Cause and prevention of heavy scaling?

[gables]

I am very green at knife making. I have been using a
coal forge to heat treat. I had my blade nice with 300 grit, normalized 3 times and then heated and quenched in 140 veg oil. The pittng and scaling is imense and I am having to start back at 80 grit. Does a gas heated forge prevent some scaling?

[robtattoo]

It can do. I'm hoping Karl or Lin will jump in on this, but as far as my memory goes; scale is formed by overheating the blade, the pitting you describe I would guess is possibly carburization. You'll often find small 'lumps' of extremly hard material on the surface of the blade which are VERY resistant to being sanded out. This is either carbon migration or carburization (the two could be the same, my memory's shot) but basically it's a symptom of overheating the steel before the quench.
A lot of folks (me included) when they first start out, think that the steel needs to be red/orange hot before quenching. As a general rule of thumb (VERY general) for the basic carbon steels (01, 1095, 1080 etc...) the color you want is a very dark red. You won't even be able to see it glowing in daylight. Turn off all the lights in your shop & it'll show as a dull red. That's plenty hot enough to quench in hot oil & produce an acceptable (not perfect) level of hardness for you to draw back.
Using coal, coke or charcoal can compound the problem due to carbon being transferred from the heating medium to the steel, through high-temerature contact. Gas doesn't completely eliminate this, but it makes it about 1000 times better.

KARL! Help me out here Brother!

[kbaknife]

Good to see you here, Rob! Where you been??!!??
Lin's info was very spot on - if you are going to use coal, you need to learn to use it CORRECTLY.
The basic cause of heavy scaling and pitting is too much heat + oxygen!
Too much heat is really easy to do with coal. That stuff will burn in easy excess of 3000 degrees!! And, that's hard to see if you are in daylight.
Then, not only does the heat literally eat the steel and carbon, but you get EXTREME grain growth - like Lin said - which totally defeats the purpose of what we are trying to accomplish in bladesmithing, which is grain reduction.
Carbon ALWAYS wants to EQUALIZE, in other words, that word you have probably heard - MIGRATION. It will MIGRATE with the help of heat, to areas which have less or NO carbon, and in this case, that place is your shop!
If you get blade steel down into a really hot coal fire, that forge will practically use the steel for fuel.
How we get away with this in propane forges is that we adjust our forges so that ALL of the oxygen is used up in burning the propane, and there's none left to burn up the steel.
Remember basic science class? To get combustion we need a heat source, fuel and oxygen. The carbon in the steel is a POTENTIAL fuel source, but if we have used up all of the oxygen, then we can't burn up the carbon/fuel.
But, that is in a PROPANE forge.
That's what Lin was talking about. You build sort of a cave/igloo/thing out of your coal that has an opening into it. In there, you control the amount of oxygen and heat so that you don't burn up the steel.
It's not an easy thing to learn - it take a lot of time with someone who can show you the ropes.
Rob is also correct in that probably one of the biggest mistakes in hardening blades is over heating, and/or too much time AT HEAT.
Depends on the steel.
And the heat source.
Smartest thing I have ever done in knife making is to go bother other makers at their shops and have them show me how to do things.
Go to seminars.
Go to hammer-ins.
Go take classes.
Etc.
Etc.