Fox James

I know you have tried hard to show us the issues you think you might have it is still not very clear to me what your problem is?

If you have built a 6” or 150mm stove then the riser is fine with the same diameter, you do not need to match the fire box square area to the riser square area.

There are hundred maybe a thousand J tubes running just fine with a six inch square fire box, six inch square burn tunnel and a six inch round riser with. No problem at all.

I ready don’t think fitting everything inside a barrel will give you the best performance possible but if you don’t see any smoke out of the chimney and your barrel top is reaching 250c  then you stove is probably working as expected.

If you want more performance then you need to make a more free flowing design using insulating material and a taller barrel.
Because you have used dense fire brick, the stove will take longer to reach peak operating temperature, maybe 30-45 minutes to saturate the bricks.
However if you don’t add insulation then the heat might not rise high enough inside the fire box to satisfy your needs.

Can you buy  vermiculite board where you live? It is very easy to work with and very quick to build with.

Hi Kees, where do you live, what is the air temperature?
I have had a quick run through your post but I am not sure exactly what your problem is?
Do you see white or black smoke coming out of the chimney?
What temperature do you see on top the barrel above the riser after the fire has been burning for an hour?

Sounds good apart from putting super wool in your oven, that is not a good idea due to the risk of microscopic fibres being ingested!

I have this short video from this mornings experiment …

I think you have a point however I am not so sure, it depends on how you intend to use your stove.
If it is one load in the morning and one in the evening then the slab wont take in enough heat.
If your stove is more dedicated to cooking then you could burn it for longer and saturate the slab, then it might be a good option.
There are millions of pizza ovens in use that use thick mass for prolonged cooking times but they do take several hours to saturate with heat. You can expect the face side of a 3” thick pizza oven to reach around 500c and the reverse side around 300c after 2-3 hours of heating. The good news is it takes two days to return to ambient temp.

I really dont know how effective your idea will be as you will have the opposite issue with a thin kiln shelf as it will only work while the fire is running, maybe it will still stay warm once the fire dies down to coals but not for long?

White ovens are one of those things we see regularly on this forum but rarely find out how or if they work!
It can certainly be done with a J tube that is used for continuous radiant heat but a batch box does offer some unknown challenges.

I think a thick solid mass is a good choice for the oven floor as it will hold and stabilize the heat inside the oven.
Kiln shelves are amazing and certainly have there uses although for me they are an expense option for me.
A lot of refractory materials seem to be very regionally priced, for me 25kg of casting cement would be around £25 a 12” x 18” kiln shelf would be around £75.

I think we are talking about Molar clay, it is readily availible throughout Europe and the UK.
It is a form of expanded clay and used extensively in the growing industry as it hold water well but aids drainage.
It is used as cat litter due to the high absorption rate and also for growing bonsai trees.
It is also  key ingredient for lightweight, insulating building blocks.
Because the clay is baked at over 1000c it also has refractory uses.

Well that is unfortunate for you, something obviously went wrong!
The top looks like it has been deprived of ‘water’ or ‘cement’ not something I have seen very often but it looks like the top layer dryed out to quickly however, without actually being able to witness how it was mixed and how it was vibrated, I can only guess at what happened.
The correct amount of water (normally around 4lt to 25 kg depending on manufacturer ingredients )  is really critical for two main reasons.
One is to cause the chemical reaction to cure the cement and the other is to ensure a strong end product that won’t carry too many voids.
So whatever the amount of water added, that volume of water will be replaced by air as the water evaporates forming microscopic voids through out the cured end product. That is why it is mixed so dry, if you double the amount of water you will double the amount of voids!
So the mixing of relatively small amounts of water must be very thorough to ensure every monocle  of cement dust is dampened.
Vibrating offers several benefits, it allows all the grains and cement in the mix to settle down tightly and in turn expel any excess water.
However if you over vibrate all the smaller grain and particles (dust) will fall to the bottom leaving a dry top and dense bottom!
With a proper vibrating table, with a shallow mold it will take about 10 seconds.
With DIY tools you just have to guess and keep your fingers crossed.
With a proper table, the mix is compressed very rapidly forcing the water to the surface, the water on the surface will then partially sink back down but also ensure the mix stays moist and does not dry out to quickly.
However too much water on the surface is a sure sign of, to wet a mix or over vibrating.
Your results look like a mix that has been left to dry in the sun and has dried out too quickly. I not saying you did that but that is what it would look like if somebody did do that.
Refractory cement is not an easy product to work with and the end results are very much based on correct mixing, vibrating and curing.
As it only offers about 20-30 minutes working time you need to be quick and make sure the mold is covered as air tight as possible as quick as possible. IE as soon as it is vibrated, it must be covered to keep the moisture in for the next 8 hours or so.
A big plastic bag would be ideal but anything that absorbs moisture like dry sacking or plywood will have the opposite effect !
The lower part of your cast may be ok, I would scape off the top and test it out.

That is a bit of a tease Pete, we need more pictures or even better, a video?

If you can buy vermiculite board then most of your problems will be solved.
So much depends on the materials available .