RMH, mass under the kitchen table

Nina Surya wrote:Somehow I'm convinced the bell should be covered with one single plate.
Have you tried two plates next to each other? If so, how did it work out for you?

These plates are designed to work in an assembly of usually at least 2 or more in the cooking stoves where the fire is hitting them from the under. The perimeter of the plates is sealed with refractory wool (in older times they were just laid in the bed of clay I think). They join to each other with a protruding lip. Hot gases are supposed to go the the lower pressure area which is the chimney exit. I grew up with plates like that and I do not remember any leaks or smoke coming through the dry joints never ever. I have some brand new ones that I'm going to use on my kitchen stove so I can send you close up images if you like.

Julian Adam wrote:I see you mention tomettes. Are you French(-speaking)? If so, maybe try looking for 'plaque d'égout en fonte ductile'. Based on the fact that the usual barrel over the J-tube can last for many years, I would think this would work well, assuming you put some stove rope / seal in the groove.

Hi Julian, I'm in France, so the search terms are in French, yes.
Thank you again for your input!

The thing I'm concerned about with the plaque you mention is if it can handle the very high temperatures in the bell.
In the previous setting, with a horizontal feed tube, the bunny-cage-plates held well. But my rocket didn't roar and the mass got only luke warm to mildly warm.
Now, with the J-tube setting (and a much shorter burn tunnel), the rocket is roaring and the mass is heating.
Because of the inferno within the bell, the bunny-cage-plates crack.
At the moment we're looking into refractory cement casting, as Thomas describes here -> Casting-Large-refractory-Slabs .

To be continued...

Cristobal Cristo wrote:

These plates are designed to work in an assembly of usually at least 2 or more in the cooking stoves where the fire is hitting them from the under. The perimeter of the plates is sealed with refractory wool (in older times they were just laid in the bed of clay I think). They join to each other with a protruding lip. Hot gases are supposed to go the the lower pressure area which is the chimney exit. I grew up with plates like that and I do not remember any leaks or smoke coming through the dry joints never ever. I have some brand new ones that I'm going to use on my kitchen stove so I can send you close up images if you like.

Hi Cristobal,

Thank you for the clarification! Ah yes, that sounds excellent for a wood stove! I'll keep in in mind for if I build a rocket stove at some point, thank you!! ( VERY inspiring/tempting to modify the present version in the future like that...hmmm!)
However the temperatures in the riser and hitting the roof of the bell are very high, I've understood it's more than 1000 degrees Celsius. That's why the present plates are cracking, as opposed to serving just fine the whole winter before, when the temps in the riser were lower due to a horizontal fuel feed.

There's a link in my post here above to the solution we're looking at right now.
To be continued!

Hi Nina, I dont  think you will get anywhere near 1000c under your top plate, maybe 500c at peak burn.
Your stove may produce closed to 1000c at the base of the riser.
You present choice of wire in concrete is not a good one because metal and cement expand at different rates and often cause severe cracking when heated above 300c then the concrete its self will break down at around 250-275 c becoming weak and fragile!
So I have a suggestion that may work for you,, a ceramic glass recovered from an electric cooker hob, that is good for 750 c and very stable.

On my own J tube I use a 10mm steel plate with a centre cut out, that works very well but only because the cut out allows the top to move without the edges distorting.
I do explain this in one of my videos
I also notice that you might be using some sort of ceramic fibre to seal your present top?
If that is the case then please remove it as that is not a heathy option in your living space.

So this video shows how I managed to control a metal hot plate to stop it deforming in a negative way, I dont know how this would work with a square top … mayb e a square hole?

Fox James wrote:Hi Nina, I dont  think you will get anywhere near 1000c under your top plate, maybe 500c at peak burn.

I wanted to say the same
The masonry stoves I grew up with were all coal fired so it was easier to get higher temps and the heat was more condensed due to the stove design.
Sometimes the plate would start glowing in the center - indicating 500-600 C temperature which they withstood just fine.
Also not all cast iron has the same quality. I was using some Chinese made hot plate sold by some popular brand. It was placed on my outside experimental stove and developed a long thin crack (all way through) after a tennth fire.
Glass sounds good too and as a side effect you can look inside from above!

Fox James wrote:Hi Nina, I dont  think you will get anywhere near 1000c under your top plate, maybe 500c at peak burn.
Your stove may produce closed to 1000c at the base of the riser.
You present choice of wire in concrete is not a good one because metal and cement expand at different rates and often cause severe cracking when heated above 300c then the concrete its self will break down at around 250-275 c becoming weak and fragile!
So I have a suggestion that may work for you,, a ceramic glass recovered from an electric cooker hob, that is good for 750 c and very stable.

On my own J tube I use a 10mm steel plate with a centre cut out, that works very well but only because the cut out allows the top to move without the edges distorting.
I do explain this in one of my videos
I also notice that you might be using some sort of ceramic fibre to seal your present top?
If that is the case then please remove it as that is not a heathy option in your living space.

Hi Fox,

Thank you for your feedback!
Yes, I realised the metal wires in the concrete aren't great because of the expansion, but we're using what we have.
Will keep eyes peeled for a ceramic cooking top glass.

The fibre is "fire wool", again, the sealing agent we had at hand.

Tweaking, tweaking...

Cristobal Cristo wrote:
The masonry stoves I grew up with were all coal fired so it was easier to get higher temps and the heat was more condensed due to the stove design.
Sometimes the plate would start glowing in the center - indicating 500-600 C temperature which they withstood just fine.
Also not all cast iron has the same quality. I was using some Chinese made hot plate sold by some popular brand. It was placed on my outside experimental stove and developed a long thin crack (all way through) after a tennth fire.
Glass sounds good too and as a side effect you can look inside from above!

Thank you again Cristobal, for your input!
Appreciated :)

Ok, well I dont know what fire wool is but if it is ceramic fibre then it is not safe to have exposed in your living space!
The problem with this type of martial is after it has been heated, it is very likely  to produce millions of air born microscopic particle into the living space, if it is disturbed  or handled in any way.
These particles are not good as they can be absorbed into your body and possibly cause serious heath issues in later life.
If you use refractory wool or mat, then make sure it is sealed inside the stove and if you ever need to disturb it, wear a mask and hoover up everything afterwards, or use a damp cloth on any susceptible surfaces.
Fibergalss rope or titanium rope is much safer as the partials are larger however the dust from any refractory material should be treated with respect.
All refractory products sold in the EU will be reasonably safe before they are heated, they become  especially   hazardous when exposed to naked flame or high heat and then exposed to fresh air.
I would not worry to much in your case but, just be aware of potential issues.