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on becoming a kiln master

updated wed 23 nov 05


Ivor and Olive Lewis on tue 22 nov 05

Dear Tony,
Using your observations is all I can do.
First thing however is to be clear in you mind about what "Heat" is and =
what "Temperature" is.

Lets start with "Heat" Heat is an energy form that can be moved from =
place to place. It can do this by Convection, Conduction and Radiation. =
All three happen in a kiln, but mostly conduction.

Temperature is the quality of "Stuff" or "Space" that shows heat is on =
the move. As Energy moves the place it moves from cools and the place it =
goes to heats up, provided the destination is cooler than the departure =
point. Now all things have heat energy in them of one sort or another. =
When there is a difference in temperature heat always flows from stuff =
with the high temperature to where the temperature is cooler. To make it =
happen the other way you need an engine, something to drive the motion. =
In a Refrigerator you have a motor and a compressor to push heat from =
inside the fridge out into the atmosphere. In a kiln it is the fire on =
the hobs or the burners or the electric elements that act as that =
driving motor to get the heat moving

You burn wood in your firebox and release chemical energy by combustion. =
I have difficulty in understanding the next step but the way it appears =
to me is that the flames represent gas in a very agitated state, the =
greater the agitation the higher the flame temperature. This is as hot =
as things can be in that system. As the heated gas moves through your =
cold kiln the kinetic energy of the gas is transferred into things that =
are at a lower temperature. As heat transfers from the hot stuff to the =
cool stuff the cool stuff hots up and the hot stuff cools down, all the =
way to the stack.

As you get to the end of a firing the fire box and the early chambers =
are as hot as they can get at the damper setting you use and the rate at =
which you feed the wood. Open the damper to draw the hot stuff into the =
cooler chambers, to draw cool air in to calm the fire down and feed fuel =
at a slower rate (Guessing !). Keeping the early part of the kiln at a =
constant consistent heat means that less energy is syphoned away into =
the hot bricks and maturing ware (So you hope !). That heat will only do =
its work when it gets into a colder or cooler place, where the =
temperature is lower.=20

Right ! Back to the idea of Green Wood, damp or wet wood. Here the =
active ingredient is Water but there are two systems working, Physical =
and Chemical. At the beginning of the firing the physical one is in =
operation, at the end, above 1100=BA Celsius both are in operation.
You treat your water to four operational phases.
a) heat is needed to raise the temperature of the water to its boiling =
point. This is its specific heat and can be measured in Joules per =
degree C per 18 grams (gram molecular weight of water). Then there is b) =
the heat which converts water into steam, Latent heat of Vaporisation. =
You can't get any useful work from this fella, no temperature change, no =
work done but energy is needed. That's a few more Joules per 18 Grams of =
water. Now phase c). Your fire starts to warm the steam up. This =
requires a bit of heat since Water vapour has its own value for Specific =
Heat but it still needed for every degree rise in temperature. There's a =
few more Joules per degree C per 18 grams of water. So instead of =
heating the kiln your bursts of energy are adsorbed by water in your =
wood. Add to this the bit of heat c+) to change dry wood into a gas, =
which is only partly successful. These things need energy so things keep =
cool and you get a lot of dead coals clogging the system. Do you see the =
three operational phases, using energy to get gas of one form or =
Now to the top of the firing where it seems as though things are =
stalling and the last chamber will not get up there
Above 1100=BA Celsius you start to add wet wood again. What has happened =
in Phase a. b.and c plus c+ to get the wood burning still has to happen. =
An equal amount of cooling takes place to change every gram of water =
into steam. But now your Coals start a huge love affair with superheated =
steam. Marriage and Divorce. Carbon has the ability to separate the =
bonds that bind Hydrogen to Oxygen. Seems that the bond between Oxygen =
and Carbon is the stronger so there is an interchange and you start to =
flush a mixture of two lively gases, Carbon Monoxide and Hydrogen, into =
your kiln. The price to pay is that the temperature tends to fall =
because the heat has to come from somewhere. You will cool the firebox =
unless there is a stead pull from the stack, re-energising the coals. =
Now you have two energy rich gases entering the kiln from the firebox =
and they are hot and they have energy to spare. All they need is a few =
puffs of oxygen. If they get this then you get a flame front moving =
through the kiln and those flames are at about 1550=BA Celsius. The =
back of the kiln is down near a thousand degrees. Pots and those cool =
bricks are a heat sponge, eager and ready to soak up all those =
kilojoules from burning Hydrogen and Carbon Monoxide.

So friend, your theory that green wood prevents a blast off early in the =
firing and gives a blast off in the final hot stages of a firing would =
seem to have a sound basis founded on Scientific Facts.
I'll leave the Calcs to Bruce.
Anyway, its only about Economics when you think about it.

All the best, and thanks for the accolade.