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Cast dome with 2nd hand arch. Queensland.

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  • Nicholasd
    replied
    Thanks for the info on the flue, it is giving me a bit of a headache. I'll ask more about that in another post.

    I'm looking for render at the moment and was going to use your method of part home-brew render mix and part acrylic PM render "A 4:1;1 mix of sand, hydrated lime and cement is pretty good. I use that mixed with the PM render 50/50 of each with plenty of AR fibreglass fibres randomly mixed for strength enhancement."
    Bunnings seem to be out of the Davco PM render. But they have a Dunlop 20kg Fine Coat Render which is polymer modified. Would this be suitable?
    The missus would really like a penny tile mosaic over the dome. This would enhance the weather proofing as the oven is in the open. Can I do this after one coat of render?
    Thanks,
    Nicholas

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  • david s
    replied
    Here are a couple of pics that might help you

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  • david s
    replied
    When the stuff has gone white you'll think it is perfectly dry, but it won't be deeper in. Try getting a cheap garden moisture meter to push into the layer to test for moisture.
    Regarding setting the flue pipe, most builders use an anchor plate. However they require drilling into your refractory and using fixings. Both of these operations are asking for cracking to occur around the holes from thermal expansion as well as corrosion problems if you don't use stainless fixings, plus anchor plates are really expensive and take up a lot of room. I prefer to cut 3 tabs in the pipe so it can be inserted into the hole in the gallery (slightly loose fit to allow for thermal expansion). The tabs can be easily bent to adjust pipe level.
    You have plenty of blanket layers so don't really need insulating capacity from the vermicrete layer, you just want a firm substrate to render against. For others reading this an easier way to get more workability in a very lean vermicrete is to use a little powdered clay which is half the mass of cement and imparts stickiness to assist application. eg 7:1 vermicrete results in a substantially poorer insulation due to the extra mass of cement. eg at 10:1 the mass of cement is around 1.5x that of the vermiculite/perlite combo. At 7:1 it's around double.

    Click image for larger version

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    Last edited by david s; 04-13-2023, 01:26 PM.

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  • Nicholasd
    replied
    Click image for larger version

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ID:	452581 Made some more progress.
    Here insulation on. Ended up using the whole box and got 4 layers on so about 100mm insulation. CF does not reach the slab.

    Also went with the flashing around the base with high temp silicone to seal it up.
    Click image for larger version

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ID:	452582 I am terrible at silicon work!

    Perlite (coarse) and Vermiculite (grade3/medium fine?) went on soon after in a mix of 8:1 GP cement whereby I put a bit more Vermiculite in the mix (5:3 Vermiculite perlite) to help balance out the coarse perlite. Was not easy to apply from the second row and kept crumbling so reduced to 7:1 to help get it to stick. Let the first row set overnight and continued the next day with more success. Also put in a steam vent, out of 25mm/1/2" galvanized pipe which I cut with vertical cuts in the base to allow water vapor in.
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    It has been 7 days of very dry sunny weather which has been great for air drying. No fires yet!
    Render/stucco in the next few days.

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  • Nicholasd
    replied
    Will start insulation after a week of damp curing. No curing fires yet.

    Ceramic fibre, I have 1 box so what ever that covers. This will be followed by perlcrete 10:1 in 2 x 40mm layers.

    Question: How far down to go with the ceramic fibre? To the bottom of the fire brick or the Cal Sil board? I presume NOT to the slab as to avoid moisture.

    Same with the Perlcrete which I guess goes down to the slab?

    Does the pelrcrete go below the CF at the base up against the CS or should I leave an air gap all around the base for ventilation. The CS sits on broken tiles, I have weep holes in the slab.

    I like this idea taken from another user (and maybe the melbourne firebrick co!) With 0.03 Alu foil flashing and a bead of hi temp silicone around the base. I think this would be better on the outside of the perlcrete? My perlcrete should go to the edge of the slab in places.

    Click image for larger version

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    Flue questions: I plan on using one segment for the flue plus hat. I take that this just sits on top of the arch and is held in place by about 45mm/2" thickness of perlcrete 5:1 (?) and about 300mm/12" high. I have a bucket from Bunnings that will fit perfectly for this job. Does that sound about right. Does the flue sit in a small bed of home brew as well? I'll leave an expansion joint when I render.

