Primitive firing survival

[John Hayter]

I have set up a primitive Wood burning kiln. It looks sort-of like a bar-b-q pit made of rocks and clay mud. I have a sheet of steel to cover it and a piece of tin to keep the heat in on the front. I have had two firings. Both times all pieces were lost! I build a pretty big fire with lots of hardwood coals and I keep adding wood for about 12 hours and then let it burn all night and check it in the morning. I recently purchased some cones, but I haven't had a chance to try them out yet. I suspect my fire is too hot. Any suggestions or reasons ? I am using wild clay from my property. It has some small stones in it. I have started screening the clay now but I have not tried it yet.

 

[liambesaw]

What do you mean "lost"? Melting? Breaking? Exploding?

If melting you need to not fire so long, if breaking you need to make sure you're not putting wood directly onto the pottery and physically breaking it.  Not much you can do about quartz inversion temps I don't think, but maybe try going slower to begin with and most importantly, make sure the clay is BONE dry, not a lick of moisture.  A good alternative would be to bisque the pots in an electric kiln first, this will give them strength and eliminate any moisture issues.

[Callie Beller Diesel]

I will not claim to be incredibly knowledgeable about pit firing, but the once or twice I participated in one as part of a group, we used materials that smouldered more than burned: sawdust and the like. Too hot too fast, maybe?

[Chilly]

8 hours ago, John Hayter said:

 It has some small stones in it. 

 

Could be the stones.

 

Have you fired this clay any other way?  Electric kiln?

[John Hayter]

Thank you everyone for your responce. The pieces were definitely broken ,not melted or hit/crushed. I'll bet it needs a pre-heat or simply just go slower when increasing the temp. I think by screening the clay it will preform much better next time. Raw clay has rocks, roots, and worms in it. What was I thinking! 

[Babs]

Well some folk add grog..fired clay ground up  to clay for pitfiring which allows more protection against thermal shock . Check oyt what raku clay looks like. Slow start way to go. I'm guessing a crash cool would be happening, also draughts? Anyw way to blanket it after reaching top temp...soil all over it?

What are your cracks like.. this us usually a great indicator of the causes

 

[John Hayter]

Well, I have plenty of grog that I can add. Lol. I am retired and I have taken this craft up as my thing to do. I don't have access to an electric kiln I guess until I make some pottery friends. The cracks are mostly fine but long, sometimes it makes the piece completely fall apart. My method for bowls is slab construction to avoid seams as much as possible, I changed from pinch pots or coil because I was afraid the problem might be my construction.

[Babs]

See if you can get a hold of Hamer and Hamer

"The potter's dictionary of materials and techniques"

Your wild clay may need some additions.

How plastic is it?

There are some posts in these forums by folk exploring local clays.

Have you had an opportunity of firing some of your pots in a gas or electric kiln to see how they survive?

Some small electric kilns are kitchen bench size...not recommending that siting!

But can be great test items

[jrgpots]

My daughter and I used a 55 gallon drum layed on it side.  A sheet of expanded steel was used as a shelf.  I supported the shelf with 6 fire bricks.  Once the pieces were set on the shelf, we spread oxides on clay shards around the pieces.  Sawdust covered the pieces. kindling on top of that.  larger pieces were fed below the shelf.  The opening of the barrel was closed off so there was only a 6 -8 inch wide opening. We fed wood in under the shelf  after about two hours, allowing the kiln to warm up more slowly.  we fed the beast for about 4 hours.  THEN WE CLOSED IT ALL UP while it was still burning,  blocking the front completely,  kaowool insulation wet on top of the barrel.  It slowly cooled down.  no loss.

I think the thing that was successful was to trap the heat and force a slower cool down.

 

Jed

[Marcia Selsor]

I taught primitive pottery during summers in Montana. We used a 2-3' deep pit. half way up the wall was a ledge to support refrigerator shelves.  We started a fire with wood to create a bed of hot coals. We preheated the pots along the edge of the pit. When we had a bed of coals, we placed the pots , inverted, on the shelves. Then we added sawdust and dried cow manure , some thin sticks, in a pile up to 2 feet above ground level. Covered it loosely to contain materials. the flame would burn through in 2-3 hours. Then covered it tighter with more pieces of steel to cool. This is a Maria Martinez firing style. We processed the clay and added some grog. Very good success rate. I taught this class for rural teachers with zero budgets.

Marcia Selsor