Pit kiln experiment

Hi everyone


I have posted several times on permies forum about kilns and rocket stoves.  I suppose slowly slowly I am working my way backwards to see if I can literally start from scratch.   I had discussed a backyard stoneware kiln a couple of years ago, but I think I was jumping ahead of myself and will start with the basics of a pit kiln.

So I guess I am just looking for some comments on my ideas because I don't really know what I am doing.


I have dug a big old hole in the ground of my allotment.  I don't think the allotment committee would look too kindly on a big old kiln sticking out of the ground, especially since I haven't got any veggies growing yet.

So I am trying to keep everything sub surface.   Dimensions are 800mm Internal diameter, 700mm deep.  There will always be risk of rain getting in although I will build a cover, so I sloped the base of the pit towards a drain hole bored into the side of the pit at the base, which at some point in the future may serve as firebox.

The plan currently is to simply build a firepit using wood, dried cuttings, manure, soil etc, and make clay items from that same clay with a bit of grog added in.  I am also experimenting with insulative bricks to make my rocket stove, so I may want to fire those in there aswell.  I will cover it towards the end of the firing with some corrugated sheet metal.

So any comments on the procedure and anything else are welcome.

Looks like an interesting idea.  I will look forward to seeing how it works.

There are various grades of kiln, with pit firing being near the base. What you would do with this is put a mix of small wood and pots in, light the fire, and mostly cover it so it doesn't flame up vigorously but keeps burning. Uncover the next day and see what you have. This can fire pottery hard enough to use gently, though it will not be waterproof. Some pieces will probably not be fired hard enough to keep from melting in water, and some will most likely break from thermal shock. It gets better with practice. This is one of the earliest kinds of firing practiced in primitive cultures.

A more advanced kiln which can fire much hotter and make harder pots requires a fire with a good air supply underneath the ware chamber where the pots sit. For this, dig another hole next to the first, about a foot away, and large enough to stand and feed the fire without scorching your shins. Dig a hole, maybe head-sized, between the bottoms of the holes, put your pots in the first pit along with enough things to fill it to the top leaving plenty of airflow space. Build a fire in the connecting hole and slowly push it farther into the first pit until it drafts up through the pots. As the pot chamber gets hotter, it will draw more strongly. You will need to keep the fire going steadily for several hours until the interior of the pot chamber, or "ware chamber" as it is properly called, is glowing orange inside.

There is much more detail involved, but that is the essence.

Ok Glenn thanks.

I had a similar design idea in mind, so I will readdress that after I've tried firing the pots this way pit kiln stylee.


Anyone know how to rotate images within a post?. The sketch was the right way up before I uploaded it.  

Taken me a while, but I've finally managed to create a lid for this pit kiln.  When it's rained the pit has filled with water and turned into a nasty swamp.  

Hopefully the rain will keep the water out.  May still have a problem with the water table,  but we shall see.


Anyway, my question relates to brick making as this will be my first project.  

So basically I'm trying to come up with a recipe.  Because I want these bricks for thermal mass, and not insulating, the two things I feel I need to consider are addition of grog and shrinkage rate.


To create a recipe I need an exact quantity of water, and to know the weight of my clay when it comes out the ground.  

Not really sure how to do this.  Perhaps I clean the clay (remove as much sand and stones etc as possible), dry it out to a leather like consistency, weigh it, fire it and then weigh again to see what the shrinkage it.

Then I would make several bricks using the same consistency to test:

1) Pure clay

2) 5% grog added

3) 10% grog added

4) 20% grog added

Fire them, and should start to give me an idea of what amount of grog will prevent shrinkage and cracking.


The plan was to use crushed brick dust.  But I also read that sharp river bed sand can be used as a grog.  What do you think?

I'm also having trouble filling the molds.  When the clay is too dry, you get a lot of air pockets.  When it's too wet it sticks to the mold.  

Pure clay shrinks quite a lot when fired, most potters use a 15% sand mix and use a pug mill to blend well.

I talked to Acme Brick Works and they say a 20% sand 80% clay will shrink 15% from wet to fired. ("The more sand the less shrinkage occurs but don't go past 30% or there will be fatal cracking during the fire")
They also said the clay mix should be just firm enough to hold a ball shape (I really don't know what that means)
I did get to watch some forms being filled and from that I gathered that the clay should be wet for filling the mold, wet being wet enough that when you slide a finger along the surface of a ball you not only leave an imprint but also take some material with you because it stuck to you.
Since I am intending on playing a bit with making some bricks myself I talked with them about what to make my home molds with and they said wood would be my best bet. Untreated 2x4 material with screws holding the pieces together was the recommendation.

One tip they gave was to use Vaseline to coat the inside of the mold. It acts as a mold release so the mold will slide off the new brick, also freshly formed bricks should sit in the mold for 24 hours minimum.

They didn't have any helpful info on a pit kiln, no one was really familiar with those.

Hope this is of some help to you.

Redhawk

Brilliant info Redhawk

You make no mention of grog.  i.e. crushed up old brick/pottery.  So for brick making, you don't add any grog, just sand right?  And can I confirm it is what we call over here "sharp sand" which has been dredged from the river bed?  

And what about the clay itself.  Is it just clay from the ground with stones removed, or has it be sieved through a screen/mesh?

Ironically, when I was originally processing my clay, i removed as much sand as I could from it.  My friend used the stuff out the ground on her pottery wheel and said it was too rough on the hands due to the sand, so I sieved it out down to 120 mesh.

The guys at the brick factory didn't tell me about using grog, they use river sand brought in by the rail car load, the clay they use is comprised of pretty fine stuff so I imagine it is really clean clay.
Currently all my own experience is with a potter's wheel and making adobe bricks which are a whole different animal since I use straw, sand and 30 mesh clay from my land to make the adobe.
I dig my clay then spread it out thin to dry so I can break it up and remove the "junk", it is more of a terracotta type of clay on my land with lots of sandstone and shale type stuff mixed with it right out of the ground.

for pottery I use packaged clay powder, it is screened to 150 mesh (on the bag) then I add in a sand used for sandblasting to get the structure of the clay right for firing.