Old 08-20-2008, 11:43 AM
Join Date: Jul 2008
Location: Norway
Posts: 121
Default Minimising risk of cracks

I am soon to start the curing process in my completed oven taking care to use the procedure for gradual heating and multiple, increasingly larger, fires. Looking at other threads showing cracks during initial firing and reading about different practices w.r.t. having insulation on or off during the curing part I can see the advantage of being able to spot cracks with the insulation off. But, will the risk of cracking increase with the insulation off due to higher temperature gradients through the bricks and mortar? The temp. gradient causes stress which again causes cracks. With a slow fire the bricks and mortar should have a better chance to equalise in temperature with less gradients? I guess that holds for all later firing as well, - start a bit slow and increase the fire as the temperature climbs?

regards from Karl
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Old 08-26-2008, 10:15 AM
Join Date: Jun 2008
Location: Cambridge, UK
Posts: 97
Default Re: Minimising risk of cracks

The other advantage to insulation on, is that if it does crack, you won't see it and worry about it .

Seriously, I'm not sure the heat loss from the firebricks is fast enough to make a real difference when firing, but I may be way off base about that.
Matt S, Cambs, UK
42" Pompeii

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Old 08-26-2008, 10:51 AM
Join Date: Apr 2008
Location: Australia
Posts: 20
Default Re: Minimising risk of cracks

When I cured my oven I had all the insulation on - two layers of ceramic blanket plus vermiculite. My thinking at the time was basically to delay any heating for as long as possible, to allow as much cement curing under ambient air temps as possible. It probably gave the dome another week or more drying before it saw any heat.

Then I started small fires etc as recommended. After the first couple of fires I tried to keep the fires going as long as possible (eg all day) without allowing the fire to build up too much. This is a little contrary to the suggested build the fire up and let it die down again fairly quickly.

I think by leaving the insulation on it took longer to drive out the moisture, as I could feel some exterior hot spots, which eventually went away.

Inside the dome I cannot find even a hairline crack anywhere in the mortar - as much as I can see inside when the oven is white hot, and getting my head inside when it is cool. I have a hairline mortar crack on the outer arch which closes up again as the oven cools - really annoying but apparently nothing to worry about.

So my vote goes for delay curing fires as long as possible, do it with insulation on, do long moderate fires, no rush.

All the best
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Old 08-27-2008, 11:28 AM
Join Date: Jul 2008
Location: Norway
Posts: 121
Default Re: Minimising risk of cracks

Thank you for your comments and advice. I will apply the "no rush" philosophy and the "keep-it-warm" at moderate temperature steps. It fits very well with my schedule for curing. Beeing a weekend commuter I intend to heat it up to a modest temperature on friday evening. Try to keep it until noon saturday and then ramp it up slowly some 50C and keep it there for another 24 hours. I will manage one more ramp-up and "hold" until sunday evening before I "box it up" and leave it until next weekend. The next weekend I will do the same, only at higher temperatures, and hopefully complete the drying and curing process.

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