Drying Fires

[RickH]

I started my Drying fires, 1st 2 days I kept the walls at about 300 degrees, days 3 & 4 around 400 couple spots got to 450, tonight and tomorrow I was going to go for the 500 - 550 range.
and then 700 the last day.. Is this a good approach? I read somewhere that the black on the bricks was from moisture, I thought the black is there until you reach at least 700 degrees.
currently my whole dome is black. I appreciate any input.
Thanks,

[SableSprings]

Rick, you might want to see that you've driven off most of the water before you go +500. With the vermicrete/perlcrete insulation, there's a lot of water retained. Take a piece of clear plastic and put it over the top of the dome (overnight works pretty well after you've shut down for the evening). If you're getting much condensation, slow down, you're still driving internal water out!. Remember that water will expand ~1,500 times when turned to steam...no sense rushing at this point. The black on the inside of your chamber is from carbon build up which is a good visual indicator that your bricks are not getting hot enough (~700°F) to burn it off. Directly related to moisture still remaining, which keeps the temp down. Every time you fire up the oven (after the cure), black soot/carbon is deposited on the inside surfaces until it is burned off...called clearing.

Normal pattern for new ovens is a spot at the top of the dome that clears first and then as the rest of the dome clears down from that as the oven bricks reach that ~700°F mark. Common for the first chain or two (nearest the oven floor) to be the last to clear...sometimes it takes them quite a bit longer to clear than the upper dome areas. The number one issue new oven owners face is the rush to go through the final curing/drying process...relax, go slow, and remember there will plenty of time to enjoy the oven AFTER this step.

[RickH]

I kept the fire at around 500 degrees last night, then covered the oven with plastic, this morning there was some moisture but not a lot, really not sure if it was just condensation.
I am confused when you say that the carbon build-up is directly related to moisture in the bricks and that the bricks are not getting hot enough. at what point do you think it is safe enough to bring the oven to 700+ degrees? I currently have 4" of ceramic blanket and 1 - 2" of perlite ( that I keep losing more every time I cover and uncover the oven) I would like to get stucco over this perlite, but I want to make sure it is dry enough, I also let it sit for a week before I started the fires.

[SableSprings]

If masonry (bricks & mortar) still contain moisture, it cannot reach a high enough temp to burn off the carbon. If masonry contains moisture and is heated too fast, the expanding stream generated by that moisture can cause breaks or cracks. If your covering or base insulation is damp/wet it will keep the masonry cooler...it needs to be completely dry to insulate.The carbon burn off begins starting at the hottest spot of the oven - the top (the first area of combined insulation/masonry to completely dry). Contained moisture is pushed out during the initial curing/drying process from the top down. As the oven dries and you gradually increase firing targets, you'll first see that cleared spot appear at the dome apex. During consecutive firings, that spot will clear faster and enlarge (moving down) as moisture leaves both the masonry and the covering insulation An oven that's almost dry, will often show a dark ring of carbon bricks around the lower ring that usually takes the longest to clear.

Once you've driven off all the dome masonry & insulation moisture, you'll be ready to fully use the oven to its potential. Every time you fire the oven, it will turn black initially, the clearing spot will appear at the top and expand downward. Since you no longer have moisture retained in the masonry/insulation structure, you simply see the oven coming up to temp as indicated by the soot/carbon burn off. If you have water/moisture find its way into your oven down the road (for example: rain through stucco cracks, leaks into the insulation seam with the slab, high humidity and long periods of not being used, etc.), you'll need to do another set of drying fires to bring the oven back up to its peak efficiency.

Sorry to be so wordy...I hope that helps you understand the firing process

[david s]

If masonry (bricks & mortar) still contain moisture, it cannot reach a high enough temp to burn off the carbon. If masonry contains moisture and is heated too fast, the expanding stream generated by that moisture can cause breaks or cracks. If your covering or base insulation is damp/wet it will keep the masonry cooler...it needs to be completely dry to insulate.The carbon burn off begins starting at the hottest spot of the oven - the top (the first area of combined insulation/masonry to completely dry). Contained moisture is pushed out during the initial curing/drying process from the top down. As the oven dries and you gradually increase firing targets, you'll first see that cleared spot appear at the dome apex. During consecutive firings, that spot will clear faster and enlarge (moving down) as moisture leaves both the masonry and the covering insulation An oven that's almost dry, will often show a dark ring of carbon bricks around the lower ring that usually takes the longest to clear.

Once you've driven off all the dome masonry & insulation moisture, you'll be ready to fully use the oven to its potential. Every time you fire the oven, it will turn black initially, the clearing spot will appear at the top and expand downward. Since you no longer have moisture retained in the masonry/insulation structure, you simply see the oven coming up to temp as indicated by the soot/carbon burn off. If you have water/moisture find its way into your oven down the road (for example: rain through stucco cracks, leaks into the insulation seam with the slab, high humidity and long periods of not being used, etc.), you'll need to do another set of drying fires to bring the oven back up to its peak efficiency.

Sorry to be so wordy...I hope that helps you understand the firing process

That’s a great explanation Mike. It is also precisely at this point that the structure is most vulnerable to cracking. With the top of the oven becoming hotter and drier, the chamber becomes more efficient leading to a more ferocious fire and a jump in temperature which of course also leads to greater thermal expansion in the top half of the dome. Meanwhile the temperature and the corresponding thermal expansion of the wet base of the dome has been held back because of the excess moisture present. It is the enormous difference in the thermal expansion that sets up stresses that can cause cracks. If you have an IR temperature gun it will show this big difference in temperature at different parts of the dome. Once the water is driven off the differences become more tolerable for the structure. But it is also another reason to always allow your oven to reach temperature slowly rather than racing to pizza temperatures as fast as possible.

[UtahBeehiver]

Plus one on both Mike's and David's comments, this is the point where we see builder go to hot and fast and damage their hard work. Turtle wins the race not the hare.