Pdx 42" update

[david s]

Macrinehart


From what I can see in your pic, the top arch bricks shouldn't be very far off from the taper. I agree with JRPizza in his post #176. I too like a thicker mortar joint on the outside of the dome. You will probably find that you can compensate when cutting the dome brick to intersect the arch.


Thanks david s,
I wouldn't want anyone to confuse my advise for a home brew mortared brick dome with the refractory calcium aluminate cements which have a 1/8th" tolerance. I'm also sure that 2"+ home brew castables need a little more attention to their ingredients. However, as far as steam spalling is concerned for a home brew mortared brick oven......... that is what the curing and drying schedule is for.

Given that most castable refractory products contain burnout fibres as well as instructions for first firing schedules is an admission that steam spalling is a common problem for thick joints and why slow firing schedules alone are insufficient. When firing with wood a slow controlled rise is almost impossible to achieve unlike gas electricity or oil that can be finely metered. Why else would refractory mortars limit the size to 1/8"? A small brick oven will have gaps on the outside of the dome that may well exceed 2". The larger the radius the smaller the gap, so the problem is reduced, but certainly always exceeds 1/8" Filling these gaps with brick wedges is a way of reducing the thicknesss of the mortar joint as well as utilising waste offcuts.
From my experience the homebrew recipe's high clay proportion sometimes leads to shrinkage cracks, so I halve it. I know the clay imparts a degree of refractoriness, so while the lower clay proportion fixes the shrinkage problem, I'm not sure if it has other drawbacks, so don't take my lead on that idea.I do however, always add some burnout fibres which also hold the mortar together well.

[Gulf]

Macrinehart

......I was concerned about shrinkage and cracks but based on your guidance I can get away with a little extra mortar or using some waste material......

From what I can see in your pic, the top arch bricks shouldn't be very far off from the taper. I agree with JRPizza in his post #176. I too like a thicker mortar joint on the outside of the dome. You will probably find that you can compensate when cutting the dome brick to intersect the arch.

Commercial refractory mortars recommend that only really thin joints are suitable. I am not totally sure why, but suspect they are covering themselves for warranty issues due to failures caused by steam spalling. As the same issues arise with steam spalling in a thick homebrew joint a simple remedy used for castable refractory would seem a sensible solution. The addition of fine polypropylene burn out fibres to both commercial refractory mortar as well as homebrew mortar will certainly reduce the risk of steam spalling, The drawback is that they require extended mixing time for proper dispersal. insufficient dispersal results in the fibres clumping and does not produce the adequate network of mini pipes that allow moisture to escape. This is something that does require some experience regarding dosage and mixing times.

Thanks david s,
I wouldn't want anyone to confuse my advise for a home brew mortared brick dome with the refractory calcium aluminate cements which have a 1/8th" tolerance. I'm also sure that 2"+ home brew castables need a little more attention to their ingredients. However, as far as steam spalling is concerned for a home brew mortared brick oven......... that is what the curing and drying schedule is for.

[david s]

Remember that home brew mortar is fairly forgiving if the gaps can be filled with mortar. A half to 5/8ths inch is ok imo. Anything over can sometimes be stretched with a sliver of waste cut placed in the middle of the bed joint.

Commercial refractory mortars recommend that only really thin joints are suitable. I am not totally sure why, but suspect they are covering themselves for warranty issues due to failures caused by steam spalling. As the same issues arise with steam spalling in a thick homebrew joint a simple remedy used for castable refractory would seem a sensible solution. The addition of fine polypropylene burn out fibres to both commercial refractory mortar as well as homebrew mortar will certainly reduce the risk of steam spalling, The drawback is that they require extended mixing time for proper dispersal. insufficient dispersal results in the fibres clumping and does not produce the adequate network of mini pipes that allow moisture to escape. This is something that does require some experience regarding dosage and mixing times.

[Macrinehart]

Good point, I was concerned about shrinkage and cracks but based on your guidance I can get away with a little extra mortar or using some waste material.

In addition to the days getting shorter, weather is getting cold now with snow possible later this week. In anticipation I've taking the canopy down and covered everything with tarps. Will have to see if weather cooperates.

Cheers,

Mac

[Gulf]

Remember that home brew mortar is fairly forgiving if the gaps can be filled with mortar. A half to 5/8ths inch is ok imo. Anything over can sometimes be stretched with a sliver of waste cut placed in the middle of the bed joint.

[Macrinehart]

At the end of the day I've been able to mark up all but the last 2 blocks in the arch. The 5 blocks at the peak of the arch, including the keystone, were cut two deep on the overcut so will have to be redone. The rest can be salvaged.

[Gulf]

Should be close enough for "brick work"

[Macrinehart]

I thought it would be a challenge to measure the OD, but I'm doing pretty well with a yardstick and "compass" attachments. 4 bricks in, it seems pretty consistent that the OD intersects the outside of the dome at the depth of 4.5", which happens to align with the depth of the oven wall bricks. Pictures to illustrate...

[Gulf]

There are some simple but, precise methods for marking the OD. The link that I shared shows one method. This one was made from a coping saw that I took apart temporarily for the job. There are others. Any offset that will allow you to mark around the corner and attached to the pivot point to gauge the distance from it will work. The staged pic below only shows one brick being marked. However, on this build, the OD and ID for the entire arch was actually marked at one time while it was dry stacked.

[Macrinehart]

Hehe...did I say "every other"? meant "every".

I think my method for measuring the OD would be a bit inprecise. But I will mark every brick first and then rebuild the arch and check all the measures before I start cutting.

[Gulf]

It sounds like you may have it now. Though I did not quite understand why you would only ".....mark every other brick".
I find it less confusing to completely mark every brick on all for sides before going to the saw.

Also, "Finally the purple lines can be connected on the top of the wedge, yielding the yellow lines from post 189."

It may be just the wording, but that is a little backwards. The Outside diameter scribe (yellow line) from should be established first. Then the tapered cut (purple lines) can then be drawn by connecting the intersections of the Outside Diameter And the 1.5" Reveal

[Macrinehart]

Gulf and UtahBeehiver, I think pictures from post 177, 189 and 191 are painting a clear picture. I guess where I'm getting confused is that the undercut that I actually cut in the picture from post 189 follows the ID of my dome. But the overcut I did was intersecting with the undercut at the inside face of my arch brick, which is the error. Also didn't help that I cut those overcuts first. But I think I understand now.

So, when I was positioning my arch form, I set it so that the Keystone has OD and ID intersections almost exactly as Gulf has illustrated on 191. Looking at the post 189 again, I think my undercut doesn't need to change. Instead, I should measure the distance from bottom of the undercut to the top of the undercut on my keystone (about 1.5") and then mark that line on every brick in the arch. That should reproduce the blue line on post 189.

From there I can then mark a 4.5" line on the side of each brick, staring at the blue line and intersecting with the top of the wedge at 4.5", which is the depth of my brick. That will produce the purple line shown on post 191.

Finally the purple lines can be connected on the top of the wedge, yielding the yellow lines from post 189.

Thanks for your patience!

[Gulf]

Here is one included inside the link I previously shared.

EDIT: It may not be clear in the photo. But, the OD scribe mark is on top of this brick. The purple line is the actual tapered cut. The purple line is drawn from the 1.5" reveal to the OD scribe.

[UtahBeehiver]

Gulf's pic pegged the issue perfectly. The blue line should be the ID of the dome and the yellow line should be the OD of the dome as shown in my pic in post 177 and also shown in JR's post 117.

[Gulf]

Hopefully this rough mark up will help