Results from a DIY cast batch box

Hello all,
Some 5 years ago I came to "rocket science" when I was researching  designs for a heated bench to prolong my garden season. Especially the evening use of our roofed-over patio.
I liked the design of Peter's batch box the most, because it offered a longer hands-off operation than a J-tube, but still reasonable simplicity to build it.
It got a 200 liter barrel and a 2,5m long bell-bench made from hard fired bricks
internal bell dimensions: about 240cm by 33cm by 25 cm, or  94in long by 13in high by 10in wide.

Last weekend I dismantled it for curiosity and eventual mainenance...

Core and riser: I built the mold for my 4" core out of plywood and sealed the edges with wax.
Somewhere along the materials discussion extruded clay granulate was proposed as insolating filler for refrac cement, so I thought I might give it a try.

The first refractory cement I mixed w/o filler to seal the inner walls of the fire box (6 to 8mm thick / about 1/3")
Then I used 'Seramis' brand that is availabe locally and also some round LECA aggregate in the mix the to fill the mold.

For the riser I used a different approach: I got a 105mm / 4" cardboard tube centered within a 150mm / 6" plastic tube.
The space between was filled with Seramis only wetted with the cement to a lean mix and lightly tamped in.
The idea was, that the brittle riser would slide easy from the plastic tube and the cardboard could be burned out.
But in reality the cardboard swelled enough to take it out without resistance after drying, but the plastic had to be cut. (theory vs. real world )

The riser was then covered in 4" mineral wool (not the yellow glass but the dark grey hi-temp. variety) and some wire mesh.

The bench was used for 5 years now, every so often. It works well, even with snow around, like St.Niclas or new years eve.
I would estimate a hundred to 130 fires for several hours, not a very intense use...

As you can see the core is in pretty good shape. It only lost some of the mortar at the bottom of the riser.
The little crack at the right side was there from the casting time, it has'nt expanded at all.

The mineral wool was clad in some kind of oven paper supposedly heat resistant and to protect it from dust and residue.
It totally burned off at the upper two thirds. The wire mesh is also quite scaled in the upper part, but still holding together.

Inside, the riser got a pinkish tone to it, which I attribute to the Seramis. The surface is hard, but a little dusty to the touch.
Almost no wear.  

During construction the upper lid of the core had an accident, so it was substituted with some vermiculite board cut into 3 stripes for the sides  and a cover.
You need to caution, because this stuff is not abrasion resistant at all when scraped with the firewood. More so, it startet to disintegrate at the outside.
Probably from heat shock, because it spalled only where it is open to the air - not inside where hot gases are on both sides.
Another maybe could be UV degradation, because is less on the shadowed side.  

Above the core I used an angled rebar to bridge the gap since there is only one more layer of brick and it is a corner.
This is the only place where I got a small crack in my brickwork (note the soot coming out of the joint).
Metal expansion might be the culprit here.

Since the patio is roofed, could not led the flue straight up.
It starts about 5cm / 2" above the bottom of the bell in the corner where the bell connects to the slightly deeper housing of the core (open 270° horizontally and 180° vertically).
From there it goes some 2m / 85" up and then another 7feet almost horizontal with only 7cm/2.5" raise. Depending of the wind I can add another ellbow with 1m / 40" extra vertical.
This last rise makes no difference in draft, but keeps the steam from being blown under the roof.  

The bench is a little on the big side for this small 4" BB.
Therefore the heater starts slow, especially in wet or damp weather with temps above 10°C/50F.
When the bench is warm at the heater side (about 40mins after a cold start) it got lots of draft.
After an hour the whole bench is warm and the fire is almost out.

My firewood comes from the garden and the forrest behind it, mainly magnolia, pine, birch and some linden and beech.
The first three got quite some oils and resin so the inside of the bench is black.

The effect is heavier, when reloading the heater before the coals are burned out: The heat gassifies the oils but there is not enough oxygen to burn all of it.
So wait for it.

Thomas;
This is a great report on your build!
Thank you for sharing it with us.