High Peak Pompeii Oven build

[gayterman]

Hi all

I thought I'd relay the nightmare I've had with diamond-tipped angle grinder cutting discs (115mm). I bought three at £4.95 each from a local builders' merchant (the make was 'Mexico'), and the first one did sterling work, getting through all the bricks I needed to cut for the circular base, as well as the arch springers. However, after about 30 bricks it started to glow red at the tip and to leave a thin grey line. I therefore swapped it for the second disc, which to my compete surprise lasted about two bricks before giving up the ghost. Surmising that this was a faulty disc, I swapped it out for the next disc, which lasted all of one brick! No amount of angled grinding or persistence (on the assumption that there was a diamond-free layer I needed to get through) made any difference, nor did soaking the bricks, nor holding the grinder lightly). I subsequently ordered three new discs (a different make off eBay) at £4.95 for three, with a very similar story playing out, two of the discs lasted about three bricks, but the last one did about 8 bricks. I then tried yet another make (at £10.95 for three off Amazon), reasoning that more expense meant a longer-lasting disc. The first one lasted 3 bricks, but the second and third fared much better, with the last one cutting about 20 bricks all told, with some heavy work on a lot of them.

I still have no idea why I experienced the massive difference in longevity. I did wonder if some of the bricks were especially hard, given that there are distinct differences in coloration, some being a soft yellow and others being almost orange/red. As a scientist, I should have gone through a rigorous experimental approach to ascertain which make of disc cut which bricks the best, but by this time I was losing the will to live! Unfortunately, the really poor discs co-incided with the upper dome courses, where the bricks needed the most work (cutting), so I inevitably took short cuts, with fairly predictable consequences (losing bricks that I bolstered too soon) and opting to use a mix of uncut and cut quarter bricks to reduce the total number I had to shape. The latter has resulted in, shall we say, a 'somewhat less than symmetrical' couple of courses, that looked simply hideous from the inside, so much so that I even knocked about eight out and replaced them when I realised that I had a 'good' disc. I couldn't face back-tracking two courses, so it's still not great to look at from the inside, but nevertheless seems quite strong.

Still no idea why!

Cheers, Ash

[bry]

4 inch grinder blades are normally very thin and only have a small amount of diamond tip. 9 inch and stone saw blades have much chunkier blades with alot more depth/life in the actual diamond section. Diamond blades are one of those things where buying cheap is a false economy, decent stone saw ones can cost £100+, that being said my mid range £50 ish blade has been going strong along time now

[keithw]

Would the disks last longer if you had water trickling over the bricks when cutting them? Might be messy but would keep the dust down also.

[bry]

I havnt received my bricks yet so don't know how hard they are but yes water should keep the disk cooler and prolong its life a bit. General concesess iv seen is to soak them for a minute in a bucket before cutting them. As you say will also produce bit of messy sludge so make sure your not firing it out over your nice clean car or something...

[gayterman]

Aug 15, 2016 19:30:47 GMT bry said:

4 inch grinder blades are normally very thin and only have a small amount of diamond tip. 9 inch and stone saw blades have much chunkier blades with alot more depth/life in the actual diamond section. Diamond blades are one of those things where buying cheap is a false economy, decent stone saw ones can cost £100+, that being said my mid range £50 ish blade has been going strong along time now

Hi Bry, you live and learn! I'd just like to know why some discs literally expired after one brick, whilst others (same make) did 20!

It will be interesting to see whether you see a big variation in the hardness of your bricks. I did take to dunking mine in water for 30 seconds, but to be honest, I don't think it makes a lot of difference other than the propensity to create a pool of sludge, as you are soon through the relatively wet layer into dry brick.

I did take the precaution of parking my car 25 yards away, my son however, wasn't so lucky ;-) The garage is absolutely covered in brick dust (nothing of value in there), and though I've swept up at the end of every session, I'm clearly going to have to take every single thing out of there, clean it off and put it back in :-(

Cheers, Ash

[gayterman]

A few photos of the arch-intersection and the next course above. It looked reasonably good at this point, but later on I discovered that I'd dipped down slightly at the front, which meant that the courses were not as concentric as they should have been, which became more obvious as I went higher and the hole diameter reduced.

Also, because the diameter of each course becomes smaller, if you are using half bricks, no matter where you start the course (i.e. with the half brick squarely over a joint) as you work around you will inevitably end up with joints precisely over joints on the course below. This kind of grates on me, but I'm pretty sure it doesn't make much difference to the overall strength (at least I hope it doesn't)!

Cheers, Ash

[keithw]

Any more progress gayterman?

