Clay plaster and mesh

[morticcio]

I've sourced some left over clay from a local pottery which I hope to use for clay plaster. Can anyone suggest the sand/clay ratios for suitable for plastering the firebrick bell and also what is the best mesh to use - jute/fibre glass/other?

[matthewwalker]

Clays are all so different Morticcio, you'll just have to do some small batches until you get it how you want it. If it's truly pure clay, I'd start with something like 1:4 clay:sand. That still might be clay rich. Don't know about the mesh.

[Robert]

1:4 might be to much or just right. But yes i confirm do the test patches and see how it is reacting. and add a little bit of fibres like straw. 3 cm long pices. but Donkey should confirm.. he is the master of COB.

regarding the mesh... once i used the metal one and it behaved quite ok... some ppl said it might be a problem with expansion because of temperature, but i have not seen it during many burns of the stove...

someone suggested me the jute mesh, but i did`not use it.

also i have seen that Joris from Belgium is platering without a mesh,but thats kind of brave

again would be nice to see what Donkey has to say about that.

[radek]

i do agry with matthewwalker and Robert.I would start my test patch 1/4 clay sand ,but because its pure pottery clay im pretty sure sure you will have to go higher with sand.Some smole fiber would be good too,but it will make the plaster more insulative.I do like to add some flour paste some times for the finnish layer as well,but it all depends how the basic mix of clay-sand feels to me.The mesh i have used for the double bell batch box was some special fiber glass masonry heater mesh -up to 550C, but not expensive.I know in old days people use to used chicken wire mesh,and some builders around here are using it till today.Somewhere i have seen copper wire mesh ,but never worked with.I would not go for the yuta.

[Donkey]

I personally would never use the mesh. It might work out, but it may also expand/contract different than the plaster which will crack it all to pieces and make it harder to repair later. Remember to douse the area that you will be plastering with clay slip first, it'll help insure a good bond. Also, press the plaster in firmly but NEVER slap it. Slapping the plaster is most likely to separate it from the substrate.

My order of business would be to first make slip out of the clay. In a large barrel, I would mix the clay with water using a paddle mixer on a strong drill motor. Add enough water till the slip has the consistency of a thick milk-shake with freshly melted ice cream in it.. When you put your hand in to the mud, it should come out evenly "gloved" with a coating of smooth slip. This can be screened if necessary to remove chunks of un-hydrated clay and other impurities (leaf drop, sand, and other crud).

From this barrel of slip, you can very quickly, mix out any test and/or finish batches that you may need.

1:4 sounds like a reasonable starting place. You really need to make test patches to be sure. You can, in a very short time, whip out several tests in a small bucket and apply them to a surface that is similar to the object that you will be plastering. You could just put the tests on the object itself if you don't mind scraping it off again when it's time to do it for real. Bigger tests patches will give you a better idea of how a mix will perform. I like patches around a meter square, though 2/3 of a meter will do. Smaller patches will often not reveal negative properties of a mix, it won't show how a larger piece cracks, etc..

Fibers in your mix will help hold it all together, there may be some insulation in it but it is worth the strength that the fibers lend. Use short fibers, the ones found in horse manure are about right (roughly the width of a finger) for finer work.
Natural fibers like manures, chopped straw and cattail fluff might char slightly and discolor a plaster that is in a really hot spot, but I find this to be very rare.

[Robert]

Donkey.

That is very interesting. No mesh at all? just really want to confirm. I will be adding about 3 inch thick layer of plater on the stove made of fire bricks... people suggested to use a jute mesh... but you said you would never use it... and i think i have seen Joris from Belgium also was not using any mesh. Just applying a mix directly on the stove. Thats nice

[morticcio]

Thanks for the suggestions. I'll try some test areas starting at 4:1 and see how it goes.

Ref mesh, here is an MHA article using mesh on a masonry heater.

[Donkey]

I don't use it and I've never had problems.
Others do use the mesh and claim to not have problems..

