From the title of this article I thought that I’d find out how to tell how hard were random pencils from around the house. This shows the danger of lumping nouns together and leaving the reader to sort them out. It isn’t “testing the hardness of pencils,” it’s “testing the hardness of things using pencils.”
If you are thinking of using a paint or plastic coating, you can test how hard it is by using a graded set of pencils. See “The Pencil Hardness Test” by Mac Simmons. You test the coating over wood.
…the pencil hardness test is only one of many tests used to evaluate coatings. There is obviously more than the hardness of any coating to be considered, so do not judge a finish entirely on the basis of this test.The test is very simple to do, will give uniform results, and is dependable because the pencils are graded. The grade of the pencil is determined by the amount of baked graphite and clay in its composition.