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Ellie 01-19-2009 10:53 AM

Mason says 30 day cure?
The mason building my oven says it should cure 30 days before I heat it up. I did a search here, and see 7 days as an average.

Anyone think it should be longer than the 7 days?


Jed 01-19-2009 09:12 PM

Re: Mason says 30 day cure?

The only cement that will be effected by the curing fires is the cement used in the 'high heat mortar' used to place the fire brick in the dome. All of the other cement products are effectively protected from the heat of the fire by the insulation.

The books will say that cement in the mortar will reach nearly full strength in 28 days (for this conversation - the same as 30 days) after mixing and placing. The cement in the concrete (or mortar) chemically reacts with the water, it turns the powder cement (and sand and what ever) to a solid material. The reaction is slow. Most of the 'curing' happens at the first part of the cycle, and the rate of 'curing' slows over time. Over the course of the 28 day cycle, the largest part of the reaction is complete. At 28 days, you can make assumptions about the maximum strength of your mortar.

I expect your mason is referencing this part of the puzzle, the curing cycle of the cement in the masonry.

When you start your curing fires, you will essentially stop the chemical reaction in the cement (now mortar). The cement needs water to do it's magic (powder to solid). The curing fires will drive the moisture from the mortar, and you will not allow the mortar to achieve it's full maximum strength.

The choice most of us make is that after about a week, the mortar is strong enough. The design of the dome in the Pompeii is an arch, in three dimensions not just two dimensions. The choice is to let the mortar cure fully or decide that good enough is good enough, and because it is a dome / arch structure, it isn't going anyplace... and the mortar doesn't need it's full strength to be 'good enough'.

So, it will be your choice.

It is correct that you should let the mortar cure fully, but my dome didn't fall down because I started my curing fires after about two weeks.

I was glad to see the form on your base changed to keep the concrete continuous.. Is your plan to have a 6 inch thick pad to as a base for your structure?

See you,


Les 01-19-2009 10:06 PM

Re: Mason says 30 day cure?

I think your mason is stating rule of thumb. Concrete takes forever to fully cure. I recall reading that the "mud" used to make the Hoover dam is still in the process - could be folklore, don't know. You are just trying to avoid the proverbial cracks - almost impossible. David let his oven naturally hydrate for months - still got cracks. I used next to nothing for mortar - still got cracks. I say let it dry for a week, bring little fires to big, after a week, call it done. Just my opinion (but I respect it :))


Xabia Jim 01-19-2009 11:40 PM

Re: Mason says 30 day cure?
Good discussion Jed ... the natural curing gives off heat so adding heat drives the reaction the wrong way.

The mantra from me is "the longer the better" as you can't wait too long but you can certainly get them started too early.

but you get so damned excited with these WFO, you want to see them going!

....and cracks happen....everybody has one!

nissanneill 01-20-2009 01:16 AM

Re: Mason says 30 day cure?
are you planning on using high temperature refractory adhesives (mortar) or the 'poor mans' 1pt hydrated lime : 1 pt portland cement : 1 pt fireclay : 3 pts brickies sand as your mortar?
I used the latter, commenced my drying fires as soon as I applied the outer insulation and had 4 fires and I HAVE NO CRACKS!!!! Anyone is welcome to come to check. Check with Tim and Benny who have inspected it lately quite closely! Benny also took several pics inside the dome only last Friday!
So, my advice is to follow my build BUT don't go stupid with your fires and get it dried and used as soon as practicable!



dmun 01-20-2009 04:16 AM

Re: Mason says 30 day cure?
Because of winter, and the construction of an overly ambitious chimney, my oven sat fully six months before I started the curing fires. I pretty much got the same cracks as everyone else.

Ellie 01-20-2009 09:50 AM

Re: Mason says 30 day cure?
Well, this is a most informative discussion. I really appreciate your teaching on this one.

I do have the time to wait the month if it needs it.. though, it looks as though if I really want to use it and have some fun, two weeks is the average time. Is that correct?

I don't know which mortar he is going to use for the fire brick. I will ask him tomorrow morning. I did wonder that myself. I did not notice any lime in the materials list though. Hmmm, wonder what he is going to use? He is used to doing fireplaces but has never done a wfo before.


glennb 01-20-2009 03:49 PM

Re: Mason says 30 day cure?
Make sure he is using refractory mortar, not just ordinary sand/cement.

Ellie 01-20-2009 04:12 PM

Re: Mason says 30 day cure?
Thanks, Glenn. . Refractory mortar is the fire clay mixture that is in the plans and on the forum, right? Will he use it on the floor and dome?

Thank you,

dmun 01-20-2009 04:40 PM

Re: Mason says 30 day cure?

Refractory mortar is the fire clay and lime mixture that is in the plans and on the forum, right?
That mixture works fine, but a mason who's used to building fireplaces will probably want to use a commercial refractory mortar like Heat-stop.

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