ceramic shell as guts of a rocket stove

[smarty]

Hello, has anyone else ever tried using ceramic shell to make a rocket stove? I have a small foundry and so I thought I would give it a go - ceramic shell is normally used to make moulds for bronze casting.

The resulting shell is about 1/2" or 13mm approx thick. I used it to make a 6" tall x 8" wide firebox with the same sized feed and an 8" round chimney. I wrapped the chimney with ceramic finer blanket and lit it and it seemed to roar away.

It seems fairly abrasion resistant and by the time the whole thing is encased in vermiculite and so on I would think that it would be fairly durable.

It has the advantage over castable refactories in that it is thinner, it has a fair degree of insulating value of its own, it can be added to if for example you want to add details like trip wires to the insides, and it very resistant to thermal shock.

I made the pattern from cardboard painted with a thin coat of wax and then burned this out after building up the shell over the top. complex components can be made of whatever size and shape you can make out of cardboard.

I'm wondering what is the downside... there has to be one somewhere.

Any thoughts on the matter?

Regards,

Martin.

[pinhead]

Pics???

[Deleted]

The obvious advantage is that you can build virtually everything imageable with very fine details.
www.youtube.com/watch?v=8zJTxGkog1I

I cannot see any downside.
The shell is hard but brittle, which is no problem at all if it is covered by additional thicker layers.

[Donkey]

It sounds very interesting to me.
I've never heard of ceramic shell till now. When I saw your post last night, I immediately looked it up.
I've got no idea how it would work though it looks promising as all get out. It should be pretty easy with this stuff to play with all kinds of shape(s) that have been difficult to do in the past.

I do wonder at it's durability.

All I can say is try it and let us know how it works.

[Deleted]

Jan 7, 2013 8:09:29 GMT -8 Donkey said:

I do wonder at it's durability.

That depends on the used ceramic mix and how well it is wrapped by additional layers.

Ceramic shells for castings are created to be easily destroyable.
For gold often pure gypsum is used for the schell, after casting it can be washed away with water, so that not even small amounts of the expensive stuff will be lost by abrassion.

Thus I would not recoment to use any mix that is specially created for ceramic shells for castings.

Any kind of clay slip can be used to build a shell.

[smarty]

well in response to Karl the stuff I use is a slurry of colloidal silica and molochite flour which does break up fairly easily when hit with a hammer and kind of cracks up around the casting anyway when the mould and casting shrink. At a guess I would say that its tougher than insulating firebrick but nowhere near as tough as cast refectory concrete or dense firebrick.

A couple of things in its favour are that it is quite porous and so resistant to spalling if it gets wet. and provided that you don't hit it with a hammer or bash it about with lumps of firewood too much it is fine... so far anyway.

I wade a mini version with a couple of cardboard tubes from the toilet roll coated them up with shell wrapped some ceramic finer blanket round that and coated this up with shell as well it worked a treat so I wen't on to do the 8" version that hopefully should be attached.

I have my doubts about its durability around where the wood is being poked around but i think it should resist fairly well the problem that firebricks in the top of the burn tunnel have of burning out.

[kevinofmorris]

Smarty,

I'm also liking the idea. Do you have to fire the shell in a kiln, or otherwise "bake" it to set it?

Cheers,

Kevin

[smarty]

yes Kevin, you do. However as opposed to most ceramics it is so resistant to thermal shock that you could probably 'fire' it by running a welding torch all over it - although this might take some time. I myself do the initial burn out with a large propane torch to get rid of the cardboard and after baking it up with some more coats and the fiberglass I then pop it in an oil drum lined with ceramic finer with a hole cut in the side and one in the top and bake it in there with the propane torch till its glowing hotter than red hot which takes about half hour or so.

If you know any friendly local backyard bronze casters they will probably help you out. It is extremely delicate until its backed up with the fiberglass and baked so be warned. It also requires patience to let each successive coat (four or five) air dry before applying the next.

The good thing is though that since it is quite quick and easy to make the cardboard formers you can make six or seven pint sized variations on a theme and shell them all up together. They could then be tested against each other.

I think the jury's maybe out on how durable it is as a full scale finished article but for rapid prototyping of different scaled down models it's fairly unlimited in its versatility.

I took up another poster's suggestion about using clay slip as a slurry and I'm going to be trying burning it out tomorrow so I'll let everyone know how it goes.

Martin.

[kevinofmorris]

Thanks Martin,

How are you applying the slip/liquid ceramic? I'm guessing by pouring/drizzling it over the form?

After reading and watching a few lost-wax-casting videos, I came up with some pretty wild ideas based on the fine detail that could be added to the liner that just wouldn't be practical with other casting techniques.

As you mentioned "details like trip wires to the insides", I thought of adding a small diameter (3mm) wax ring around the heat riser, about a third of the way up, with 8-16 3x3mm wax spacers between the ring and the riser form. Then several more vertical wax rods hanging down from the wax ring and and spaced 1-2mm from the riser with ceramic spacers. The bottoms of these wax rods would be bent out 90 degrees from the riser.

Hopefully you're still with me. What you end up with after building up the shell (preferably by dipping the form in the alternating slip and air-floated sand baths) and melting out the wax is a internal ring of preheated secondary air inlets entering at the hottest part of the heat riser allowing additional oxygen to burn any un-burned wood gas in the exhaust. The passages would be ceramic instead of metal, so no danger of their melting. The lower ends (bent out from the riser) could extend far enough from the inside of the riser to be connected to metal tubing to be later buried in the insulating material. The ends could be connected to an external manifold where the operator could adjust the volume of secondary air entering the system.

In my mind this is applied to a cooking stove, but would work in RMHs as well, just more difficult to fine tune without seeing the exhaust.

Sounds crazy, but in might work.

Cheers,
Kevin

[Donkey]

.. interesting idea.
Injecting secondary air into the heat riser has been tried by several people. So far, the results have been zero to poor.
The heat riser may well be the place to inject secondary in an Aprovecho style cooker, but it appears to NOT be with RMH.
In RMH, the place is down in the burn tunnel, what is essentially the function of the P-channel.

[smarty]

Well kevinofmorris yes I do just drizzle it over the form I don't have anything as sophisticated as a fluidized dipping thing for the granules so I just throw and sprinkle them over the slurry.

I tried the clay slip version today and it fired out with no problems. the resulting shell was moderately tougher when it came to abrasion than the normal shell using colloidal silica.

The slip I used was porcelain slip because it fired at lower temperatures. I'll have to pick the brains of a potter to see if there are better options. I'll probably try a slip made from fireclay next but I worry about whether I'll get it hot enough to fire it properly.

If you can't find molochite granules to build up the shell thickness then coarse grog will probably do. that way all the materials are easily available to anyone to try.

I'd tend to agree with donkey about the secondary air thing, but don't let that put you off coming up with ideas. For example the inlet for the p plate that runs in metal along the top of the batchbox chamber could possibly be incorporated into the thickness of the top of the burn chamber to get much hotter pre-heated air.

Another thing worth trying is lining the inside of metal heat risers to protect them from burning out since I've noticed that most of the cracking from expansion and contraction when casting metal tends to be on the outside of the form.

Martin.

[Deleted]

Jan 13, 2013 2:23:11 GMT -8 smarty said:

I'll probably try a slip made from fireclay next but I worry about whether I'll get it hot enough to fire it properly.

Mixing fireclay with a smaller amount of another clay with a lower melting point
could be a solution.