Blue edges

You turn on your high-speed grinder. You offer up the bevel of your carving tool to the spinning wheel. Sparks fly. All’s going well, then — oh no! Suddenly, before you can react, a part of the cutting edge turns blue…

Bluing edges is a calamity: the blue colour marks where the temper — the hardness that the manufacturer imposed on the blade — has been destroyed. The blued part of the cutting edge is now soft; it will dull very quickly, even bend, next time you carve.

What can you do?

Other than re-temper your carving tool, nothing — except grind the blade back beyond the blue colour into fresh metal, re-shape and re-sharpen. And that’s a lot of work. You definitely don’t want to blue your edges!

Physics 1

The blue colour means the temperature in the metal has risen too much. The temperature rises because of heat entering the metal.

Heat enters the metal because of friction, and friction arises for any or all of several reasons. Pressure: Pressing too hard onto the fast moving surface. Time: Leaving the blade in contact too long before cooling. Speed: A standard 150mm grinder rotating at 2850rpm passes around 23 metres of surface across your blade every second. Wheel surface: A finer wheel presents less air and more surface grit across the blade than a coarser wheel.

Physics 2

Besides friction, the other factor that leads to a rise in temperature is the mass of metal. There is far more heat in a hand-warm dinner plate than in the tiny fragment of a spark, even though the spark has a much higher temperature. The upshot is that the thinner the metal — the less mass — the faster the rise in temperature from the heat of friction.

Thus it is that bluing happens to corners and thin edges. The point of a skew is particularly vulnerable.

How to avoid bluing your edges

In a nutshell, you have to keep the blade cool. A water-cooled, slow-speed grinder is the obvious way to achieve this, but these machines are quite expensive compared to the simple highspeed dry grinders that most carvers use.

I’ve used a high-speed grinder for years without ever bluing my edges — well, hardly ever. From experience I know that if I blue the metal I’ll spend a lot more time repairing the damage, so this is what I do to avoid it.

I grind in the normal fashion, but follow this practice. Fixed close to the grinder, at both ends, are lidded jars of water. I hold the blade in my fingers. Frequently, and I mean every few seconds, or less, I slide the blade back into my fingers and check the temperature.

If the blade is warm, or if I even think the blade might be warm, or even just out of habit, I dip the blade in the water and stir it quickly about. I don’t wipe the water off but go straight back to the grinding.

A summary

• Check the temperature of the blade often and regularly

• Do not let the metal get above warm before dipping. Dip even if you are unsure whether it’s warm or not

• Be patient!

• Have a light touch; lay off the pressure

• Take your time. It’s much faster than a benchstone, whatever!

• As the bevel metal gets thinner and you approach the finished edge, take even more care

• Check the temperature even more often

• Use the coarser wheel

• Buy a slower speed dry grinder if you can.

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