[gayterman]

Hi Keith

Just returned from Cornwall, so here are a few more photos heading up to the dome closure. My mistake earlier with not keeping the upper courses truly level and concentric came back to haunt me here. I managed to make the situation even worse by trying to avoid cutting bricks (due to the disc-cutting nightmare) through using unshaped quarter bricks and then filling in between with shaped bricks - needless to say, this made the concentricity problem even worse. Fortunately, I then won first prize on the disc lottery with a really 'good' disc, so I knocked the offending ring out and started again using tapered (cut) bricks. This produced a lot better result, though I can't say that I'm thrilled with the overall aesthetics - there's no doubt that it is strong, but the last few courses are not pretty when viewed from the inside. Others on this forum have done a much better job (envious) and though they will of course be hard to see, I know they are there!!!

Cheers, Ashley

[gayterman]

Three more photos showing the last brick in the dome, both from above and below. This went reasonably well considering the struggles that I'd had a few courses beforehand. Although I managed to correct matters so that I ended up with a reasonably symmetrical plug at the end, this was definitely at the expense of aesthetics (when viewed from the inside).

Cheers, Ashley

[gayterman]

Started to build the chimney base out of fire bricks (four courses), but with a clay flue liner and brick surround still to add on top, I started to get a little concerned about the weight, which is pushing down on a single column of fire bricks (through the springers) at the front. I therefore built a column of half fire bricks on each side for extra support (not convinced about the look of them), and have also taken some advice given earlier to drill through the arch supports and strengthen them with rebar. I also added the first few courses of the cosmetic arch at the front, which I've made from reclaimed hand-made bricks, as I wanted an irregular look rather than the effect you get with machine-made bricks.

Cheers, Ashley

[keithw]

Nearly there now, it'll be Pizza time soon.

[gayterman]

A few more shots. Finished building the decorative arch from reclaimed, hand-made 'slips'. Not sure whether this is the name used everywhere in the country, but essentially they are half height bricks so are thinner, giving a smoother arch. I then put a layer of reclaimed quarry tiles on the chimney (as an architectural feature) only to discover a slight mis-match between the centre of the arch and the centre of the chimney cavity. It's about half an inch out, so I'll be correcting the chimney top, which will be a red clay flu liner (in the photos) surrounded by reclaimed bricks. This will effectively mean that the firebrick base will be slightly out of alignment with the arch and the chimney proper, but by the time it's covered in render, which I'll make thicker on one side, I'm hoping that it won't be noticeable. I mentioned earlier in the thread that I was getting worried about the weight of the chimney, so took the advice given and drilled a hole right through the arch supports and inserted a length of rebar on each side. Better safe than sorry!

Cheers, Ash

[gayterman]

I'm now giving some thought to rendering, which will be directly over ceramic blanket and chicken wire. I plan to do a scratch coat (about an inch thick) followed by a finish coat (maybe half an inch thick). Looking on this forum, as well as elsewhere, I was planning to use the following mix for the scratch coat: builder's sand (3.5 parts), sharp sand (3.5 parts), cement (2 parts), lime (1 part). The finish coat would then be: builder's sand (4.5 parts), sharp sand (4.5 parts), cement (2 parts), lime (2 parts). Does this sound sensible? Maybe I can simply go for 4:4:1:1 on both layers?

Also, I've practically given myself a migraine wading through the internet trying to understand whether or not to add waterproofing agent to lime render or not. Basically, there appear to be four options:
- No waterproofer at all (some folks state the you should NEVER mix waterproofer and lime)
- Waterproofer in both layers (some folks always do this and others think they are mad)
- Waterproofer in the scratch layer only (seems to be the most popular)
- Waterproofer in the final layer only (seems the least popular)

Note that I plan to paint the dome in a waterproof masonry paint at the end.

Any advice?

Cheers, Ashley

[bry]

Wouldn't put building sand in render, it's too fine and will make it more likely to crack, also inch thick is hell of a thick even for combined scratch and top. People say you shouldn't put Feb in with lime but we all ways put a little in has never personally concerned me, what makes you want to add lime. When I render mine I plan on doing something like 12/2 scratch 13/2 top, plastering sand/cement although may go coloured for top so something like 6 plastering 1 ginger 1 white cement depending on local sand colour gives a light beige if sands fairly yellow probably put 1 lime in as well as it normally puts some nice aesthetics when floated, will put waterproofer in regardless of others views. Main reason we put waterproofer in is because it tops the top coat pulling into the scratch coat too fast, we always put in both coats. What ever you put on it your likely to get at least a few minor hairline cracks because of the expansion and contraction of the dome

[gayterman]

Hi Bry

Thinking on what you say, I'm looking now to make the dome as round as possible so that I can keep the render (scratch and top coat) much thinner. I went on a few sites which recommended mixing builder's sand and sharp sand (1 to 1), but again, after due consideration, I think I'm just going to go for plasterer's sand, so it will at least be one last thing to mix. The reason for using lime is that many sites claim that it is more flexible than straight portland cement render and can thus stand the thermal expansion better. I'm a total amateur in this regard, so can only repeat what I've read!

Cheers, Ashley