Curing fires

[RickH]

Can someone clarify when the best time is to do your drying fires? I was with the understanding that you build and insulate the dome with the blanket and perlcrete, Then let the whole thing sit for a week to cure.
Then start the curing fires, once that is complete, I can stucco and waterproof the dome. I was watching some videos, and some people were starting a fire after the brick is complete, before the blanket.
Which way is recommended? Thank you

[rsandler]

Your understanding is correct; there's lots of unwise stuff you can find on YouTube. Insulate first, no question. You could apply gentle heat (e.g. from a halogen light) before applying insulation, but it is not wise to introduce actual flames before insulation. Insulating will keep the temperature of the dome relatively consistent inside to out; a big temperature gradient is more likely to lead to cracking.

[Giovanni Rossi]

RickH it is correct to distinguish between curing and drying. For those new to the Forum, concrete products harden by a chemical process in the presence of water, curing. The drying process is used to work moisture out of all of the refractory materials, bricks, mortars, pericrete, etc., as well as any moisture in any other insulation materials.

Proper curing (achieving the correct harness and density of concrete products) is accomplished by managing the components of the concrete product, the ambient temperature and the time moisture is retained.

You can certainly dry the oven before adding the blanket insulation, but that would have to be done really slowly. If you read enough builds on this Forum, you will notice a theme of how a lot of builders start the drying fires with the intent of going "low and slow" but they often lose control and push the temps higher than intended. As descried by the experienced builders on this site, the risk of firing too high and too fast without insulation on the dome is that a large temperature differential can be created between the inside and the outside of the dome, causing uneven expansion and risking too many or large cracks. Taken to the extreme, catastrophic failure could result. This makes sense if you think about how conductive refractory is. Heat builds up on the inside of the dome and conducts to the outside. Without insulation, the heat is lost to the atmosphere, keeping the outer part of the dome cool. There's a better chance of keeping the differential smaller with insulation.

You still need to go low-and-slow, but your chance of success is increased with insulation. Also, if you haven't incorporated a vent in your design that will allow moisture to easily pass from the blanket layer though the outer pericrete layer, you may want to investigate how builders have accomplished that.

Good luck!

[RickH]

Thankyou, that makes a lot of sense

[david s]

Thanks Giovanni for that very good explanation. For years both builders and manufacturers have been using the term “curing” and applying it to the drying process which only serves to confuse. Curing by heat, relating to a WFO does nothing chemically because between ambient and 500C there are no chemical changes, only the elimination of mechanical water. This can cause steam spalling because water expands to over 1500 times its volume when turned to steam. The process should be referred to as drying. Allowing a week of sun and wind on the oven, if available, before any drying fires, is easier and safer.