    Thanks for all your help on this! Maybe this will help someone else with their oven.



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  • Nicholasd
    replied
    More work done last week here is the update.

    Formed up the door with xps foam as it was easier to shape than wood. Piled up bricks and sand again to shape the door and gallery.
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    Here is the result
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    And after patching.
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    Didn't add any colour to the mix.

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  • david s
    replied
    I prefer white because it reflects light better, giving you a good view of what's in the oven. The white contrasts well with the black soot. When the oven approaches pizza temperatures all the black soot will have burnt away., right down to the base of the dome. This is my primary temperature gauge. For roasting or baking I stop firing when the crown of the oven goes white, but the rest of the dome is still black.

    I just noticed that your oven is pretty much level with the garden behind it. As moisture is your enemy once the oven is finished and dry, It would be a good plan to put a drainage channel behind the oven. Moisture wicking into the slab can be quite a problem. Also, if you've not already cast some weep holes in the slab you can drill some in from underneath. You'll feel it break through when you hit the insulation.

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  • Nicholasd
    replied
    Very busy, so can't rush it at all.

    Quick question, can I add colour to the home brew mix. I think this would look good for the door opening and be better around the brick look of the arch. I was thinking of black as the red probably wouldn't match the red of the arch. I know some of it will get black from soot but the lower sides wont.

    Thanks.

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  • david s
    replied
    Looking pretty good. I think you’ve got the back broken now. Just don’t get too excited and rush things. How are you planning to do the insulation?

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  • Nicholasd
    replied
    Made some progress and got the dome done about five days ago. It took at least three weeks for SIKA to send the packet of fibers, thanks to the shop who ordered them as they don't normally stock products like this.
    Turned out well (I think!) with the occasional void to fill which we did the following night with some sieved sand home brew mix.

    It is very hot here at the moment around 30c/86F during the day and about a bit less in the evenings. Have kept it well damp with a wet towel and misting hose and covered for five days and will continue to do this while I plan out the door and joining the arch/gallery. Click image for larger version

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    Attached Files

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  • Nicholasd
    replied
    I'm searching for the Sika fibers on the Gold Coast/South Brisbane to no avail. I have called Sika Brisbane and they gave me a mobile nr of the concrete guy, but he hasn't replied to my messages yet. I would be grateful for any leads on this. Just put the name of the company and I'll search it. Sika list distributors for ALL products so you get Bunnings, tile and flooring shops for their other products.

    The sand mold door area/gallery is not finished the door plate is just there to hold the sand up. I want to cast the door entrance after the dome is cast.

    The gallery is actually doing my head in, but now the sand mold is done the visual aid of this is helping.

    Here is a pic of the arch/gallery I bought 2nd hand. Kind of regret it now after seeing your arches which look great and practical. The bonus was 4 segments of flue pipe which I can sell/give away to another builder/neighbour. I also got a set of oven tools in fairly good condition. Click image for larger version

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    If you look at the back side of the arch with steel door frame (NOTE: I will not be using this door at all, I will make one out of leftover Cal Sil. The door is however similiar size but not the "Bell" bits) I will shape the dome down towards this.
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    Perlicrete will then fill the area above the door to the top of the arch. I also plan a thermal break out of perlcrete to join the arch to the cast dome/door.


    Anyhow, first I nned some fibers and get the dome cast!


    Cheers,

    Nicholas


    Attached Files

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  • david s
    replied
    Looking at your pics, it appears that you won’t end up with a flat face on the front of your oven mouth for the door to seal against the rebate. The front mould plate should be as big as the outer size of the inner dome casting, while the sand should be the inner internal shape. Ie smaller than the mould plate.

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  • david s
    replied

    Built the sand dome which looks good and gives me motivation to keep on going![*]Questions: If I don't finish the dome (by leaving the gallery to do afterward) how long do I have? Is it the next day or can it be a few days later? Also how do I finish the edge of the mud so that it joins again when I do the gallery.[*]Is it worth putting a join around the top 1/4 as a natural expansion/crack line. Someone else did it here and it seemed to work. Could this be done by simply putting a piece of paper/card board between the bottom 3/4 and the "scull cap" top piece which would burn out leaving a join?[*]I put strips of wet newspaper on the dome, but it seems to slip around and has wrinkles. I have seen another FB build where they cast the dome on the sand with good results.[*]I have one bag each of lime and clay will this be enough? I have several bags of cement and a pile of sand. 85cm/33" dome.[/LIST]Fibers. I don't have fibers yet as I'm not sure which ones to get. I looked at the Sika website and found this SikaŽ ConfibreŽ 19F fibrillated polypropylene, melting point 160c. Are these correct? Or these ones? SikaŽ ConfibreŽ 51F. Jim, the guy I got the oven supplies said they are difficult to handle and its not really worth it.

