What pattern for the hearth bricks?
My stand is finished and now I wait for the inclement weather to abate before starting on the hearth. I have two questions?
How smoothly should I finish the refractory concrete hearth in preparation for the bricks. My inclination is to make it quite smooth, but if I do perhaps the refractory mortar won't adhere to it well enough.
In the Forno Bravo instructions they talk about a herringbone layout for the bricks, but the picture shows a traditional overlapping pattern. Photos that members have sent in seem to be exclusively overlapping pattern. It seems to me that the herringbone would be better for sliding things in and out.
When I figure out how to do it I'll send some photos in of my oven to date. :p
Level and smooth and diagonal herringbone
Davy, ultimately the degree to which you trowel your slab, in my opinion, depends on only one thing--will the perimeter not covered by the oven show? However, keep in mind that when you trowel your sand/fireclay bedding mixture to set your hearth bricks, you're only using an 1/8" notch trowel, so the ability of the bed mixture to compensate for a highly rough surface is limited. In my case, I steel troweled the surface so it was pretty smooth. That makes it much easier to keep your hearth as flat as possible. Marcel is correct that the two do not adhere per se, so that isn't a consideration. Your dome mortar and cladding will lock everything in place. If you have any other questions, feel free to email.
Thanks for the guidance
Thanks Marcel and David for the prompt reply to my queries. (I think this website is a fantastic resource.) Your responses make lots of sense, and I'll follow them to the letter.
In the Forno Bravo instructions it says that I can use either a regular concrete mix (I guess that is 1 part cement and 5 parts navvy jack) or refractory concrete (I'm not sure of the ingredients and ratios here.) Would either of you (or anyone else) recommend one over the other?
Consider latest thinking about inverting the slab layers
(M) Before you commit to any proportion for your hearth floor, I think you might want to consider following a thread which has been discussed by James Bairey, this Forum's moderator, and Jim ( aka ColCorn76) who built the first "igloo" Pompeii oven metnioned on this site. The reason I suggest you do so, as others also feel that perhaps the initial plans of a bottom layer of perlcrete followed by a layer concrete may be possible to reverse and come out with an improved design. There is also some discussion about an inner island. I believe some builders have already reversed the layers, or created an island.
(M) Since I have finished mine in the traditional layer order, I'm not in a position to help you much on this but if I were to start another oven, I'd do a Search on this Forum for key words like: Island, Layer, and maybe Slab. I'm pretty sure that a few Gurus will step up to the plate and give you some feedback soon. Suffice to say that many builders who used the traditional Pompeii order of layers felt that they had a hard time keeping their oven temperature high enough without burning a great deal of wood.
(M) Someone, and I don't remember who, questioned if it would be better to pour the re-bar reinforced concrete layer first so that the hearth bricks would sit on some insulation. That was thought to be a way of heating the bricks quickly as the heavier concrete would not be drawing away too much heat.
(M) I believe someone else modified that to have a separate island of Perlcrete (insulating concrete) surrounded by the heavier concrete. It is all too technical for me but I think it is something you owe it to yourself to at least investigate. Their logic seems solid enough but there haven't been enough ovens built yet with the reverse or modified layering to make any evalution valid.
(M) Take it slow. Make this oven a fun process rather than a goal. Once poured you've got it "cast in concrete". If you spend a few more days exploring, you'll still have most of the spring to bake pizza.
(M) btw: Many forum members don't include their general location in their profile. I think that's a shame because geography Info. would help us advise for things like frost heave, wet wood, etc. I'm in central west Oregon where it rains a lot but I didn't have to set footings in fear of freezing earth.
Island hearth or not?
Thanks again Marcel for your observations; they are very helpful.
I visited the local supplier today to pick up 280 bricks and refractory mortar and calcium aluminate cement. They have fire bricks, but not refractory bricks. They have no refractory mortar or calcium aluminate cement and no fire clay! Which is all just as well for I am forced to slow down now. As you imply, enjoy the journey and not just the destination. Good advice.
Because I'm a newbie here I now see that I have been asking questions that have already been answered in the past. I saw one posting that said it was preferable to use standard concrete over refractory concrete, so I will take that advice.
Unless I hear from others advising against it, I intend to go with the "island hearth" concept probably using 4" reinforced concrete, 2" vermicrete and 2" non-reinforced concrete as an island surrounded by vermicrete.
