bento
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Post by bento on Dec 5, 2015 18:06:53 GMT
So...good and maybe bad! Good - Managed to get out today and completed the first full top course. Maybe bad - when I mixed the home brew for the first course attached to the arch, I did a 3:1:1:1 But I mixed too much so bagged up the dry mix. I then used the dry mix with and extra ratio of sand to make it 4:1:1:1 All good so far. Last week I mixed up another dry batch of 4:1:1:1 and used it last week and today. Once I finished the course today, where I had begun this course last week, I knocked a brick and dislodged it. The home brew I mixed up is taking a long time to set - it's crumbly if you scratch it, similar to what it was like in the summer after a day (this is about seven days later)! I would have thought this should set quicker as there is less sand (even in this weather). Questions: - Have I made an error of mixing a dry batch with builders sand, and the moisture in the builders sand has caused the cement to go off, which is why it isn't now setting? - Is it not setting quickly because of the weather - cold and moisture in the air (although it really hasn't been all that cold recently)? - If it doesn't ever correctly set, as the cement breaks down at high temperature, does it mater if it doesn't set as long as it holds the brick in place? This is the whole top ring where I have made a dry mix and then added water as I am having to lay a brick at a time now. You can see from the second pic I didn't taper the bricks upwards to make them not visible looking through the opening - I didn't notice this until it was too late. I will add some slips to finish this nicer once the dome is complete.
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simon
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Post by simon on Dec 5, 2015 20:07:33 GMT
Builders sand (soft sand) will not have been dried like kiln dried sand, or any specific measures taken to keep it dry. It will almost certainly therefore have some amount of water in it. For this reason, you should only mix it with cement when you intend to use it. Did you notice a difference in the mix a week later (like it was lumpy)?
The cold does slow down the set and has the added benefit of giving you a longer working time. Heat will dry the mortar, but drying is not the same as setting. Cement based materials like mortar and concrete harden through a hydration process. There's a chemical reaction with the water. This is not the same as drying. Many people think that the hardening is a drying process.
As I understand the home brew mixes, you're not relying on the cement when using the oven, only when you're constructing. It's the lime and the clay that ensure you have strength at high temperatures.
From what I can see, I think you'll be OK, but don't use the old mix again, or perhaps add a little fresh cement to it. It's a hard call over the internet!
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bento
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Post by bento on Dec 5, 2015 21:18:26 GMT
Thanks Simon. When I did my first 3:1:1:1 mix it appeared to remain fine, so thought it would be a good time saver. The batch of 4:1:1:1 was not quite as fine as I was mixing a larger batch and was more difficult to mix, but as there was more sand I expected it to be not as fine. It certainly wasn't what I would call 'lumpy'. I did consider adding more cement if I were to continue using it, but maybe I'll put it to one side for emergency use...or just mix a scoop in every time I mix a fresh wet load up. I plan on coating the whole dome with a ready mixed fire mortar at the end anyway, and if I am concerned about this one course, I guess I can scrape an inch or so out once the dome is complete and re-pack it?
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bento
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Post by bento on Dec 19, 2015 20:15:26 GMT
I am very close to completing the dome, maybe next weekend. Once the dome is complete I plan to apply a coat of ready mixed heatproof cement over it.
Q. Would you cure the oven before applying the ready mix, and then again when the ready mix is on, or cure it just the once after I have applied the ready mix?
As always, appreciate the advice!
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Post by chas on Dec 20, 2015 13:20:07 GMT
I am very close to completing the dome, maybe next weekend. Once the dome is complete I plan to apply a coat of ready mixed heatproof cement over it. Q. Would you cure the oven before applying the ready mix, and then again when the ready mix is on, or cure it just the once after I have applied the ready mix? As always, appreciate the advice! Are you planning to skip the insulation layer then? Or is the layer of render the insulation rather than 'waterproofing'? Is the cure you have in mind a succession of small fires? Particularly at this time of year, I think I'd get some of the residual damp out of the dome brickwork before sealing it in with anything waterproof. So, if you've left a couple of weeks for it to chemically cure, I'd light some small fires to dry it a bit, then add the render coat(s). Of course, if your delux gazebo shelter is still in place, much of this ramble is irrelevant! Cheers, Chas
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bento
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Post by bento on Dec 20, 2015 17:27:05 GMT
I am very close to completing the dome, maybe next weekend. Once the dome is complete I plan to apply a coat of ready mixed heatproof cement over it. Q. Would you cure the oven before applying the ready mix, and then again when the ready mix is on, or cure it just the once after I have applied the ready mix? As always, appreciate the advice! Are you planning to skip the insulation layer then? Or is the layer of render the insulation rather than 'waterproofing'? Is the cure you have in mind a succession of small fires? Particularly at this time of year, I think I'd get some of the residual damp out of the dome brickwork before sealing it in with anything waterproof. So, if you've left a couple of weeks for it to chemically cure, I'd light some small fires to dry it a bit, then add the render coat(s). Of course, if your delux gazebo shelter is still in place, much of this ramble is irrelevant! Cheers, Chas Hi Chas, This is just going to be an inch or less, just to add a bit more strength to the structure as I'm a bit paranoid of bricks giving way! My delux gazebo shelter is indeed still in place...just...having been torn down by high winds a couple of times, so the dome is actually quite dry, although I soak all my bricks for a good week before cutting them, so certainly the top three courses will still have moisture in them. Is it advisable to leave the dome a couple of weeks to chemically cure then before lighting the first fire? I was going to do the following in order: Finish Dome Leave one week Cure/dry oven with the first fires Coat in fireproof cement Leave one week More fires Insulate - blanket Insulate - vermicrete More fires? Waterproof tender 1 Waterproof tender 2
Thanks. Also, should I build my entry and chimney before curing the dome?
