[Clayart] The watered-down-kiln

Robert Harris via Clayart clayart at lists.clayartworld.com
Wed Feb 11 22:54:06 EST 2015

Interesting thought-provoking post, thanks!

It definitely seems to me that when considering water addition to firings
we need to make a real distinction between what might happen in gas kilns
versus solid fuel (i.e. wood) kilns.


On Tue, Feb 10, 2015 at 5:50 PM, L TURNER via Clayart <
clayart at lists.clayartworld.com> wrote:

> Water, gas, and other musings about fired kilns.
> The watered-down-kiln discussion has been interesting. Here are some
> thoughts at least should stir the fog.
> None of the wood firing reports that I have read contained any quantitative
> data regarding the probable air flow into the kiln.  Making sense out of
> their claims about water, and other manipulations is very difficult.  Us
> (that includes me) potters seem to be lousy at experimental reporting.
> When water (or steam) comes in contact with the glowing coals of a wood
> fired kiln there is a reaction between water and the char in the coals.
> The reaction products are: carbon monoxide, hydrogen, and cold (and
> smaller) coals. (The reaction is endothermic - meaning that the reaction
> removes heat from the environment of the coals to produce the reaction
> products).  The overall effect can however be useful by turning the coals
> into a gaseous product that will later combine with the air entering the
> kiln and subsequently burn. If the kiln does not have sufficient air
> remaining to burn the CO, then no more heat will be released in the kiln,
> only an increase in CO.
> Effectively at a given temperature, the ratios of Hydrogen, Water, Carbon
> dioxide, Oxygen, and Carbon monoxide are fixed at their thermodynamic
> equilibrium values because for the vapor state the interchange reactions
> are on the order of one millisecond.  Nitrogen in the air is just a
> diluent.  The math is tedious, but not terribly messy and the chemistry is
> third week second semester high school level.  The equilibrium constants
> are also available online from many sources.  A alternate way to gain
> insight is to do the Minimum Gibbs Energy calculations for a simple gaseous
> fuel and air such as methane and air.  Vary the moisture content of the air
> in the simulation and examine the results.  There used to be several online
> combustion calculators available from both university and commercial web
> sites. There is also the NASA CEA software that can be run online:
> http://www.grc.nasa.gov/WWW/CEAWeb/  and http://cearun.grc.nasa.gov/
> The heat transfer between the combustion gas and the kiln ware is via
> radiation and convection, with radiation being the more important in fired
> kilns.  However, the emissivity of the combustion gas depends on the
> concentrations of carbon dioxide and water vapor.  All the other components
> are effectively diluent.  The math is messy but adding water to the
> combustion gas will cause an increase in the gas emissivity resulting in
> increased heat transfer to the ware. For more details see Perry's Chemical
> Engineers' Handbook - 7th edition, chapter 5 starting on page 32.
> Adding water to the combustion gas will lower the density of the combustion
> gas as well as increasing the total mass flow.  The effect is to increase
> the velocity past the ware.  Increased velocity increases the convective
> heat transfer to the ware.  Math is messy.
> Diffusion of water into the surface of molten glaze (be it applied or wood
> fired) will change the optical and other characteristics of the melt.  Only
> a few 10's of ppm are sufficient to produce noticeable changes. The glaze
> must be above its transition temperature for significant amounts to diffuse
> into the surface.  For more background dig into the melting of hydrous
> magma being studied by a number of geochemical laboratories.  If you add
> water to the kiln, you have more water than just from the combustion and
> you will get higher moisture levels in the glaze.
> This effect could be the main difference between electric kiln work and
> fired kiln work.  Be a good advanced student project.
> Regards
> LT
> The Woodlands, TX
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