The "gradient" is the densities that vary in the pan. At the inlet from the float box you will usually be around 2% sugar, since you keep adding at the same place as the "sap" is boiled down, the further along the route of travel it moves, the higher the sugar%. When you get to draw off you will be at about 66.5% sugar. This gradual thickening in the pan is known as the gradient.
Start at as said, 1.25-1.5" over the flues and syrup portion of the pan and set the float to keep it there as you boil. Make it boil as fast as you can. Use dry wood, split it fine and after starting the fire, add wood by the clock. Many use 7 minute intervals. Have the wood ready to go in before you open the door and fill the firebox to about 2/3 full and close the door ASAP. An open door stops the boil, the less time it is open the faster you boil. The faster the boil the better the syrup.
Concerning overnight shut down. Don't light your first fire until you have at least 3x your expected hourly boil rate. After 3 hrs the sugar in high enough in the pans to keep them from freezing and damaging the pan. A slush will form but it will not freeze solid.
As you gain confidence boiling, gradually reduce the depth by adjusting the float control until you get down to 1" deep. That should be your goal for your 1st year. In another year you can remain at 1" or reduce it to 3/4". The lower the level the faster the boil.
Have fun!!!!
Warning, this is very addictive and there is no cure. However the best therapy is The Maple Trader regularly.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.