Why does home brew need to cure?

[mrchipster]

I have been thinking about what I have read regarding home brew mortar and I cannot determine if home brew mortar really needs to cure like regular mortar.

Here are my thoughts.

Portland cement develops it's strength over time by damp curing. But we are under the assumption that the Portland will be burned out and loose it's strength over time anyway because of the high heat of the oven.

The full strength of the home brew comes from the sand fire clay and the strengthening of the lime due to the high heat.

Therefore the Portland is only really adding a water resistance until the lime takes over.

So I ask again is the one to two week delay in starting to cure the oven really needed?

Chip

[david s]

Re: Why does home brew need to cure?

I have been thinking about what I have read regarding home brew mortar and I cannot determine if home brew mortar really needs to cure like regular mortar.

Here are my thoughts.

Portland cement develops it's strength over time by damp curing. But we are under the assumption that the Portland will be burned out and loose it's strength over time anyway because of the high heat of the oven.

The full strength of the home brew comes from the sand fire clay and the strengthening of the lime due to the high heat.

Therefore the Portland is only really adding a water resistance until the lime takes over.

So I ask again is the one to two week delay in starting to cure the oven really needed?

Chip

I'm not sure about how long the lime requires to cure, but I think the waiting period is also to allow the brickwork to dry somewhat. Many builders wet their bricks before laying. Firebricks being more porous than solids will take up quite a lot of water. Even one week of drying will not remove all of the water. If you have time wait even longer.

[Tscarborough]

Re: Why does home brew need to cure?

The home brew can be considered to be a heavily gauged lime mortar. Here is a break down of what each component does:

Portland: Gauges the mortar, that is gives it an initial set time that is not measured in days or weeks, as well as providing both adhesion and compressive strength.

Lime: Provides bond strength, waterproofing, and workability to the mortar.

Fire clay: Also gauges (to a small extent) and assists with workability of the mortar but primarily provides heat resistance.

Sand: Adds body to the mortar and prevents cracking when graded correctly.

[cynon767]

Re: Why does home brew need to cure?

Letting the lime convert is part of it. It's also to allow the fireclay to dry out more fully before firing. Just as in firing ceramics, if there is too much moisture in the clay when it is heated the expansion of the escaping steam will cause cracks. I let mine dry out a couple weeks before starting a very slow heating schedule, and haven't regretted it.

[mrchipster]

Re: Why does home brew need to cure?

Yes, my question was really more one of should I begin to try to dry the oven before a long curing cycle. I believe I have my answer and that is that the lime actually cures similar to Portland cement and I need to wait at least a week before beginning a drying cycle.

As we have approaching winter here in Minnesota, I am concerned about possible freezing of my bricks before getting a full cure on the oven.

I will be leaving the oven covered with plastic and a blanket and on cold nights putting a 100 W light bulb in the oven to keep it from freezing.

Chip

[Tscarborough]

Re: Why does home brew need to cure?

Mortar freezing is only an issue while it is still plastic.

[mrchipster]

Re: Why does home brew need to cure?

Must be very cold in Minnesota.

Since I have lived here the lowest temperature I have experienced is -36 degrees F. (-38 C). and that was walking off a plane after being on a trip to Hawaii. So when I got on the plane it was 85 degrees F (29 C) and when I got off the plane 7 hours later it was -36; a 121 degree swing.

The lakes here freeze to a thickness of 4 feet in January.

I have been reading up on the topic of freezing of refractory brick and it seems that freezing the wet bricks is just as hard on them as expanding steam. The reason I looked into this was my recent Very difficult experience with my oven. I built it from some bricks that I had acquired on craigslist a few years back.

The bricks had been left outside uncovered for several years and had experienced many wet freeze thaw cycles. I completed my oven Dome brickwork about 5 weeks ago and had started my curing fires 2 weeks ago. When the temperature reached about 400? after the 5th day of firing I experienced a fairly significant crack vertically from the base up to the 9th chain directly through the bricks not on my mortar joints.

