Wildcat Creek Brick Company

Saturday, January 2, 2010

Salt glaze is a tricky thing!

Im pretty pleased with the pots and pieces near the flue. The salt worked its magic there. The front had almost no glaze effect. It may be that the chamber temps were lower at the front of the kiln. It may also be that most of the churning flames the salt vapors ride on were nearer the flue side. Either way it means I still havent mastered this yet. Back to the drawing board! Any suggestions? Im thinking of closing up the flueside chamber floor duct and expanding the frontside duct. My hope being that it will force the salt vapors to travel across all of the pots in the chamber, not just the rear most.

A two handled pot and a candlestick holder. I may put the holder back in one more time. Dont like the lack of glaze in the fluted areas.

I am now settled on some facts and limitations I have to abide by.
1. Creekbed clay is out for now as I cant reach the temps needed to flux it out. Nor does salt vapor seem to have any effect on it. Maybe the low iron oxide content in it.
2. Field clay from the Cutler - Burlington site has a high iron oxide content and salt glazes easy to a near black glaze.
3. Field clay from the Flora site has a somewhat lower iron content. It also has something else in it I havent identified that gives a grey green to it. It also salt glazes well if not quite as good as the other field clays.
4. I want to seperate the bottle glass I use as flux in the clay bodies by bottle colors. Dark browns, greens, and clears will be divided to help control the resulting colors in firing the clays.

Another first for me is that I logged time/temps this firing. I may be able to use this to work out some improvements in the kiln or my stoking methods.
I can say that I did use about 1/3 less wood this time than the last. It did come up to temp quicker as well. So closing off unused chamber space when firing a small load does have a positive time and fuel savings effect for my kiln.


  1. Hi Richard,
    I was wondering if you could post a simple sketch of the layout of your kiln so that we could understand the way the fire circulates through the kiln from the firebox to the chimney, and approximate sizes. It might help solve the riddle of the heat distribution in the chamber.

    I was re reading an article in a really old Ceramics Monthly magazine this last night about a potter that had modified his wood fired kiln to have forced air through the firebox. The kiln had previously relied only on chimney draft and worked well enough, but he wanted to improve the efficiency further. One thing he believed in was keeping the flame in the chamber, and he had a simple spy hole into the chimney that was level with the chamber roof. If he saw flame going up the chimney past the spy hole in the chimney, then he adjusted his stoking and/or air to keep it from climbing higher. He reasoned that the hottest part of a gas flame is out near the tip, and it is cool near the burner, so thought that letting the flame climb out the chimney was a waste of heat and energy. The combination of the technique he used with forced air and keeping down to chamber top hight made the kiln easier to fire to temperature, and also saved wood.

    I wonder if some of the problem you are having is that a lot of the combustion is taking place out of the chamber rather than in the chamber itself? The hot exit flu might confirm this. I know from some of your photos of firings that your firings are rather spectacular with a great display of flame out the chimney! It could be that all you have to do is to slightly reduce the forced air pressure, or to add a little less wood each stoke.

    Another issue could be the length of the firebox and chamber. Wood seems to need room for the flame to develop. If the firebox and chamber combination were too short, you may also have trouble with most of the combustion happening outside the chamber.

    Something to ponder anyway as you deal with winter snows!

    Hope you are able to keep warm... you may need to keep firing the kiln yet!
    Thanks for your comments on my site, much appreciated. I've just put replies on my site to a couple of your comments.

    Best Wishes,


  2. Hi Peter, Thanks for the swift reply. I too have learned that flames out the chimney are wasted heat and fuel most of the time. I have been working on my stoking to keep the flames in the ware chamber as much as possible. I use a chimney spy hole like you do. That is my first most indication of how my flames are doing.
    Second is sound of the burn. There is a crackling rumble when I get the stoking just right. It actually quiets down if I over stoke it.
    Third, this is a newer tool for me, is the use of my pyrometers to watch temp rise and fall at stoking time. I have a commercial pyrometer probe as well as homemade thermacouples placed in ares of the ware chamber. I also use a deep fry themometer at the chimney brick wall above the flue exit from the ware chamber.
    I can watch the temperature in the kiln ware chamber rise and fall 50 deg F diring stokings.
    I will try to put together a picture and post it here of the flame paths in my kiln along with my temperature/time records from the last firing.
    Thanks again for your helpful input Peter.
    Yes its cold here right now too.
    Will be that way until at the least mid march.


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