November 20, 2012 - 6:50am
The floor of my oven scorches the bottom of the bread, but the oven only gets hot enough to bake one load. I've got 6 inches of loose vermiculite fill on top of the oven (in addition to two inches of vermicrete plaster over the brick), and no insulation underneath. Next day oven temp is 200F. Thoughts?
Vermiculite is an insulating material but will not hold heat in it's mass. What kind of mass do you have to hold heat on the inside of the oven? And... how long are you heating it and with what material? How long has it been drying? (It might not be dry yet.)
It would be helpful to know a little about the construction of your oven (thickness of thermal mass in the hearth and dome for instance), and how you are measuring temperature in your hearth and dome. I'm also assuming that it is a wood fired oven.
I agree with Mini Oven that if your oven is still not cured properly you will not achieve a good result. Having no insulation under the hearth is certainly not helping you out.
You must have insulation under the hearth also, otherwise it beomes a heat sink, passing the heat into your slab and support structure.
What material are using for the cooking floor? (fire bricks, soap stone, pavers, metal)
This is a problem that is more complex than hearth floor insulation. In fact, if your hearth floor was underinsulated, you'd have the opposite problem -- burned tops and undercooked bottoms.
The goal in firing a WFO is to dump an approximately equal amount of heat energy into the oven roof as the oven floor. The "hot spot" problem is often the result of a combination of too little total heat (too short and/or too low of a burn) and insufficient "soaking" -- the time needed for the heat to travel from the masonry surface deep into the thermal mass. Short or cold burns may result in dumping more heat into the hearth than the dome since the fire can transfer heat to the hearth through direct conduction. Trying to bake prior to properly soaking the oven often yields poor results since the surface temps start too high, but drop rapidly as the heat sinks deeper into the masonry. Assuming the oven has no major design flaws, the solution usually involves longer and/or hotter fires to raise the total heat energy in the oven's mass and allowing more time between raking the fire out and the start of the bake to allow the oven to stabilize. If you record your oven's surface temps over time, you'll see that they fall pretty quickly once the fire is raked out until they reach a point where the temp decline slows down. The time from raking out the fire until the surface temp drop settles to a nice, slow decline is the time you're shooting for as a soaking period. (If you have a thermocouple deep in your masonry, you'll note that as the surface temps drop, the deep probe temps rise until you reach a point where both sets of temps start falling slowly in approximately equal amounts -- a stabile oven.)
As a seasonally-appropriate analogy, think of trying to roast a turkey in 30 minutes by cooking it at 1,500° -- you'll burn the outside to a crisp but the inside will be raw. To heat a turkey -- or an oven -- evenly and thoroughly, you need to heat it at the proper temperature for a long time. The bigger the bird -- or oven -- the longer the time to get the heat to "sink in".
There is no set formula for this since each oven is different. My (small) oven needs about 90 minutes of bright fire, followed by allowing the fire to burn down to coals which are spread out over the oven floor for an hour or so before raking out. Then close the door and wait for 30-45 minutes for the oven to equalize. Check temps. For bread, I shoot for surface temps (as measured with an IR gun) of around 450° or a bit higher at the start of the bake. To get this, I need to get the surface temps up around 900° - 1,000° during the firing phase and the entire procedure -- from striking the match to loading the bread -- takes about 3 hours. Lastly, damp mopping the deck helps drop the floor temp a bit and has the added benefits of cleaning the surface and introducing just a bit of steam to the oven. Don't go overboard with this though, or you may stress crack the deck.
Finally, the number of bakes you can get out of a single firing has little to do with how "hot" the oven is and much to do with how much your oven weighs -- it's the mass of the oven, not the temp, that will determine how long it will stay hot. But heating any oven for sufficient time to fully heat the mass through its entire thickness will maximize the time the oven will stay at baking temps.
You have some fun and tasty experimenting to do. ;-)
PS: I see your avatar is a dutch oven -- a dutch oven is a great way to get some extra milage out of your coals when you rake them out. You can cook up a nice stew or chili in the dutch oven using the WFO coals while your bread is baking. Fresh bread dunked in hot stew -- can't beat it!
Thank you, ClimbHi, for your most thoughtful and most helpful comments.