noticeably lower in comparison to the hardness of a diamond. The confusion with most consumers is often used by manufacturer because the customer thinks they are talking about the Mohs scale, even thought the actual number refers to the pencil scale.

Pencil hardness

Pencils are made with different grades. This is done because a pencil with a different grade gives a slightly different darkness when used while drawing. It also give the user more control over the shade by using more or less pressure. The difference is done by softening the graphite in the pencil. When a pencil with a very low hardness (grade 9B) is pressed on a hard surface (such as paintwork) the graphite will deform under the pressure. If the pencil is hard enough, it will permanently damage the surface it is pressed on while the pressure is transferred onto the surface underneath. This damage might be very small, depending on the pencil and the grade. Pencils are graded in 9 number and 3 letters. The range goes from 9H (hardest), 8H 7H, 6H, 5H, 5H, 3H, 2H, H, F, HB, B, 2B, 3B, 4B, 5B, 5B, 6B, 7B, 8B and 9B (softest).

Determining hardness via pencil scale

The pencil scale gets its score from the hardest grade pencil that does not leave a permanent mark on the surface when pressed firmly onto the surface under a 45 degree angle. Some claim the pencil has be moved, others claim it shouldn’t moved and only pressed down. There doesn’t seem to be a definition on what the correct technique is. For example, a panel of paintwork is coating with product X and is left to fully cure. A pencil with hardness HB is pressed onto the surface on a 45 degree angle, but even though the pencil breaks, it does not mark the surface. The test is repeated with increasingly harder pencils. Eventually a pencil with hardness 5H is used, but leaves a very slight mark on the surface. The 5H pencil is the first one to permanently damage the surface. Therefore, the highest hardness grade on the pencil scale would be 4H. Because this is the hardest pencil that did not leave a permanent mark on the surface.
When a surface is so hard that even a 9H pencil is (by far) not capable of scratching the surface, the material will still get a score of 9H, as this is the highest score possible on the pencil scale. However, some manufacturer like to exaggerate the performance and will claim it has a hardness grade of 10H or even 11H. In general, a manufacturer will do several different tests, but only publish the ones with the highest number, leaving out the type of grading, to use the consumers lack of knowledge.

Putting hardness in perspective

Even though certain manufacturers put claims on their products, stating it has a hardness of 9H or more, it doesn’t mean that it is harder then a coating that offers 6H. For example, if manufacturer A uses the pencil scale to determine the hardness, but manufacturer B uses the Mohs scale to determine the hardness of their coating. The coating with 6H will be much harder then the coating with 9H.
To put hardness into perspective, we would need to know how hard a pencil is on the Mohs scale.
In general, a pencil falls between 2 and 3 Mohs, depending on a few different variables. The softest pencil (9B) can sometimes be as soft as 1.5 Mohs, and can differ slightly per manufacturer and thickness of inner core. The hardest pencil can go up to 3 Mohs and is noticeably harder then most other pencils. To put this in perspective, a human finger nail is around 2.5 Mohs. So, even a 9H grading on a coating would be about 3 Mohs in comparison.
The problem with the pencil scale is that it doesn’t go higher then 9H, so a coating with a hardness of 7 Mohs would still score as 9H in the pencil scale. Seeing as 9H in the Mohs scale refers to toughened steel, it would be very implausible to believe that any pencil is hard enough to score this high. Even more so, general paintwork on a vehicle will score around 4 to 5 Mohs at best and will easily score 9H on the pencil scale. Seeing as the Mohs scale grows exponentially, a 7H hardness would be twice as hard as a 6H grade. A 8H would be twice the hardness of the 7H.