|
Post by iamronen on Oct 24, 2011 8:19:23 GMT -8
We are at the end of a long journey of our building our first all brick rocket stove (we promise to show and tell about it at length soon!). We are now stuck at the finishing stage (which is not just about aesthetics but also about sealing it tightly so no gases get out).
All of our attempts at various clay/sand mixes have failed ... when we fire up the stove they dry, crack and fall off.
This is our first encounter with cob (actually our second - the first failed too) ... so we are not yet experienced with it.
Can anyone suggest how to create a cob plaster that will adhere to the firebricks (the whole stove is made of firebricks)?
|
|
|
Post by Donkey on Oct 24, 2011 16:36:53 GMT -8
Umm.. Where is yer clay soil coming from?
Typically you'd use some good clay rich soil, put it in a barrel, add water, whip it with a paddle mixer, screen out the stones and whatnot (I use a metal fly-screen on a frame) and mix THAT with other stuff to make your plaster. Said other stuff could be sand, chopped straw, horse (or cow) manure and a list of other stuff including pigments etc.. If your clay soil is worth a damn, all you should need to do is wet (well) the surface to be plastered, then smear on the stuff on and pet it smooth. If you've wet the surface WELL, applied a bit of pressure on application, not applied it too thickly and let it dry properly, it should stick plenty on it's own. If you've done all of the above and the stuff just won't stay on, you need to either mess with the mix till it does (you've used too much sand or something) or find a different clay soil. If the dirt you've been making cob with makes good quality, tough cob... It should make a serviceable plaster.
Almost forgot, it's helpful when plastering firebrick to "slip" the brick before plastering, which is to paint on that wet clay stuff you made the plaster with, then apply the plaster. Extreme heat WILL break up plasters.
|
|
|
Post by iamronen on Oct 24, 2011 21:51:10 GMT -8
thank you very much ... we'll try again today ...
our soil is very clay-rich ... so that shouldn't be a problem.
we didn't wet the bricks at all ... so maybe that will help.
regarding drying ... would you fire up the stove immediately or let the plaster set a while before heating it up?
|
|
|
Post by Donkey on Oct 25, 2011 18:28:49 GMT -8
I've done both.. Probably best to let it dry completely.
|
|
|
Post by iamronen on Oct 28, 2011 9:03:28 GMT -8
Thank you donkey.
Here is what finally worked for us ... we didn't have time to experiment step by step so I can't say if what may be redundant: - we used the same clay mix that failed the first time - we added to it chopped hemp hurds (we don't have direct access to straw and wouldn't know how to cut it down to size if we did) - we also added some "acrylic glue" left over by the workers who fixed our cob walls a few months ago - we wet soaked the fire-bricks thoroughly before applying the plaster to them.
We let it dry overnight without heating the rocket. The next two days we gradually heated it up and as we did some we caressed cracks that appeared with a clay slip ... and today it is fired up completely and looking like its going to hold up.
We will check back here when we post in detail about the rocket we built.
|
|
|
Post by iamronen on Nov 19, 2011 7:22:59 GMT -8
|
|
|
Post by crazy1944 on Dec 28, 2011 11:56:23 GMT -8
I liked your idea of the raised fire box. Also liked the brick surrounding the bottom of the riser. Looks like an good way for the cooling gases to exit into pipe for a cob mass. I think I will use a 55 gallon steel barrel instead of bricks when I get around to building one in my greenhouse. I just have to build the new greenhouse first. This is what is so great about this forum, people sharing their ideas and projects. Thanks for sharing your project. I'll have to take pictures when I get around to building mine so I can share too. Jim in Calgary Alberta Canada
|
|
dvawolk
Full Member
DSR2 125mm open system (actual project)
Posts: 254
|
Post by dvawolk on Mar 18, 2013 4:21:08 GMT -8
Donkey: what would stand for "extreme heat"?
Based on this post it looks like it should hold if plastered on the first bell of rocket stove... If so, i need no more...
Regards, Klemen
|
|
|
Post by Donkey on Mar 18, 2013 7:02:36 GMT -8
My plasters seem to do fine. I think it has a lot to do with the kind of clay soil you have. Some clays do great in the heat and some just crumble, mine are on the plus side of the equation.. Umm, plasters for truly extreme heat should NOT have straw (or natural fiber that can burn) in them. Sand would be used instead and it would be applied in successive, thin coats, much the way they did it above.
|
|
dvawolk
Full Member
DSR2 125mm open system (actual project)
Posts: 254
|
Post by dvawolk on Mar 18, 2013 12:41:22 GMT -8
Sand would be used instead and it would be applied in successive, thin coats, much the way they did it above. Hi, Donkey. Just to clarify: If it is not extreme: i need sand/clay mix with fibers. If it is extreme: i need (more)sand/clay mix. Adding iron mesh would probably help? My clay is very clayish. I tested it with various ratios from 1:3 to 3:1. 3:1 and 2:1 (sand:clay) seems too much - with 3:1 it is very hard to form anything... 3:2 and 1:1 (sand:clay) seems good. So.. If i use natural fibers. Do i need to use 3:2 or 1:1 and then add fibers OR should fibers instead of part of sand?
|
|
|
Post by Donkey on Mar 18, 2013 17:55:07 GMT -8
Ohh... Good question, and the answer is... It depends.
Clearly, the clay will only take so much stuff, of any type. In some cases, you CAN wipe just clay slip on with a sponge; as long as the layer(s) are VERY thin, they shouldn't crack much or at all. Working back from that position, both sand and fibers in a plaster will prevent cracking. Both materials have their obvious properties and both have their trade-offs. Plasters made with only sand or only fiber (clay is assumed) each have their purposes and you can back off on one ingredient to add the other, mix and match to suit the job. Fiber for tensile strength, sand for hardness. Sandier mixes tend to dry faster too.
I almost always make my base plaster out of nothing but chopped straw and clay-slip. It holds together REALLY well, it's sculptural so I can refine shapes and build detail with it, and when I want to fill a REALLY big space, I'll mix in a bunch of sand to make the mix stiffer. Over this fiber rich base, I usually go for a sandier (or all sand) top or finish coat. Sand will fill in over the texture of the straw, leaving the finish smooth and of a regular color. Sandier mixes tend to "dust off" when rubbed after dried, so often additives like wheat paste, cactus juice and/or white glue (Elmers exterior grade) can be used to make the mix hold together better.
|
|