Plastering stage

[rhyddian]

Hiya all,
Joined the forum again after a gap of about five years. It's moved on a peg or two.
I'm well happy with the stove; it's an eight inch system and runs great.  Now, the devil being in the detailing... i'd like to finish it well.

Have some experience of earthern and lime plaster & renders but none where heat is concerned.

The bench is pretty much dried out now after quite a few firings.  You'll notice some corbelling on the bench (extends about 2 inches).  There are also some fairly large gaps between final bench height and current.  Maybe even up to 2 inches in places.

I'd love to do a decent job of the plaster making (should have some experienced help in applying) and would welcome learning from any mistakes you may have made on previous project here!

Here's the stove: www.flickr.com/photos/mango-coconut-papaya/sets/72157648233804016/
(BTW my main learning points for this build is that i would create ducting predominantly from the red storage heater brick for future builds for increasing longevity and decreasing cost.)

[nemo]

Hello to all! Since I was wondering the same questions I thought of posting them here.
How to do the finishing layer. I'd like it to look smooth and shiny without using linseed oil as i understand i' toxic till it dries.
Also how not to have cracks. I have been using cob with straw and everything to plaster my half barrels.I understand straw is insulative and hardware cloth would be a replacement? Does it help cob not to crak or it creates just adherence?. Tell you, in the placesi did not use straw it cracked badly but wit straw..close to none. I dont want ghat happening to my finishing layer.

[gjh42]

You can use straw in the outer layers of the cob for a stronger surface. In those places it will not get hot enough to char the straw, and used modestly it will not add a lot of insulation.

[matthewwalker]

I've been having great luck lately with very finely screened clay, or fire clay if you want the easy out, pure sand, cow poop, and flour paste. Roughly 1:3:1 by volume, clay slip:sand:poop. I use about 1 cup of flour to 2 quarts of water, cook in the flour, then use that to make the clay slip. Mix in the poop, then add sand until you get the right consistency. Might need a bit more liquid as you go.

The poop works well, but the absolute best batch I've made I used the contents of the stomach of a bull that went in my freezer this fall. Perfect fiber if you can find it.

[nemo]

Matthew, what do you mean by two quarts of water? Can you tell me a percentage of how much flour mixed with water would be out of the whole mix? And what percentage the flour takes out the water or somethin like :"a cup of flour to 500ml of water". Sorry I have trouble understanding this:(.
And I understand that I have to mix the water and flour up go a boiling point?
One last question: which of the elements will give the mix a shiny look and make it dust free if any?

[gjh42]

Two quarts (US) equals 1.9 liters. You would add the cup (237 ml) to this much water.

Or you could just use 250 ml of flour to 2 liters of water.

[matthewwalker]

It's a rough mix, don't worry too much about measuring anything. Just keep going until you get it right, it will be different with different clays and sands and such anyway.

[patamos]

ONe way to make the wheat paste mixing go well is to mix 1 part cool water with 1 part flour in a tall pot, and mix that blend thorougly so as to get all the lumps out. YOu can get into it with a drill and small paint mixing bit for 10 minutes or more.
While doing that i put another 4+ parts of water on to boil in a big pot. ONce it boils i add in the cold mix right away and put the drill mixer into it so things keep moving.

So 1 cup of flour blends with 5+ cups of water all told.


Another thought. If you want to do a bit of thickening of areas to smooth out the lines of the bench. YOu can first apply a browncoat plaster with a wooden float. The this plaster is basically cob with short fibre straw (2cm and smaller). Maybe a little wetter and/or more clay rich so that it moves around easier.

cool looking building

have fun

[rhyddian]

Good stuff so far.... thanks for responses/questions.  Browncoat plaster is a new word... but i hear where your coming from.  I've got a few buckets of the type of straw you mentioned as i chopped with a strimmer before Christmas so will go with a browncoat (stipple?) layer first; this'll give me a chance to get to grips with the flour paste proportions. That'll keep me going for a while as there's plenty of other work on.  In the meantime, consider my eyes and ears open.  Thanks again.

[patamos]

Cool

For brown coat i tend to use 4 parts sand to 2 or 3 parts thick clay slip and 2 or 3 parts fine straw. The recipe is pretty flexible. ALthough if too wet or dry or fibre rich it is hard to work with a float.
Always good to apply clay slip to the cobb substrate just before applying the plaster.
Good also to keep it above freezing while it dries if you can

[daniel]

Hi Patamos,

Do you have any advice about plastering the top and bottom of the bell I just made, it has the lower half about 1m of red brick layed flat and upper half of refractory brick on edge, I measured the temperature on a medium burn at the upper most row of refractory bricks at 100 degrees Celsius, the lower part had about 50 degrees Celsius within about one hour. These temperatures could get higher.
Also I am worried if the plaster in the upper part should be attached to the firebrick or there should be a separation area as to allow for contraction etc, in that case I don't know how much of the heat will be transfered to the outside.

[patamos]

Hi Dainel,

I am not entirely sure at what temperature the straw will char. Recurrent heating might also have an effect such as it does with wood. But i think you are pretty safe at 100c. The again, if the temperature keeps rising…

Those two types of brick might behave a bit differently, so the seam between them could be a high stress zone. Also, the higher up bricks are more likely to expand and contract.

On the whole i'd say adding straw to the brown coat mix is worthwhile. You are not in the hot zone where Donkey would suggest fiberless cob. ANd even if the straw burns out it will still have achieved much of its purpose in preventing cracks as the plaster dries in the first place. At the same time, the unfired clay-sand still offers much resilience. This is why it is the preferred mortar of many old time masonry heater builders.

Where major movement of bricks is bound to happen, i have gotten in the practice of setting strips of fiberglass mesh into the finish plaster. This is the same mesh sold at building supply stores for drywall taping. Easy to work with. Fun even. I have pics of the first time i did it on drop box linked to my early threads. (too luddite to add them here).


hope that helps some

[daniel]

Thanks Patamos, do you have some proportions for different layers of clay mixtures?

[patamos]

Donkey has done a very good write up on this. I think it is in the reference library. If you can't find it let me know and i will dig up some notes.

In general though. Brown coat: 1 part thickish clay slip, 2 parts mixed grain sand, 1 part well chopped straw. I run straw from bales once through a garden mulcher.

Thickish slip means about 65% clay 35% water. This can be thinner if the sand is dry, thicker if wet. You will have to experiment til you find the most workable. If a mix turns out a little soggy you can add some straw. But too much straw and it becomes unworkable in a different way.

Matt's recipe (above) sounds like a good finish layer. Clay, fine sand and horse dung will work well too but not be as hard as cow dung. Wheat flour paste is a hardener/glue. The sheen has much to do with how and when you compress the plaster as it is drying. The flat part of a plastic yogurt lid works well. Cut off the edges and rough spots and make a little loop of tape on the coloured side for your fingers to hold it. Compressing too early and the plaster will still move around. Too late and you will be fighting it all the way.
Usually i wet down the (dried) brown coat quite thoroughly to buy time as i apply the finish plaster…
Lots of details that hare hard to explain in writing...

Sodium silicate is a good hardener and water repellent, but in my experience tends to darken the colour a little unpredictably. Boiled linseed oil work okay to give sheen, strength and water proofing. But you want to be careful of breathing it for the first 24 hours as the chemical dryers evaporate. I wear an organic filter full face mask and coat my exposed skin in burdock root salve. Also have a burdock root bath afterwards. Feels great