wood-fire ceramic kiln

Hello, I think I need some advice, please.
I wanted to built a wood-fire garden ceramic kiln. I had a small clay hill (made from a pond that was dug near its place 15 years ago). I dug an horizontal trench through it, to the middle of the hill at ground level (about 2 meters long) then set up a vertical chimney (about 2 meters high, 50cm on 50cm opening). I covered the trench with cement blocks (creating a tunnel of 50cm high and 35cm wide) and isolated the top of the chimney and the vertical trench with soil.
I placed a thermometer inside and lit a bonfire in the front opening. Immediately the fire went horizontal and made this burner/rocket sound. I have added wood and the fire was very strong. Temperature went up to 710 Celsius.
BUT, the operation is not total success:
1. The opening becomes very clogged with charcoal. I needed to clear it twice, making the temperature drop and "restart" the heat.
2. As much wood I have added and fierce fire I made, I could not get it more than 710 Celsius.

Can anybody please give me some advice on a better front fire design? and how to reach higher temperatures?
These are some photos of the process:  https://goo.gl/photos/EyWCc615TtBv7fir9
Thank you.

I just fired my wood-fired cob kiln yesterday. I wish I had more time right now - I would love to talk shop, but I am late for heading out of town for two weeks (selling the pottery I fired, and making and firing more).

Using all small wood will give a very fast hot fire, but the sticks turn to coal and collapse and smother each other before they can finish burning. For most of the firing you will want wood of 2-4" (5-10cm) diameter equivalent. Split wood burns better. At the end when you want to get the last bit of temperature rise you can use small wood.

My recent firing got to cone 06 (1828F) at the top and cone 1 (2078F) at the bottom. (An updraft kiln is always hotter at the bottom, and cools toward the top.)

If you are using your vertical portion simply as a chimney, the size of it when heated by a good fire will make an extremely strong draft that may pull air through the firebox faster than it can be heated. A damper on top to regulate airflow would be a simple method that might let you get higher temperatures. A permanent solution would be to make the chimney smaller, no more than the cross section of the firebox. You might still want or need to make the ware chamber (where you put the items to be fired) larger than firebox or chimney, to let heat linger and eddy around the pieces.

Thank you Glenn for the wonderful reply.
You made it very clear, and your kiln implementation looks really nice. Hope you had good sale day.
Yours is the second such kiln style that I see working very successfully.
I read (in Rocket Stove literature) that to create the air flow one must make to chimney exit area larger than the entry opening.
I understand now that in a kiln implementation one should not strive for a strong heat flow and keep it slow so temperature can build up.
Thank you

I fixed my kiln so it is now as Glenn's model (more or less), but I am slightly stuck with creating the floor on which the works will sit. My heart is square (not personally, the kiln's) of about 20" sides. I thought of creating a cob polls (about 2" think), about 25" long, dry them outside and put them as floor, stuck into side holes that I will creat. Then I will run empty fire to dry and strengthen them. Does this sounds practical? What is the best cob mix l should use? Shall I place strew in the mix? Build it around long twigs?
Any hints would be appreciated.
Thank you.

Just back from my two weeks selling pottery and doing medieval recreation things... thanks for the nice comments, and for taking my work as a model.

You could make firebars of clay, dry them, and set them into the sides of the kiln for support. A more direct method is to make an arch of a green flexible stick, poke it into the sides to be stable, and wrap cob around it so the bottom surface is an arch and the top surface is flat. The stick supports the clay until the clay is dry enough to support itself. Using this method, I have numerous times fired a kiln for the first time loaded with pots sitting on the firebars - no need to prefire the kiln. It just takes a bit longer to do the first firing as you are drying and firing the kiln too.

Glenn Herbert wrote:Just back from my two weeks selling pottery and doing medieval recreation things... thanks for the nice comments, and for taking my work as a model.

You could make firebars of clay, dry them, and set them into the sides of the kiln for support. A more direct method is to make an arch of a green flexible stick, poke it into the sides to be stable, and wrap cob around it so the bottom surface is an arch and the top surface is flat. The stick supports the clay until the clay is dry enough to support itself. Using this method, I have numerous times fired a kiln for the first time loaded with pots sitting on the firebars - no need to prefire the kiln. It just takes a bit longer to do the first firing as you are drying and firing the kiln too.

Thank you very much for the great help.
I made some bars and they are almost dry. BUT they are from cob, with strew and twigs. they seem very strong, more than 2" think.
Do you think they will suit the purpose? (if I would fire empty kiln just to make them stronger)?
Thank you

If they dried without cracking, they will probably work fine for light loads. I might fire them first before loading them, unless it is with just a single layer of tiles.

Update: It is only when I made the kiln more like the above model, ie the fire is really under the pieces loaded, that I manged to get the kiln to a proper temperatures.
Now I try to learn and find a way to probe the temperature, without  melting the thermocouple. It seems that they are sold, either exposed, without any cover, or too short to stick them in the kiln but still isolated and away so their handle would not melt in the process...

Just thought I would add this kiln here. I think he is getting closer but needs to implement some of the ideas you all have come up with.

His kiln is good; the only major thing I would change is to have more layers of tiles on top, so the upper part of the kiln chamber is not so exposed to the atmosphere. That would let it get hotter, and keep it from cooling off so fast when the fire dies down. (Firing pottery needs to heat up and cool down slowly (hours) to avoid thermal shock.)