Cast dome with 2nd hand arch. Queensland.

[david s]

That’s fine. Either side up, it doesn’t matter. It is the gaps between the tiles that you want to create.

[Nicholasd]

Very hot here today (for the gold coast), not going outside until some shade is over my work area.

My question is about the sand part of the homebrew.

I got some brickies loam for the cast before xmas thinking I would get some casting done and because the shops shut over the holiday period. Now I am not sure if it is correct after reading some FB posts about brickies loam/sand for homebrew. One post said that it already had clay in it. Is this correct? Do I have to adjust the homebrew recipe or look for different sand?

[david s]

Basically any sand will work ok and actually a mix of grain size is better than sand that has the coarser grains sifted out. Some brickies loam does naturally contain a little clay, but probably not enough to cause you any problems. Too much clay can impart more shrinkage than you want, leading to shrinkage cracks. You could test the clay content of your sand by simply filling a glass jar 1/2 full and topping it up to 3/4 full with water. Give it a decent shake and let it stand for 24-48 hrs. The sand will separate from the water with a small layer of clay in between, if there's any there at all.
The homebrew already has a high clay proportion and extra could impart the shrinkage cracking. However, as it really helps to bind all the other ingredients together to make it easy to apply, apart from imparting refractory qualities, it is an important ingredient.

[david s]

Because of the hot weather, I suggest you time your application of the dome cast for the late afternoon, say around 4 pm. That way the casting will be out of the sun and will be slower to go off. Also the use of chilled water will help. In summer I have to always do this to avoid wet casting material going off too fast. We’re in Townsville so the heat is an even greater problem. Two people applying the wet mix helps get the job done quickly, but make sure you wear rubber gloves or you’ll regret it. Cover the casting the next morning, but don’t use black plastic which will also increase temperature if the sun hits it.
The addition of the fine polypropylene fibres, apart from their use as burn out fibres to assist water elimination, also assist greatly in reducing early shrinkage cracking which is their primary function for standard concretes.

[Nicholasd]

Now I'm back from camping, time to get into the oven again.

Here is a pic of the sand in water after letting it sit over night. I can see a very narrow band of clay on top (about 1-2mm) Do I need to adjust the homebew mix to account for this?.

[Nicholasd]

Laid the fire bricks down and cut the the pieces with an angle grinder (125mm). Soaked the bricks for a couple of hours while I exchanged the correct cutting disc, the inner diameter was incorrect. One disc cut about eight bricks before slowing down. Very happy the bricks are very flat with no edges catching on a trowel to test it.

[david s]

There is negligible clay in your sand so no need to reduce clay in the recipe.
When cutting firebricks the inhalation of brick dust is a dangerous activity, you really need to use a wet saw. Even after soaking the bricks for ages it still makes a large amount of dust as you no doubt found. However it seems like it's too late now. Also a cheap dust mask is inadequate as you've probably also already found out when you blew your nose.

[Nicholasd]

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!

• 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 Sika® Confibre® 19F fibrillated polypropylene, melting point 160c. Are these correct? Or these ones? Sika® Confibre® 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!

[Nicholasd]

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

[david s]

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 Sika® Confibre® 19F fibrillated polypropylene, melting point 160c. Are these correct? Or these ones? Sika® Confibre® 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.

[david s]

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.

[Nicholasd]

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.

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.

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

[Nicholasd]

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.

[david s]

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?

[Nicholasd]

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.