Longmont, CO new WFO build - casted over sand

[david s]

When you have completed the dome casting, remove the mould door plate, but not the sand behind it. You can then form more sand in front of the oven as a mould for the flue gallery. You will need anothermould plate for the front of the flue gallery but won't need one for the back.Don't forget to cover the sand mould with strips of wet newspaper so you get a nice inner surface. Forget the manufacturers recommendations on water addition and just mix in enough to make it nicely workable. Too much and the mixture will slump, too little and you'll get more voids. Don't go more than 2% of dry weight of castable for the ss needles or you'll have trouble getting a nice workable mix. Don't mix up more than 1/2 bag of castable at a time as the stuff goes off really fast. Chilled water in hot weather will give you more working time.

use a piece of plastic pipe or bend some sheet metal for both the inner and outer bits at the top of the flue gallery.

[cnegrelli]

david s - Appreciate all the hand holding. I am not 100% sure I follow your terminology of "Mould door plate".
I assume you mean a model of the opening size/shape that would be place into the hemisphere shape such that the front opening is formed. And I can trowel up against it to create the "inner arch" essentially, with a flat vertical edge. Or I could just do wood forms?

Also unclear on your mention of plastic pipe or sheet metal, etc....

Here's a pic of the shape of gallery I plan (assuming that is the correct term as well). I plan to use an anchor plate and duravent chimney system, and create the flue with the castable up to the flat part that the anchor plate would attach to. Bought the anchor plate in fact today from our host. Would like to maximize the funnel shape to help pull the smoke up as I've read.

Does anyone know where this picture came from? Wife found it somewhere....

[cnegrelli]

Removed the forms from my stand today (finishing the wood storage area removal in the dark) but feel it came out pretty good. I don't see any obvious problems such as voids or non-straight edges.

The 10" Wet Tile Saw I planned to borrow to just cut my floor bricks is unavailable because the owner is in Africa! I will only be cutting the floor bricks, so it would seem that I don't need to be doing Taper cuts. Is a 7" saw enough for the "round" cuts I will be doing? Prefer to keep it wet, as I am pretty sensitive to dust/particulates.

[UtahBeehiver]

I recalled that you said you were doing an enclosure so there is no need to do "round" cuts and precisely fit the floot, just make sure all your floor bricks are on the CaSi/FoamGlas insulation. If they go proud in length, no issue, they will no be seen once the enclosure is up.

[cnegrelli]

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.

[david s]

I think you will have to recalculate. The volume of a sphere is 4/3 Pi r cubed (not squared)

[cnegrelli]

I think you will have to recalculate. The volume of a sphere is 4/3 Pi r cubed (not squared)

The mistake was in my verbage above, not in my calculation! I had it cubed. Does that sound like I'm in the ballpark?

[cnegrelli]

In order to level out the FoamGlas and Insulation board, I've seen a 50/50 sand mix with fireclay used. Is the fireclay required? Is the thought that it would get hot? Since I'm doing a casted oven, I wasn't going to bother with homebrew mortar and thus no fireclay has been sought out. I do have a bag of high temp mortar for patching, etc. Probably have plenty, but I have no idea how much I'll need for that portion.

What is the thoughts on the proper way to level out the 4" of insulation board I have for under the firebrick floor?

[UtahBeehiver]

You may not need to level the FoamGlas/CaSi if your hearth was poured flat. You will "not" see significant temperatures on the bottom side of 4" of insulation. If you need to so some leveling, a damp sand mix grooved with a notched trowel will suffice.

[cnegrelli]

You may not need to level the FoamGlas/CaSi if your hearth was poured flat. You will "not" see significant temperatures on the bottom side of 4" of insulation. If you need to so some leveling, a damp sand mix grooved with a notched trowel will suffice.

Thanks Russell. I laid down the FoamGlas last night (and one piece of CaSi) to get a feel for it. My Hearth has a low spot near the back that pools water slightly, so I need to level a bit. I will also need to cut off a few corners of each of the materials above so they don't extend to my enclosure walls. What did you find was the best way to cut each? I noticed you mentioned a jigsaw (blade type) and scoring the FoamGlas with a shard (I assume a shard of FoamGlas)?

[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..

[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

[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]

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

[cnegrelli]

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