Fox James

Coydon Wallham wrote:

Fox James wrote:There was a stage when ceramic fibre was used as a roll inside a tube but that has proven to be a health hazard and no longer recommended.

Can you elaborate on what the hazard is and what proof was offered?

Last I heard, Superwool was unhazardous as sold so safe to use in construction, but obtained the qualities of regular rock wool once fired to the temps seen inside a rocket riser. A mask was recommended while performing maintenance to avoid lung irritation. I haven't seen mention of evidence that the problem extends beyond that in this forum...

You might be getting confused with Super Wool and ceramic fibre, Super Wool  is said to be much safer to use than traditional ceramic fibre products.
However, its resistance to sintering  depends on the actual product you use and how hot it gets?
So the first super wools on the market were just as liable to sinter as ceramic fibers at around 900-1000c.
Then we had ‘Super wool plus’ and that is said ‘not’ to sinter below 1000c continuous use but….If you exceed the maximum continuous use limit, or push it past its classification threshold, the fibers ‘will’ undergo crystallization!
Now we have the relatively new on the market, Super Wool XT and that can handle 1400c before it sinters and becomes toxic.
So we know that racket stove riser temperature can reach 1200c even inside a 6” rocket stove riser and I think some have recorded very near to 1400c in 8” versions?

So Super Wool XT is a much better bet and very useful in certain areas but still not ideal for very hot areas in the flame path.

PS all this info is readily available via Google or AI
The headline will probably be promoting Super wool as non cancerous but, read on about max temperature before sintering.
Superwool XTRA by Morgan Advanced Materials, seems to be the best available at the moment.

You can fit a damper and see if you can find a use for it but there are a few facts to bare in mind, in order to burn cleanly, rockets stoves need to burn hot and fast so air flow is a very important aspect.
I like to have as much control over the air flow as possible however with a J tube you are stuffing the main air inlet with wood so you have to be careful to leave enough space and not cram it all in.
There is a very helpful addition you could have added called a ‘P channel’ which is a simple metal plate that allows a guaranteed amount of air into the burn tunnel.
Can I just ask what sort of cement have you used in your build?

Well anyone who has used it in a flame path can easily see why it is a hazard to health, you only have to slightly move it and the result is a cloud of microscopic dust that has been sintered into a non body soluble product.
As far as I know all ceramic fiber products suffer the same fate…. often sold as a non toxic, body soluble packaging but, they don’t mention that once heated, it changes its structure into a a brittle none soluble, sintered fibers.
If you remove a used piece that has been in use, in a hostile high heat  environment, simple hold it up to the light, give it a light shake, you can then see for yourself the amazing result …. a literal cloud of fine dust that then goes on to settle on your clothes and any surrounding objects. Would you want that in your home or work environment?
Much safer if used out of the flame path where the temperature does not exceed 500c but even then, it is a horrible product to use one the binder has been melted away.
Of course most refractory products are pretty toxic if turned into dust and breathed in but I don’t know if fire brick dust will stay in your lungs for years waiting to come back and bite you?

Fire brick is the best long term material to use but perhaps not the most efficient.
Any high mass dense material will take longer to heat up often meaning a bypass has to be used, so use split bricks on edge and use some way to hold it all together.
If you are really keen, then an eight sided riser with all cut and mitred joints surrounded by a 10” pipe and backfilled with loose vermiculite will be very cool.
There was a stage when ceramic fibre was used as a roll inside a tube but that has proven to be a health hazard and no longer recommended.
As I mentioned in your other thread, you could build a riser less core and avoid having a riser!

Hi, living in Ireland you will have access to calcium silicate board which is available in 8x4 sheets.
That will be a perfect backing to protect the straw wall.
With that is place you can go pretty close, maybe 200mm  but ideally you would stand off the board 50mm so you can monitor the back side heat. If you can justify Magply (trade name for insulation board) they are about £120 for an 8x4 sheet, then do that as the boards has a very high heat rating and insulating properties.
You will also have access to vermiculate board (not readily available in the states) that will offer lots of uses. Vermiculite actually looks like compressed straw so if you are that way inclined, may suit your need best?
You may want to consider a space heater as well, with the use of two barrels and no mass you can heat a large area very quickly and can be built quickly and cheaply.
Of course a mass heater is going to be the best however, I would suggest you seriously consider a batch design and bell or even a J tube and a bell.
Basically a Bach rocket will be far easier to operate in a workshop as they don’t require such regular feeding of the fuel but, are considerably more complex to build.
The latest of Peter’s designs is perhaps the best as you can burn multiple loads in a day without polluting the atmosphere!
One last option is to build a Shorty core (up date batch rocket design) in a bell and also have a basic space heater single barrel J tube for additional, fast radiant heat!
A shorty and bell combo will only need lighting once a day to stop temperatures dropping to much overnight and a space heater J tube built with a single barrel and vermiculite board can be assembled in a day for little money but throw out heat like you won’t believe!

I find it fascinating how little changes can have considerable effect!

I have built J tubes that allow you to see the fire from three directions ……

Well unfortunately it would have to be high temperature cement board, the one I use is rated to 1200 c.

Still finding small improvements…..