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Old 05-16-2011, 06:02 PM
Leao's Avatar
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Default Using Shims

Hi everyone at Forno Bravo Forum,

I'm about to embark on the construction of my Pompeii dome... I have a little question about using shims.

I understand that I need to use a shim to prop up each brick to the desired angle, but then how do I fit in the refractory cement if the shim is in the way?

I have seen lots of photos on the internet of a dome construction covered in hundreds of shims (e.g. Mad coyote Joe), then the next photo in such sequences narrating the construction of a dome features the dome covered in a coat of refractory cement. What happens in between?

I would guess the best way to construct a dome is to put a layer of cement on the first brick and then place the next one on top of it using the cement itself as a leverage to create the desired angle.

If anyone could shed some light on this I would be most grateful,

leao
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  #2  
Old 05-16-2011, 06:30 PM
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Default Re: Using Shims

After the mortar sets, you pull the shim out and fill in the void made by the shim w/mortar.
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Old 05-16-2011, 07:01 PM
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Default Re: Using Shims

Thanks Tusr, do you mean that you add a little mortar at the start - then a shim... wait for the mortar to set and then remove the shim and add more mortar where necessary?
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Old 05-16-2011, 07:23 PM
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Default Re: Using Shims

Hello Leao,

You can make narrow shims that will allow you to pack in a little mortar around it. The mortar sets fast enough that you can make good progress and go on to the next bricks.

You can also make the indispensable tool so it will hold the brick in the correct position. That's how I did it. I used about ten shims for the entire build. Just insert the shim far enough to hold the brick at the proper angle and release the tool, then go on to the next one. I made enough mortar to do six bricks at a time and that worked out good for me mixing in small batches.

I will post photos of my Pompeii build soon, lots of editing to do.

Cheers,

Cheers,
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Old 05-16-2011, 07:39 PM
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Default Re: Using Shims

I only used shims on one course, and since the brick shims were purposely left in, I mortared them in normally. For each of the remaining courses I mortared the bottom and side of each brick prior to pressing it into place being careful to line up the bottom and face of the brick with the adjoining bricks. I like this process better than subsequent backfilling (pastry bag?) because the single mass of mortar creates a really sticky bond that I feel will result in a unified, consistent joint.
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Old 05-16-2011, 07:47 PM
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Default Re: Using Shims

I used shims for the first few courses as well. I trimmed the 5 degree angles for the bricks which made nice shim material. I thought maybe the brick material shim would give more strength to the dome so I left the shims in place. It became a pain to mess with the shims, so I just hold the bricks in place for about 30 seconds and the mortar sets pretty well. I'm using the homebrew.
Leigh
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Old 05-16-2011, 07:51 PM
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Default Re: Using Shims

The mortar you're going to use really isn't made to span the large gaps in the firebricks (if you're not beveling all the bricks). The mortar also does not have the same properties as the firebrick.

If you trim the bricks at all (I'm only trimming sides - not top/bottom), you'll have plenty of shim material left. I'm not only leaving all the shims in - I'm adding firebrick material in as many joints as I can squeeze it in.

I'm doing this for two reasons. First - it fills as much space as possible with firebrick (not mortar). Think of having the entire dome made of firebrick instead of 80% firebrick and 20% mortar. I suspect the all firebrick oven maintains heat better. Second if (and when) the mortar degrades, the wedges have everything locked in together pretty tight. So - I don't expect any movement in the bricks - plus you use a lot less mortar.

Anyway - there's my two cents. You're mileage may vary.
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Old 05-16-2011, 07:53 PM
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Default Re: Using Shims

I used the brick leftovers as shims and left them in to use less mortar. The mortar is not technically supposed to fill large sections of space, so I thought the bricks would create a smaller joint. I am still working on mine, but it seems to be working.

Mike
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Old 05-17-2011, 04:05 AM
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Default Re: Using Shims

Thanks Mike & everyone else,

If I leave the shims in and fill in the remaining gap with refractory mortar could this cause any probs? For example, a tiny (2cm x 1cm x 3mm) shim of broken terracotta.
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Old 05-17-2011, 09:27 AM
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Default Re: Using Shims

I wouldn't use terracotta shims, it will have a different expansion rate than the fire brick and would cause problems. The brick and the mortar have a similar rate of expansion.

Mike

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