Karen Deasy on wed 1 aug 01
I am looking to try some wood ash glazes and have access
to several differant species of wood. My question
relates to the chemical properties of the ash. I know
that the temperature at which the wood burns varies for
seasoned vs. green wood, would this have any effect on
the chemical properties of the ash?
Tony Ferguson on wed 1 aug 01
The best test is to test it. If wood is left
intact--whole, the oils will be preserved in the
celluar structure of the wood as well the minerals.
With time, the oils break down but the minerals stay
the same. Burn some wood and test the ash. If its
green, it will take a bit longer to burn. Of course,
depending on the minerals in the soil where the wood
was grown will affect the content--but don't worry,
just test and you will find what you like. It's in
Tony Duluth, MN
--- Karen Deasy wrote:
> Hello all,
> I am looking to try some wood ash glazes and have
> to several differant species of wood. My question
> relates to the chemical properties of the ash. I
> that the temperature at which the wood burns varies
> seasoned vs. green wood, would this have any effect
> the chemical properties of the ash?
> Any thoughts?
> Karen Deasy
> Sterling Design
> Cabot, Vermont
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Hank Murrow on wed 1 aug 01
>I am looking to try some wood ash glazes and have access
>to several differant species of wood. My question
>relates to the chemical properties of the ash. I know
>that the temperature at which the wood burns varies for
>seasoned vs. green wood, would this have any effect on
>the chemical properties of the ash?
Try some of the 'Tennessee Ash' (from Poplar and Oak) which is sold
by Axner in the East. They have an analisis and the ash is finer than 100
mesh (mostly 200m) and unwashed, so the solubles will help in any shino
formulations you might try.
Cheers, Hank in Eugene
Dave Finkelnburg on wed 1 aug 01
The temperature at which wood burns has a profound impact on the ash
produced. My understanding is wood ash from a small, cool fire such as a
campfire, fireplace, etc has more flux than that from a kiln because of
In a very hot fire the alkali metals (sodium, potassium primarily) tend
to vaporize, which gives that lovely flashing and great surfaces in
wood-fired ware. Unfortunately, since the ash lost some alkali metals in
the hot kiln fire, it will not provide as much flux in a recipe.
This is, of course, difficult to quantify. How much flux vaporized
depends on the burning conditions and the wood itself. Thus the advice to
test the ash you plan to use is very important!
You may want to read "Ash Glazes" by Tichane or "Wood-fired Stoneware
and Porcelain" by Troy for further information.
Dave Finkelnburg in Idaho
karen terpstra on thu 2 aug 01
Maybe someone else on the list can answer your question more
specifically. There are so many variables...I suggest reading Jack
Troy's book, Wood-Fired Stoneware and Porcelain. He has extensive
information on ash glazes and wood characteristics in the Appendix.
> Date: Wed, 1 Aug 2001 16:14:53 +0000
> From: Karen Deasy
> Subject: wood ash composition
> Hello all,
> I am looking to try some wood ash glazes and have access
> to several differant species of wood. My question
> relates to the chemical properties of the ash. I know
> that the temperature at which the wood burns varies for
> seasoned vs. green wood, would this have any effect on
> the chemical properties of the ash?
> Any thoughts?
> Karen Deasy
> Sterling Design
> Cabot, Vermont
Alford Wayman on thu 2 aug 01
There is a Chart in the Book The Potters Dictionary of Materials and
Techniques. Table 21 it's very useful.
However if you had a hardwood mix it would be hard to do a brakedown and
Hank Murrow on thu 2 aug 01
Karen Deasy wrote;
>> I am looking to try some wood ash glazes and have access
>> to several differant species of wood. My question
>> relates to the chemical properties of the ash. I know
>> that the temperature at which the wood burns varies for
>> seasoned vs. green wood, would this have any effect on
>> the chemical properties of the ash?
>> Any thoughts?
I would offer this advice: Make each ash supply into two equal
volumes. Wash one of the volumes three times, decanting each time until the
water tastes semi-palatable. Then screen both volumes (unwashed and washed)
separately to about 60-80 mesh. Then dry each out and put in containers for
testing. Test each ash to see for yourself how much a difference the
I think eash ash should be tested on a standard Currie grid tile,
as that will reveal the most information to you, and serve as a later and
continuing reference. My first tile suggestion would be a "C" corner of 40%
ash and 60% feldspar of your choice. Thjis WILL reveal interesting
differences between washed and unwashed as well as between different ashes.
Check out "Revealing Glazes" by Ian Currie, and/or go to his website for
more information. The Currie Grid tile works great even if you have no
analysis of your material, in fact, the tile will serve as a rough analysis
in time, and as you gain experience.
Here is Currie Info:
Copies of the book "Revealing Glazes - Using the Grid Method" and also
"Stoneware Glazes - A
Systematic Approach" can be purchased by
credit card via the author's website:
Good Luck & Good Hunting! Hank in Eugene
Craig Martell on thu 2 aug 01
Although mel thinks all that I make are celadon glazes, I do use an awful
lot of woodash.
