Following this NP pizza obsession road, I’ve been working on a dry stacked oven similar to Michael130207’s. I’ve fired it a few of times now and have been learning about fire management and how to control the heat, get the firebrick saturated, etc. My questions have to do with opening shape and size. Because it’s dry stacked, I ended up with a 18” wide by 8” high opening into a 32" x 36" x12” oven chamber, creating a ceiling/opening height ratio of 66%. This set-up seemed fine during my initial firings and despite some smoke exiting the outer opening at times, I was cool with it.
Then Chris (sub) posts his amazing collection of authentic Neapolitan WFO’s and I notice how all the openings are formed into half rounds. Of course I’ve seen these openings before at NP places but figured they were mainly decorative. It never struck me about how important they may be to the overall functioning of the oven. With reference from Chris’ post of Stefano Ferrara design drawings I built an insert to close off my opening to mimic what the masters are doing. After trying it out yesterday I noticed that the rate of combustion in the chamber seemed slower, the flames, gentler. I only fired for a couple of hours as a test so I don't know if it was heating the chamber any quicker or slower. Since the area of the opening changed from 144 square inches to 100 square inches I understand how that would effect the burn rate. The other effect this had was the lowering of the “smoke layer” since the circular shape closed off more of the upper part of the original rectangular opening, the exiting smoke is forced through a narrower area (apex of arc) before going up the chimney and it “lowers” an inch or so. I’ve also noticed how combustion is affected when portions of the fire are in this oxygen starved "smoke" layer.
So, I’m assuming the inner opening design of a Neapolitan oven evolved to balance the utilization of the wood fire to heat the chamber and to allow exhaust gases to exit while still providing room to manage the bake. It also seems the opening sizes tend to be constant regardless of the chamber size.
I do see exceptions to this though. Notably the Forno Piccolo opening design which looks larger. Despite this, Omid doesn’t seem to have any trouble producing outstanding pies from his. Does the larger opening just reduce efficiency but promote a hotter fire? I see on their site that they promote a 1 hour heat up time so that makes some sense.
So, does my experience with this opening shape mimic your experience? Is it designed to create a more controlled burn rate in order to heat the chamber more effectively? Or is it better to let more air in and get a hotter fire going considering the cold start nature of a once a week firing?
Look forward to your thoughts/comments.