The Pencil Hardness Test 硬度測試

The Pencil Hardness TestBy Mac Simmons
This article appeared in Woodwork, April 2000, p76. and is used here by permission.
A set of graded pencils
After doing some testing on various coatings for their 'pencil hardness', I was surprised at the results I got. I thought I would share this information with those who may have their own opinions about what they consider to be hard and durable coatings. I would like you to know right from the beginning that 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.
Grading pencils come in an assortment of both hard and soft, and can be found in most art or office supply stores. The set I have consists of twelve pencils, ranging in hardness from 4H to 6B. The 'H' stands for hardness, the 'B' stands for blackness, and HB is for hard and black pencils. The hardest is a 9H, followed by 8H, 7H, 6H, 5H, 4H, 3H, 2H, and H. F is the middle of the hardness scale; then comes HB, B, 2B, 3B, 4B, 5B, 6B, 7B, 8B, and 9B, which is the softest. Another grading method uses numbers; the equivalents would be #1=B, #2=HB, #2-1/2=F, #3=H, and #4=2H. The most commonly used writing pencil is the #2 (HB grade), which is fairly soft, contains more graphite, and leaves a dark mark.
To do your own pencil hardness testing, always start with a dry, clean, well-sanded piece of wood.
Type of coatings
Pencil Hardness
Catalyzed polyester
9H
Catalyzed polyurethane
9H
Catalyzed ModifiedAcrylic polyurethane
4H
Catalyzed Acrylicpolyurethane
2H
Water-based polyurethane
3H
Water-based urethane/Isocyanate Catalyst
2H
Conversion varnish
4H
Low VOC Catalyzedlaquer [24 hrs]
2H
Low VOC laquer
3H
Urethane/Nitrocelluloselaquer [24 hrs}
F
Water reducible laquer
2H
Tung oil/polyurethanewipe-on finish
2H
Water-based polyurethanewipe-on finish
HB-F
Aerosol precat
3B
Aerosol water clear acrylic
3B
Aerosol clear shellac
3B
Aerosol nitrocellulose/polyurethane
HB
Aerosol nitrocellulose
3B
Amber (orange) Shellac1 lb. cut
3B Usually the coating thickness being tested is 1.0-1.5 mils (a mil is one millionth of an inch) and has been allowed to dry for 7 days. In some cases, a different drying schedule is used, and the chart indicates those in brackets. The ambient temperature during the test can also be a factor in the drying times and can have an effect on the coating's hardness.
Select a pencil from your set and make a line about 1/2-inch long. If the pencil you start with scratches the surface of the coating, then go down the pencil grades until you come to the first pencil that doesn't scratch the coating. Redo the test, and if you get the same results, you have determined the 'Pencil Hardness' of the coating you are testing. It's that simple. There are some coatings so hard that the 9H pencil won't scratch them-all of these coatings get a 9H rating to designate their hardness.
Use this chart only as a guide. It is also important to understand that the degree of hardness of any generic coating may not always be the same; in other words, if one company has a polyurethane with a 3H hardness, do not assume that every other polyurethance will have a 3H hardness. They won't; they will vary from manufacturer to manufacturer, and from product to product.
As I mentioned previously, the pencil hardness test is only one of many tests that are done to evaluate a coating's performance. Other tests are abrasion, reverse impact resistance, direct impact resistance, cross-hatch adhesion, oxidation, gloss retention, UV resistance, yellowing, blistering, drying times, chemical/solvent resistance (using both the rubbing and spot/time tests), salt spray resistance, humidity resistance, acid and caustic resistance, the VOC and HAP contents, and so on.
All this information is helpful in choosing or evaluating a finish. But regardless of a finish's rating, the best advice is always: 'handle with care'.
Mac Simmons is a 40-year veteran of the furniture, refinishing, and restoration trades, and the author of the Refinishing Commandments. He lives in Massapequa, New York.

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