    Thanks for all the advice!
    [/QUOTE]

    The gallery can be fitted in place well after the dome is completed. It is the same as mortaring a dry brick in place. You can simply use the homebrew mix to mortar around the oven mouth and place the gallery in position, then clean up the joint.

    you could cut some curved pieces of cardboard to create the join for the "skull cap" and leave them in place, to burn away. U think that will work well and pretty easy to do.

    To help the newspaper to stick rather than slip out of position, dip each strip in a mix of 2 parts water 1 part PVA. You don't want too much glue or it will present problems removing it.

    U think you will need two bags each of lime and clay. You can calculate the volume by working out the outer hemisphere and subtracting the volume of the inner hemisphere using 4/3 x pi x r3, remember to halve the result because it's a hemisphere.The amount lost in the oven mouth is about the same as the extra amount required that surrounds the mouth, so just work on the volume of the hemisphere ignoring the mouth.

    If you don't include the burnout fibres, you risk steam spalling and if that happens you'll have to start a new casting and probably also have to redo your sand mould. All castable refractory products that I've seen contain burnout fibres for this reason. It is not 100% guarantee against steam spalling, bur gies a long way to reducing the risk as long as you follow a slow drying schedule.
    The fibres usually come in a pack of two different fibres the short and really fine 19mm ones and the longer, thicker 45mm ones. I get Sikafiber PPM 45/19 (2.3 kg pack) Go to your local Sika distributor. These are designed for standard concrete, the longer fibres to increase flexural strength and the shorter finer ones to reduces early shrinkage cracking. So for our purposes only use the fine ones and keep the longer ones for another project or maybe use them in the outer render layer.rather than chicken wire. That layer does not reach anywhere near 160C I prefer to use the thinner, shorter AR gkass fibres there but the thicker ones will work, you may need to pull out the odd one that's sticking out.

    Not sure about carbon fibres and cement. The reason AR alkaline resistant fibres are used is because they react with the cement and degrade. The AR fibres are coated with zirconium, I doubt that the carbon fibres would be, but not sure.
    Last edited by david s; 02-04-2023, 01:13 PM.

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  • Nicholasd
    replied
    OK. just saw this which may be local (Gold Coast) for me Chopped Carbon Fibre at carbonparts. search to get the website. Is this also an option?

    Thanks.
    Nicholas

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  • Nicholasd
    replied
    Yeah those bricks are dusty, even when soaked. Hosed my self off then jumped in the pool for a swim afterward.

    Good that the sand is ok.

    Making more progress.

    Built the sand dome which looks good and gives me motivation to keep on going! Click image for larger version

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    • Questions: If I don't finish the dome (by leaving the gallery to do afterward) how long do I have? Is it the next day or can it be a few days later? Also how do I finish the edge of the mud so that it joins again when I do the gallery.
    • Is it worth putting a join around the top 1/4 as a natural expansion/crack line. Someone else did it here and it seemed to work. Could this be done by simply putting a piece of paper/card board between the bottom 3/4 and the "scull cap" top piece which would burn out leaving a join?
    • I put strips of wet newspaper on the dome, but it seems to slip around and has wrinkles. I have seen another FB build where they cast the dome on the sand with good results.
    • I have one bag each of lime and clay will this be enough? I have several bags of cement and a pile of sand. 85cm/33" dome.
    Fibers. I don't have fibers yet as I'm not sure which ones to get. I looked at the Sika website and found this SikaŽ ConfibreŽ 19F fibrillated polypropylene, melting point 160c. Are these correct? Or these ones? SikaŽ ConfibreŽ 51F. Jim, the guy I got the oven supplies said they are difficult to handle and its not really worth it.

    Thanks for all the advice!

    Leave a comment:

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