I was not experiencing any steam coming off the bricks so I believe that they were fairly dry at the time this crack appeared. My outside temperatures were in the 170? range on that day.

I believed that this single crack would not be significant if that's all I received but 3 days later after keeping the oven at under 200? for up to 6 hours each day I decided to push the temperature back up to 400 again. 2 more cracks developed one along the mortar joint above the 9th chain. And another vertically through the bricks about a foot from the original crack this one from the base up to the 9th chain again.

At this point I became very concerned about the cracks going through the bricks and not along joint lines. I filled the cracks with fire clay slurry in an attempt to patch them. Unfortunately subsequent firings produced even more cracks all with the disastrous through the brick not on the joint line cracking.

Last Saturday I tore the oven down because there were over 20 cracks going through the bricks vertically and I believed the oven to be unstable.

Last night I put a keystone in my new Dome built with brand-new bricks.

As you can see from the photos the bricks broke and the joint lines remained very solid. With the exception of the horizontal crack at chain 9.

The last photo is of the oven with 2 of the vertical and the 9th chain cracks; These expanded to over 3/8 inch width at 400 degrees F.

A lesson learned.

Chip

[GianniFocaccia]

Re: Why does home brew need to cure?

Chip,

Your story is a testamony to resilience and fortitude. Great job bouncing back! Did I read that correctly? Did you really re-build your dome in six days? Wow!
I'm pretty sure my firebrick sat out outside for a few years (in the 'seconds' lot) but never experienced freeze-thaw, only rain. Keeping my fingers crossed.
John

[mrchipster]

Re: Why does home brew need to cure?

Did I read that correctly? Did you really re-build your dome in six days? Wow!

Well I did have a few sleepless nights deciding on what to do....

Yes, 6 days and I am sure glad I used a thermal break methodology. and the flue arch was made of larger new bricks and did not not get touched.

It is amazing how much you struggle with on the first build that just seems easy on the second. Also take a look at the new cutting jig I developed for this new build. "Possible - New idea for brick cutting table" posted today.

The new jig saved me huge time in both setup and accuracy.

Chip

[Tscarborough]

Re: Why does home brew need to cure?

How hard was it to knock down? A dome is inherently stable, cracks or no cracks.

[brickie in oz]

Re: Why does home brew need to cure?

The lakes here freeze to a thickness of 4 feet in January.

And I struggle to get out of bed if we have a mild frost....

[mrchipster]

Re: Why does home brew need to cure?

How hard was it to knock down? A dome is inherently stable, cracks or no cracks.

It was in so many pieces it was real easy. I stuck a screw driver into one of the larger cracks and it came apart like a jigsaw puzzle.

I only used a hammer (very carefully) when I took apart the inner arch. interestingly the inner arch had no cracks at all and there was no cracking within 6 inches of it. I used the inward reaching arch technique developed by GianniFocaccia (John) and others it seems to be very stable so I used the same technique in the rebuild. I am also glad I used a thermal break at the transition to the flue arch.

Chip

[Tscarborough]

Re: Why does home brew need to cure?

But you did not try and stand on it or kick it down? The reason I ask is that I have seen ovens with as bad or worse cracking that performed just fine.

[GianniFocaccia]

Re: Why does home brew need to cure?

Yes, 6 days and I am sure glad I used a thermal break methodology

Would love to see some pics....!

[mrchipster]

Re: Why does home brew need to cure?

But you did not try and stand on it or kick it down? The reason I ask is that I have seen ovens with as bad or worse cracking that performed just fine.

I was not willing to finish my brick enclosure and metal roof and then later find out I needed to disassemble even more work to fix a problem I knew I had.

Well I guess it is to late now to go back..

I am happy with the choice I made to rebuild it.

I still have some cleanup to do but the dome is sound.

More photos in next post.

Chip