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u.p. hobagama wood firing #2

updated thu 3 jun 10


gary navarre on wed 2 jun 10

Hay Crew,

Forty five hours earlier the kiln got lit and the front had been to cone t=
hirteen for a couple hours after firing for thirty six or so but it was on =
that slippery slope of small mistakes and fatigue that ends in a loss of he=
at and wasted stoking. Repo the Brick Yard Guy brought another buddy over t=
o check out the kiln firing and he asked me, "How many kilns have you built=
?" At that point I told him, "Enough to know I'll never try this again!"

That was a few days ago so I've had time to recover and changed my tune so=
me, not much, but enough to stick around and un-brick the door to have a lo=
ok if any pots turned out worth looking at or using. I've always thought th=
ose Japanese potters rather silly saying they were satisfied if one out of =
a thousand pieces turned out interesting enough to keep but after that firs=
t firing I came to appreciate the thought as I had about four or five not n=
eeding a re-firing, excessive cleaning, or smashing. It's starting to look =
like a repeat of the first firing.

This time around is ending up a little more productive even though the fir=
ing range was from ^13 in front...

... and ^4 at the exit flue...

I got the front of the chamber hot after about thirty six hours and added =
side stoking for another twenty five or so but failed to bring the tail up =
to a glaze melting temperature. This procedure resulted in ash being drafte=
d onto still soft glazes in front making them rough to touch. That might be=
OK for a Dragon Pot, Spoon Pots, and Vases or the outsides of bowls but th=
is rough ash on the inside is not something I want to justify aesthetically=
so I need suggestions for a way to polish what I can and feel OK about put=
ting them up for sale in the "Art For All" show. So far I'm having some suc=
cess using 400 grit wet/dry Emery paper wet without leaving discernible scr=
atches. I'm wondering if this is acceptable because I've always tried to fi=
re the glaze to a gloss on the inside at least. I was asking a local cerami=
cs lady if she could fire some pieces for me to ^10 to smooth things out an=
d she said some of the roughness gives the pot character and she
suggested just polishing some parts and leaving other areas alone. Other o=
ptions come to mind too like getting a small electric or gas kiln to fire t=
ests and for cases like these but I don't think I can master that and build=
a display before the end of June so it looks like polishing somehow will h=
ave to do for now.

The firing album really doesn't have a whole lot of neat shots...

... mostly pictures of the pyrometers, but I'm using it as an outline so I =
can study where the videos fit in and what was happening at the time. One s=
hot of interest...

... is a setting I found with the active damper in to about 1/3 open, the c=
himney cap at 1/4 or so...

... and the passive air brick above the active damper open a bit (you see i=
t as a dot of light just above the active damper slot). What I noticed when=
stoking was if I got a lot of black smoke from the Red Pine slab wood pull=
ing that passive air brick killed the smoke by adding air so the carbon wou=
ld burn inside the stack creating the reverse effect of cooling and instead=
kept the stack draft hot yet the active damper kept heat inside the chambe=
r, at least that's what I think is happening. Due to the adjustable feature=
s of the stack design I'm sure there are other settings that might produce =
better combustion but it will take a few more firings and suggestions from =
other firers to figure them out. So thanks in advance for thinking about ho=
w you would fine tune the Hobagama #2 to make it fire more evenly.

Rick made a comment about it not getting enough heat from the grate area t=
hat made me continue thinking I might need to move the front wall of the Bo=
urry Box out another brick to give me more room for logs on the front side =
of the hobs and thinner blasting fuel right up close to the throat arch. Wh=
en I was stoking I felt the fuel was a little cramped so I'm thinking the b=
ox is to narrow. The width and height seem OK. Rebuilding the Bourry Box fr=
ont wall and Pignose is really not a problem because the hardwood logs in t=
he first firing loosened the corner bricks and rain washed some lag out of =
the pignose seams so it leaks air causing ash to still float onto the pots =
during cooling and I designed it so it could be easily adjusted. Another th=
ought could be about the throat arch but I really didn't plan on making it =
bigger. As an alternative I think I'll add another foot or two to the 14' s=
tack and see if that will help pull the heat toward the tail at high
temperature. We did a rough count on how long a stoke took to smoke and fi=
gured the rate of draft was 3.5'-5'/sec. which I think is about normal but =
if it was faster I might be able to pull the heat into the tail better at h=
igh temp and still slow it down if needed.

That's some of it in a nutshell for now so stay tuned and stay in there eh=

Gary Navarre
Navarre Pottery
Navarre Enterprises
Norway, Michigan, USA