Longmont, CO new WFO build - casted over sand

[cnegrelli]

Anyone remember the thread where the anchor bolts were embedded in the cast flue for a anchor plate / Duravent setup?

[cnegrelli]

2. Straight away after the dome casting has set (3-4 hrs)

I'm not quite ready to cast the gallery/flue area yet, so I'm probably going to have a few days between them. I want the dome to go up tomorrow as it will give me a good feel for how to use/apply. Also will give it more time to dry. Anything wrong with that plan? I assume the answer above was "Minimum time", with no maximum....

[david s]

1. You can do it straight away. Cut a stick to the length of the internal height and place it in the middle so you have a gauge to get the correct height. Adding a small amount of powdered clay to the sand will help make a good mix but not essential. You can reduce the amount of sand required by using some plastic bottles to take up some volume.You could build the sand castle and do the casting the next day but you will need to cover it and keep it damp.
2. Straight away after the dome casting has set (3-4 hrs)
3. Yes, I use 50/50 motor oil and kerosene applied sparingly but thoroughly you don't want an oily surface left on your casting.
4 A couple of inches thick
5. The water ratio is 8% from memory but your sand will probably be damp so just go by instinct.

[cnegrelli]

PIcked up the refractory this morning and the SS needles. 6 bags at 55 lb each on the refractory castable and 5 lbs of needles.

Some casting questions for david s :

1) should I plan on casting over the wet sand covered in newspaper right away? This isn't something I would spread across two days for example? Mould one day and slop on castable the 2nd?
2) How many days between casting the dome and moving on to the entry/flue piece?
3) Door Moulds - I planned on jigsaw cutting the entry shape on plywood and mounting that to another that is 2" bigger in all directions (except down). Then using sand for the rest of the dome shape but also the transition to the entry. That will give me a flat, 2" face to trowel up against. Do you cover the wood with anything for release?
4) You've mentioned skinny strips of Newspaper? What's your definition of skinny?
5) Is this sand appropriate? mix with water first in wheel barrow?

[cnegrelli]

Weak as in 5:1?
How about a Vericulite / Refractory mix for this application? I am fresh out of concrete mix.

[david s]

You may stop some heat by conduction by having a thermal break but being so small radiant heat will still transfer readily. The gaps other arguably more important function is to act as an expansion joint. If filled with an inflexible highly conductive material like an aluminium tube you are defeating its purpose IMO. Allowing it to fill with ash is probably a better option but doesn't look as good. I fill mine with a weak vermicrete mix. I think it comes down to appearance vs function,

[cnegrelli]

For a thermal break on the floor, I have seen the SS u-channel method, which is proving to be pretty expensive for me. Are there other methods? What about L shaped cutout of firebricks that would have insulation filling the void below? That would minimize the heat transfer to just the surface contact of the bricks. How effective would that be? Other methods?

I just stumbled upon a thread that used a 1/4" gap in the floor (filled with ash). Air is a good insulator. No?

[cnegrelli]

That picture is worth a 1000 words!

[david s]

Yes, a 6" flue pipe is the correct size for a 32" oven. Yes you can move the flue gallery back to keep the gallery shallow which then has the big advantage of better access to the oven allowing shorter handled tools too. See my pic. I was able to keep the depth of mine to 5" (the same as my flue pipe diam) by this method and yet still retain good volume in the gallery to collect smoke.Another advantage is that it reduces the foot[rint of the oven (mine is square).You can have any setback from the front of the hearth that you like. I only have an inch but again that was to save space to make it as compact as possible.

[cnegrelli]

Currently planning an 18" entry width that is just over 10" tall. Chimney will be 6" ID. Is this adequate for a 32" (workability vs heat loss)?

I plan to cast over sand the dome up to what would be the inner arch in a brick oven. The entry will (from a side view) be a vertical face of the above size. Then I plan to caste the gallery/flue as a 2nd piece, in place with some cardboard between them to form my thermal break.

For the gallery, I will try to funnel up a flue to a 10"x10" anchor plate. I noticed on other cast builds, the chimney flue is actually pushed back into the dome area somewhat. Is this for performance, or to just save on size/weight? These were for mobile builds such as David's or the one here at a local restaurant, but I am hoping to keep the gallery relatively short. Should I be trying to move the flue back into the dome area more?

Lastly, what sort of setback from the edge of the Hearth to the front of the oven gallery is typical? I have plenty of room on my hearth, but moving it back, gives me more counter space on each side. Moving it forward makes it easier to access...

[cnegrelli]

Or calculate twice, then post your work for peer review, then get corrected, then mix once!

[david s]

" Measure twice, cut once" or calculate twice mix once.

[cnegrelli]

Found my formula problem in my spreadsheet.
When calculating the ID, my area formula took 32" divided by 2 to get radius and then applied it to the formula above - all correct.
When calculating the OD, my area formula worked the same way, but I added 2" to the diameter (should have been added to the radius).
Turns out I was calculating for 1" walls in that case, not 2" thick - so approximately a 2x error!
David - that would have been a disaster - I owe you more than I already owed you!

[david s]

Using some handy tips I've noted on various threads via david s I've calculated I will need 3 55lb bags of refractory. 32" diameter is a 16" radius, 2" thickness. Using volume = 4/3 Pi times Radius squared, gives a inner volume. Subtracting that from the outer volume and cutting in half to get half a dome, yields about 2014 cubic inches. Which is about 33 cubic liters. Using a 85% factor, I bump that to 39 cubic liters. Somewhere, David posted 16.37 liters per 55 lb. bag. So that works out to 2.4 bags. Will order 3. For those curious, these are $46 per bag at my local refractory supplier. Prices vary widely.

The product I plan on is CastMax 28, discussed previously in this thread.

Edit: per below, the calculation for volume is cubed, not squared. volume = 4/3 Pi times Radius cubed.

Working in metric I get 0.20 m3 for outer volume (200 litres)
0.14 m3 for inside volume (140 litres)
difference is 0.06 m3. (60 litres) or 2.12 cu ft (3663 cu in)
55 Pound bag is 16.37 litres dry mix which yields 15% less when mixed wet which is 13.9 L/bag
Therefore you'll need 4.3 bags. Better get 5 and you'll need another bag to cast the flue gallery, so get 6

[UtahBeehiver]

I used a jig saw for the CaSi, be sure to wear a mask. I used my wet saw for the FoamGlas, it must be handled gingerly, it is brittle. I suspect a hand saw, or jig or coping saw will work, just don't force the cuts. It also smell like rotten eggs when you cut it. Again, since you enclosing, your cut do not need to be perfect, just extend past brick floor and dome..