As well as removing steps and/or methods that do harm, my philosophy is to also do away with things that appear to do nothing..

If I HAD to choose between types of mesh to use, I'd go with the jute. Won't hurt though it may insulate somewhat. Remember to soak it in clay slip.

[morticcio]

Earlier this afternoon I did a 4:1 mix and added water so it was the consistency of a thick hot chocolate. I had to soak the clay first for 24hrs as it was very lumpy.

I plastered this on a section approx. 3 sq ft. about 2mm - 3mm thick. The firebrick just sucked up all the moisture from the mix, but it seems to have stuck well and dried out too. Will post some pics tomorrow.

[Donkey]

That's usually the way it works. Putting mud plaster on brick (or other mud based material), the dry substrate will draw in the spare water RIGHT AWAY, which also draws in the clay with it, insuring a pretty good stick.
If you wet the brick too much (with water), it will stop drawing the mud in and instead the stuff will tend to slide off instead. So, dope the mess with a lot of slip first, while it's dry. Get slip on everything and the plaster will stick to the slip, dry or wet.

Best to let your slip set in the barrel for a couple of days to let all of the clay take up water.

Also, if you're doing a straw or manure plaster, mix it into the mud and let it ferment slightly before using. It should smell kind of like a beer wart, pleasantly sweet. If it smells like raw sewage, you've let it set too long; it will still work, just be unpleasant to use.

[Rinchen]

The fibers will give the strength instead of the mesh. So not using fibers is more prone to cracking.

[patamos]

Hey Y'All

Thought i'd chime in on this one as i have been in the business of making finish plasters from scratch of late. What may be common knowledge for most (but not all?) of us is that any type of organic fibre placed that close to the heated surface will burn out sooner than later. The reason for including it at all is to prevent the plaster from cracking while it is setting up. If using jute/burlap you had best slip it first as Donkey suggests and then sandwich it in the belly of the plaster. Also of note, the jute/bulap comes in different grades of fibre thickness and weave density. And the denser thicker fabric is a definite NO for this kind of application as it will leave a layer of separation within the plaster sandwich once burned out. I always go for the looser weave thinner fibre. Coffee shops and distributors are good places to source discarded burlap sacks (from around the world!).
When using horse manure or chopped straw i think sifting it through bug screen makes sense. Takes time but gets the manure clumps and straw knuckles out of the mix. Cattails can get clumpy - especially if picked before the seed pods are starting to disintegrate late in the fall.

I checked into this thread because i am researching the prospect of using a whole lot of left over plaster (recipe: 1 part fire clay, 1 part 20/30 grit silica sand, 2 parts 50 grit silica sand, 10% boiled flour paste, 1/4part fine straw...) as the first layer around the horizontal flues i am setting in a 6" J-feed rocket grundofen. Wondering what people think of such a recipe for said application. Also, much of the plaster has gone off (rancid flour smell), which is why i'd sooner bury it deeper in the cob. Any word on the smell getting through 3+" of that much material?

well being

pat

[Donkey]

It'll work. The fiber will be slightly insulating.
The smell won't be problem once it's dried, just about anywhere.. Plasters that smell HORRIBLE from sitting too long will stop smelling once completely dry. All that extra enzymatic action will make the plasters better too.

[patamos]

Thanks Donkey

[esbjornaneer]

Dec 23, 2013 15:17:59 GMT -8 morticcio said:

Earlier this afternoon I did a 4:1 mix and added water so it was the consistency of a thick hot chocolate. I had to soak the clay first for 24hrs as it was very lumpy.

I plastered this on a section approx. 3 sq ft. about 2mm - 3mm thick. The firebrick just sucked up all the moisture from the mix, but it seems to have stuck well and dried out too. Will post some pics tomorrow.

Hi Morticcio,
Just found this thread as I am about to render/mortar my stove. How did you get on? Is it still holding up well? Did you use any fibres or just clay & sand?
Many thanks for the thread and all the contributors,
Esbjorn