If you don't have a digital pyrometer or thermocouple handy, and want to HT in your forge, you can do it by eye, and get a good result. Smiths have done so for ages untold and all of the best blades produced before modern industrial technology made heat control practical have been produced in exactly this manner.
There really are only a couple of tricks to bear in mind.
First, this is a decent way to HT a simple steel, that requires minimal soak time. More complicated stuff, with lots of carbides will take longer to get fully austinized, and thus, will not harden fully without the required soak. 1084 is ideal for this purpose, L6 much less so!
It may be a good idea to practice with a bar of the steel you want to HT before you try on a ready to finish knife, so you can get a good idea of what you're doing.
Heat slowly, and keep your shop lights very low, so you can discern subtle differences in the color of the steel.
Once the steel gets hot enough, you will notice a temperature gradiant, with bright glow towards the hot area and dimmer in the cooler area.
Curiously, if you heat the steel to above the temperature required to austenize it, you will see, as it cools, a darker region between two brighter regions. This "shadow line" will move slowly, towards the middel of the workpeice, with brighter glow in either side of it. In a knife, you will often see two such lines, one coming in from the point, and the other coming in from the tang.
This shadow line is the area where the steel is transforming. The hotter side of it (in the middle of the workpeice, if the whole thing has gone above autenizing temp) is the austenite, the shadowed area is in transition, and the cooler side is a mix of other stuffs, mostly pearlite.
The trick is to evenly and slowly bring the whole thing up to temp until the shadow line dissapears. The problem with this is that points often take heat very quickly, and if you try to go too fast, you can cook the hell out of your point long before the body gets up to temp. I reccomend taking the blade out of the heat quite often as you work up to temp to let the point cool while the body of the blade holds its heat longer. It can also help to hold the point with your tongs, and heat from the tang end, letting the heat bleed into the point until you're just about there.
Once the body of the blade is up to temp, quickly let the point catch up, and then plunge into your quench.
If you're careful and slow with the heat, the steel shouldn't be so hot that a quick plunge into the quench would cause your oil to flash. Often, if a good quench oil is flashing (igniting) it's one (or more) of a couple of common problems.
The steel is too hot
There is an insufficient quantity of quenchant for the size of the peice you are treating
or
the blade is going too slowly under the surface of the quench.
When you heat oil with glowing hot steel, it is going to evaporate some amount of the oil. That evaporated oil hangs around on top of the surface as flammable vapors. If there's glowing hot steel hanging around in those vapors, where they are freely in contact with atmospheric oxygen, they will ignite, causing the flash fire we all know and love! Quench time is not the moment to suddenly decide that you're indecisive, or need to stop and consider. All of your consideration, planning, and decision making should be in place before the steel starts getting hot for hardening!
