Hairline Crack

[Fclark]

I've finish ed the brickwork for the dome but have not yet insulated. I let the refractory mortar dry for 5 days, and started a slow curing fire on Sunday. Tonight I fired again for step two (350F) and hairline cracks have opened up in two places, and smoke is coming out of the cracks. It appears my mortar was still not dry enough. SO - at this point what do I need to do? Can I expect to mortar over those cracks after it recools - maybe using a wet mortar to fill and seal? Or do I have to do something more substantial??? I'd hate to have to crack the beautiful shell open!!

Thanks for any insights. - Fred

[crisp]

Hi Fred,

I'm no expert but are they just mortar shrinkage cracks or movement cracks? What are the size of the joints where the cracks have appeared? A picture or two might help. What type of mortar have you used?When the content water in a mortar leaves too quickly before it has hardened it can cause shrinkage. Standardised cements are tested for compressive strength at 28 days. There are two things, mortar needs water content to harden, when water and cement are combined there is a chemical reaction, this chemical reaction needs a curing period. Retaining a moist curing period enables a mortar to harden, if a mortar drys too quickly, (drying is not to be confused with hardening) it will not harden correctly, and shrinkage can occur. If the cracks are small, things will probably be ok. There are others that have posted about this, perhaps you can search the forum about cracks and see what they did.

Chris

[boerwarrior]

Cracks are normal and your ones don't look bad to me. I would fill with wet mortar as you suggest. I have had small cracks in my oven for two years now. They have never gotten worse. Like Chris said if you search for cracks on this forum you will get hundreds of hits! Mostly by people worries about them!

[david s]

Firing an uninsulated oven is asking for cracks to develop because there is a massive difference in temperature between the inside surface and the outside surface.Much better to start fires after the insulation has been applied IMO. I'd be filling the cracks with mortar then insulating before lighting any more fires.