Longmont, CO new WFO build - casted over sand

[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...

[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]

That picture is worth a 1000 words!

[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?

[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]

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

[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?

[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]

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....

[cnegrelli]

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

[david s]

Castable achieves full strength in 24 hrs in warm weather. Start from the bottom and work all the way around making the wall about 4" high with a flat ledge on the top so the next layer has something to sit on. I gets easier once the walls start to lean in.

[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.

I did some leveling and got the foamglas and CaSi layers level. Having some slight height issues with the Fire Brick as a few seemed a bit proud of others.
Is this common and how perfect am I shooting for in terms of consistent height on the floor?

[UtahBeehiver]

We see this quite often. Try and get the brick edges as uniform in height as possible. It can be a catching point for the peel as you slide in your pies. That said, once the floor is laid but before you place on your cast dome, you can knock off the high points with an angle grinder and a diamond cup disk. I have seen some builders used a belt sander with a coarse grit bet. Just something to knock of the sharp edges of the brick.

[cnegrelli]

Well I was able to get the oven floor fairly level across. I finished cutting my bricks (there' still a lot of cuts for the floor even with an enclosure as I didn't want 4" of brick hanging out over the CaSi. I ended up just using my chop saw with a good respirator. Wet saw would have been nice, but my guy went off to Africa at the exact wrong time!

Also cut a floor gap between the dome and gallery. I plan to fill this with some ceramic braided rope and top it with vermicrete.

I then moved onto the vaunted sand dome. EVERYTHING David S has stated was true. Used a skinny dowel marked with both the ID and OD height, held up by sand castle. Had my boys helping me, so it went fast. Put the door form in place.

Then moved on to mixing the castable refractory product. When I weight out the 5 pounds of stainless needles, I discovered I got more like 3.2 lbs. Thanks warehouse guy!

This process was going smooth until my ill-supported entry form moved back when I was pressing castable onto the arch area above the entry. This caused a sand cave-in, followed by some choice words with myself. The lower levels had set pretty good and I was forced to remove sand and castable that was still not setup, and redo the sand mold in that area, before proceeding. Hopefully it will come out well. Moved some Kettle Bells in front after that with no further problems. I do believe the inner arch will be a bit off the floor cuts I did for the thermal break, but I will re-assess in a few days. I'm off on a business trip so the dome will wet cure for the next few days assuming the wife keeps it wet!

Overall, very easy to work with I thought. Half a bag at a time. The directions were much too dry but it was easy to make too wet as well (first batch was that way. We had trouble getting 4" to hold itself up and ultimately, we ended up letting it slump so our lowest few inches are much thicker than 2". That shouldn't hurt anything. After that, we found the right consistency. We ended up with a bit lumpy of a finish, but it will never be seen on the outside.

Here's some pics.

[cnegrelli]

The cave-in when my door form moved to the right!