I want to make this the best oven that I can (like everyone else)and feel fortunate that I can draw on the wisdom of those who have gone before me. What brick is best to use, refractory or fire brick, or are they the same brick but with different names? Is it important to get the specs on the bricks so one knows what temperature they can withstand?
Refractory Brick composition and availability in BC
(M) Hi, Davy,
(D) "Thanks again Marcel for your observations; they are very helpful.
(D) I visited the local supplier today to pick up 280 bricks and refractory mortar and calcium aluminate cement. They have fire bricks, but not refractory bricks. They have no refractory mortar or calcium aluminate cement and no fire clay! Which is all just as well for I am forced to slow down now. As you imply, enjoy the journey and not just the destination. Good advice.
(M) I got some Spec. sheets from Clayburn Industries, a Co. connected, I believe with I-XL. I wanted to get as close to the brick composition suggested by Jim and James as possible. They make many refractory bricks but I only got and offer these five as they were the ones my masonry dealer stocked.
(M) I ended up with their "Clayburn S". Though the critical % of Silca = 60-64% and Alumina of 29-33% are not the optimum recommended by the Jims, they probably fall within the acceptable range. Too much Alumina and your bricks actually get too hot and burn your pizza. Too little, and they retain too little heat. Be careful that you don't cross those percentages as Clayburn makes a refractory Brick for industrial applications ( their CB 70D) which reverses that important ratio.
(M) What I understood from this Forum's postings is that terms like "Low Duty Firebrick" may mislead the Newbie into thinking that such bricks are inferior where in actuality something marked "High Duty" may be totally wrong for you. Your bricks should be fairly heavy, I believe about 6 US pounds a piece. The lighter bricks are for insulation and *not* what you want for your Hearth floor.
(D) Because I'm a newbie here I now see that I have been asking questions that have already been answered in the past. I saw one posting that said it was preferable to use standard concrete over refractory concrete, so I will take that advice.
(M) I used standard premixed bag concrete although I en richened it somewhat with extra pure cement, and that only because I was running low on the straight bag concrete.
(D) Unless I hear from others advising against it, I intend to go with the "island hearth" concept probably using 4" reinforced concrete, 2" vermicrete and 2" non-reinforced concrete as an island surrounded by vermicrete.
(M) I smiled when I read "vermicrete" as I believe it was I who coined the term Perlcrete to differentiate it from pure Perlite. I like "Vermicrete" though it makes me think of vermin. As an aside, if you decide to use Perlcrete instead, be sure you get the agricultural grade and not the construction grade. It was probably paulages or Robt. Musa who pointed out the industrial grade is coated with silicon; great if you're using it to fill cores in concrete block as it flows easily, but totally wrong for our oven builders cladding as it won't absorb water well, or well water ;-)
(M) As to advice on the reverse layers of your hearth slab you'd be best off getting advice from the Jims. I only used the standard arrangement.
(D) I want to make this the best oven that I can (like everyone else)and feel fortunate that I can draw on the wisdom of those who have gone before me. What brick is best to use, refractory or fire brick, or are they the same brick but with different names?
(M) Sounds like the same brick to me, but the SiO2 and Al2O2 proportions are what are critical.
(D) Is it important to get the specs on the bricks so one knows what temperature they can withstand?
(M) Certainly the Temp. Specs. are useful but also to get some idea on the "Linear Change on Reheating". You want that to be at a minimum. These Clayburn Spec. sheets are fairly comprehensive. If you can't find their "Clayburn S" they offer a "Flame-Low Duty Firebrick" with close percentages of Silca = 62-66%, and Alumna of 22-26%. Their Tel. # is (604) 859-5288 and their Fax # is 859-0424
(M) Here follows some other useful links:
Clayburn Industries, Ltd. in Canada V2S 5C1 email address is email@example.com
Their parent Co. seems to be I-XL with a Web address of http://www.ixlgroup.com/
The URL for dealers in BC is: http://www.ixlgroup.com/dealers/bc_map.html You will see the following reference to twenty five I-XL dealers in BC but I doubt that this Copy-Paste will reproduce:
4 Dawson Creek
7 Grand Forks
15 Prince George
17 Salmon Arm
25 Williams Lake
Isolated island, with no doubts!
It is all about mass.
If you have more mass to heat you will use more calories (wood) to reach the desired temperature.
Complete slab means more (unused) mass.
I hope the pictures in annex could help you in your oven decision.
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