There is no way to know how well a tool will hold an edge just by looking at it. A tool may look nice, but may be made from poor quality steel which will not hold a good cutting edge.
So what can you do to insure you are purchasing a quality tool which will hold a good cutting edge once it is properly sharpened?
One good way is to talk with experienced woodcarvers and get their opinion. Another way is to buy name brand tools. There are a number of good wood carving tool manufacturers to choose from; some of the more common ones (listed alphabetically order) are:
Buy a tool from any of these manufacturers and you can feel comfortable the tool will hold a good cutting edge.
If you look at the specifications on any of these tools, you will find they are made of high carbon steel with a Rockwell hardness between 58 to 63. This is what makes them good quality carving tools.
What does the statement "high carbon steel with a Rockwell hardness between 58 and 63" mean?
In the process of making steel used for good carving tools, a number of trace elements are added to give the steel certain characteristics; carbon being a very important one of them. The alloy content, or trace elements, along with the carbon will determine the maximum hardness the steel can achieve when it is heat treated.
Heat treating is a very complex process where steel is heated and cooled in a tightly controlled environment. The heat treating process consists of at least two steps. Heating the steel to a very high temperature and then cooling it quickly in water or oil (called quenching) to give the steel hardness.
When steel is initially made, and before heat treating, the carbon atoms are suspended in the steel but are not attached to the iron atoms. What happens during the heat treating process to give the steel hardness is; each steel has a "critical temperature range", which is the temperature where the steel goes into a solid solution. Chemically this is where the atoms of the iron and the atoms of the carbon in the steel mix freely and bond together. If the steel is cooled quickly (quenched) the iron and carbon atoms remain bonded together. This is what changes the characteristics of the metal and makes it hard and very brittle.
To reduce the brittleness, the steel is reheated to a tightly controlled lower temperature and cooled. Reheating the steel to a lower temperature will allow some of the carbon atoms to unbond from the iron atoms and reduce the brittleness a small amount. This is called "tempering" the steel. Tempering reduces the hardness of the steel somewhat, but will produce the desired toughness required for good carving tools.
Hardness of the steel will determine how well the blade will hold an edge. If the steel is too soft it will bend easily and not hold an edge. If the steel is too hard, the edge will be brittle and have a tendency to chip or break.
The hardness of steel is usually graded by a standard called "the Rockwell scale". There are three distinct hardness scales used in the Rockwell standard, determined by the type of metal being tested; they are "a, b, or c". The small letter "c" is the scale used for hard steel (like that used for cutting tools). The Rockwell "c" scale runs from the low 40's (unhardened steel) to the upper 60's (the hardest steel can get). If a hard steel is tested and has a Rockwell hardness of 58, you will usually see it written in an easy to read format as R58c or Rc58.
The Rockwell hardness of steel is measured by a machine which pushes a diamond cone into the steel at a known force. The machine then calculates the diamond cones depth of penetration into the steel, with the depth of penetration determining the hardness of the steel. The diamond will penetrate deeper into soft steel than it will into hard steel. The softer the steel, the lower the Rockwell number.
For carving tools, a steel hardness less than a Rockwell of 58 (R58c) is too soft to hold that razor edge needed for carving. Over R63c becomes too brittle and will chip easily.
From my experience, the range of hardness desired in a carving tool is between R58c to R63c. Buy a tool with that specification and you can feel comfortable you are getting one which will hold that razor edge necessary for carving.
Good steel in the blade will insure the tool can hold a keen edge. However; to insure the tool will slice through wood easily, it must not only have a keen edge but must also have the proper blade contour. Blade contour determines how much wood will be displaced as the tool is driven through the wood.
My video "Sharpening Simplified" shows you step-by-step how to put the proper contour on your carving tools, and a razor edge, so your tools will flow through wood with minimal effort.
I'm busy working on my blog posts. Watch this space!