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Post by oblertone on Dec 21, 2015 11:06:43 GMT
If it helps (probably not) I used my oven dome for a year before getting around to an entry arch and flue, so the latter are not critical components but are certainly desirable.
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Post by chas on Dec 21, 2015 14:42:33 GMT
Is it advisable to leave the dome a couple of weeks to chemically cure then before lighting the first fire? I made the distinction because it's sometimes forgotten that cement sets by chemical reaction and not by drying. If you dry out cement artificially quickly you risk being left with a weakened bond. As opposed to a Weekend Bond which is a totally different animal. It's generally reckoned two weeks 'natural' setting gets you as set as you need to be. As an aside, it won't hold back setting if the brickwork gets wet. It would set underwater if you could stop it washing out - in fact new cement and concrete work is sometimes sprayed with water or covered in wet cloths to keep it damp in hot sunshine. Chas.
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bento
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Post by bento on Dec 23, 2015 17:39:10 GMT
Close to finishing, now cosidering the landing arch. As in the photo below, a full length brick butted to the front of the oven leaves a gap of 20mm to the entry arch. Is this gap too big for a heat break, or can I make it work. I've seen the fire rope mentioned, but don't know much about it or what thicknesses it is available in?
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bento
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Post by bento on Jan 8, 2016 22:26:28 GMT
If anyone can advise on the above post ref: the gap, it would be appreciated. Didn't get much done over Christmas but hopefully get close to finishing the dome this weekend.
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Post by chas on Jan 9, 2016 8:50:34 GMT
I'd resisted replying, having no experience of using fire rope or creating heat breaks, but an observation or three which I hope useful, or if not useful, encouraging while you're waiting for an informed reply:
Any straight joint in brickwork - heated or not - is likely to crack. Don't get downhearted by this. A flexible seal of course anticipates this by being a pre-crack. Heat-resist silicone gunned in for a couple of inches of the outer joint might be more permanently useful and water resisting - any open joint can allow water to track into the interior. Single-skin brickwork has little mass to resist distortion - could you beef it up to 9" work? It'll have more joint profile to help keep heat in. This brickwork mass my be useful when it comes to the flue/final opening if you're thinking of an arch. Arch ends need a bit of beef to bear against.
Cheers,
Chas
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bento
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Post by bento on Jan 9, 2016 11:29:37 GMT
Hi Chas, thanks for the reply.
I have one issue with beefing up the thickness - I am down to my last 19 firebricks which is exactly enough to create a single skin entrance with archway - I could double up but would need to purchase whatever normal finishing brick I intend to use when I build up around the rest of the oven (I don't think I live close enough to anywhere to buy a dozen more proper fire bricks (shipping would be too expensive)).
When you say "gun in heat resistant silicone for a couple of inches of the outer joint"; by this do you mean use homebrew to join the outer arch/wall to the inner arch/wall as normal, but don't pack it to the edge - leave a one inch gap all around and fill with silicone?
Would this siliconed joint be visible?
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Post by chas on Jan 9, 2016 19:22:58 GMT
Hi Chas, thanks for the reply. I have one issue with beefing up the thickness - I am down to my last 19 firebricks which is exactly enough to create a single skin entrance with archway - I could double up but would need to purchase whatever normal finishing brick I intend to use when I build up around the rest of the oven (I don't think I live close enough to anywhere to buy a dozen more proper fire bricks (shipping would be too expensive)). When you say "gun in heat resistant silicone for a couple of inches of the outer joint"; by this do you mean use homebrew to join the outer arch/wall to the inner arch/wall as normal, but don't pack it to the edge - leave a one inch gap all around and fill with silicone? Would this siliconed joint be visible? Well, you don't actually need firebricks for the entrance - a reclaimed clay brick will do just as well, so maybe a hunt on Gumtree will yield some cheap if you'd like to go down that route. And yes, whatever brick you use, I'm suggesting is a flexible joint where it's visible, so the outside inch or so depth siliconed with the remainder built up with homebrew or whatever mortar you fancy. You could achieve that by bricklaying as per normal, then scraping out the mortar to a depth of an inch before it sets, and replacing it with silicone. If I'm completely out of line on this I'm sure someone will jump in to put you (and me) right! Chas
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Post by downunderdave on Jan 10, 2016 4:18:02 GMT
".... I'm suggesting is a flexible joint where it's visible, so the outside inch or so depth siliconed with the remainder built up with homebrew or whatever mortar you fancy. You could achieve that by bricklaying as per normal, then scraping out the mortar to a depth of an inch before it sets, and replacing it with silicone. If I'm completely out of line on this I'm sure someone will jump in to put you (and me) right!" Chas Don't be expecting too much from high temp silicone. The best I can get is rated to 700 F (371 C) intermittent (let me know if you find one with better performance) and will certainly not last long if anywhere near exposed flame. The flame will usually lick around the top of the oven doorway and into the flue gallery. Home brew or proprietary high temp mortar mixed with a high proportion of perlite is an alternative option, but is not flexible. Perlite is good for up to 1100 C, and will compress, but it won't really expand again like silicone. Every other high temp caulk I've tried cooks hard and does not remain flexible.
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Post by downunderdave on Jan 10, 2016 4:33:41 GMT
Some folk mortar the first course, some not. It really makes little difference. Mortaring will ensure they are level. the IT should be at floor level. If it is higher you will end up with the dome taller than a hemisphere. In fact it doesn't really matter either, you would not notice any difference in the way the oven fired or performed if it's slightly (4") taller or shorter than a hemisphere.
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