My thoughts on ash composition in relation to burning temp are as
follows. If you get an incomplete burn due to green wood, bad stove,
crappy draught, whatever, you wind up with ash that still has a fair amount
of combustible material. This will affect the weight to usable oxides
ratio. What I mean is, you weigh out 100 grams of ash and get 80% or so
usable material that will work in and affect the glaze properties with an
incomplete burn but maybe you up this to 90% and above with a more
completely combusted ash. These percentages are an example, not an actual,
proven ratio. Also, most of the combustibles that are unburned will screen
out if you use a 50 mesh or finer. The burned ash particles are usually
As far as the oxides of sodium, potassium, and calcium being volatilized
from the ash at higher temps, I don't think this happens, or is of any
concern. Calcium and potassium are stable and don't volatilize at even the
highest temps we all fire at. Sodium won't start to volatilize until about
2000F, and really doesn't become unstable until cone 8 and above, according
to everything that I've read. According to what I've seen with soda fluxed
glazes that go to cone 10 and above, there's really not much
lost. Occasionally you do have some problems with outgassing causing glaze
faults. The woodash that Axner sells from Tennessee is burned at about
1800 F and still has a good deal of potassium, sodium, and calcium. Very
little carbon is found in this ash so the weight to usable oxide ratio is
really close to 1/1. I've talked to the engineer at the plant where this
ash is burned and he's sent me an analysis so that's what my data here is
If we are going to use woodash in glazes, it's important to realize that
what you have in your hands is not going to be the same thing you see on
paper. Woodash requires a great deal of hands on, empirical testing and
constant test fires to insure you are going to get the result you
expect. I save burned ash from the stoves over a period of perhaps six to
seven months and blend it, and then test in my ash glazes. I will then
have a large supply of something that I can trust to yield "fairly"
predictable results. If you want consistency on the molecular level, use
something other than woodash. If you want a great natural material that's
a bit of work to use but will often give wonderful surprises and results,
later, Craig Martell in Oregon
Russel Fouts on fri 3 aug 01
Another way to approach this:
When I was in school, in the '70s, I wanted to experiment with ash
glazes made with local clays (Michigan, glacial, earthenware).
I got my ash from a local wood stove shop, they just put all their ash
in a 55 gallon drum for me to pick up. That amound would last a LONG
time. I didn't care what was in the ash, I had a good consistant source
(55 gallon drum, remember).
The clays I dug up from the ravines and gravel puts around Grand Valley
Just using simple line blends, I came up with some really beautiful
glazes. Fired them in gas, wood and salt. Beautiful.
Less analytical, more fun, beautiful results, learned a lot. I'd do it
that way again, if I was going to do it again.
Russel (yes we had all those kilns in the '70s, built them ourselves.
Billy-Eddie ran a really special program)
Mes Potes & Mes Pots
Tel: +32 2 223 02 75
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Ian Currie on sun 5 aug 01
Found your post very interesting. I can possibly add to some of it, and
query some too....
>If you get an incomplete burn due to green wood, bad stove,
>crappy draught, whatever, you wind up with ash that still has a fair amount
>of combustible material. This will affect the weight to usable oxides
>ratio. What I mean is, you weigh out 100 grams of ash and get 80% or so
>usable material that will work in and affect the glaze properties with an
>incomplete burn but maybe you up this to 90% and above with a more
>completely combusted ash.
This is quite correct, though I believe the leftover combustibles (charcoal
etc.) are sometimes much greater than 20%.
>As far as the oxides of sodium, potassium, and calcium being volatilized
>from the ash at higher temps, I don't think this happens, or is of any
>concern. Calcium and potassium are stable and don't volatilize at even the
>highest temps we all fire at.
This may be substantially correct in balanced and stable glazes that have
adequate alumina and silica. However an unexpected spin-off from some
systematic testing I did with grid tiles was an ability to compare
vaporisation of fluxes. It was the result of firing grid tiles in stacks,
separated by no more than a centimetre (three-eights of an inch?). After
the firing there is a vapor imprint on the back of each tile from the 35
glazes in the grid underneath.
Because of the design of the experiment I was able to directly compare the
vaporisaton or volatilization of several different fluxes. (The results are
laid out on page 39 of my first book: "Stoneware Glazes - A Systematic
Approach" - Second Edition). Here you will see that several of the C-corner
glazes (the recipe consists of 100% flux materials) give off so much
vaporized fluxes that the corresponding spot on the back of the tile above
is completely glazed, similar to a salt glaze. An example is the C-corner
glaze for the 0.8 Limestone set which consists of 58% potash feldspar and
42% whiting. Incidentally, if you take this glaze and alter the
feldspar/whiting ratio, the more whiting, the more volatiles... presumably
because this means less alumina and silica (from the feldspar) to sog it up
or fix it into the glaze.
I have a colour-slide showing this for each of the base glaze sets in the
"Stoneware Glazes..." book. These were fired in the same stack, and
therefore had virtually identical firing conditions, and are on the same
porcelain clay. I will show it to you when I see you in September.
The results show clearly that nearly all flux materials fired to stoneware
vaporize (to greater or lesser degrees) if the glaze is overloaded with
fluxes, that is, low in alumina and silica. (This is what causes some
glazes to "flash" nearby unglazed clay.) The thing with ash as you are
well aware is that many (most?) wood ashes are precisely this sort of
material, low in alumina and silica. For this reason, and considering the
results quoted above, I suspect that there is volatilization even of
potassium and calcium from wood ash fired to stoneware temperature.
It is possible that this is one reason that "natural ash" deposited during
the wood firing is so different from the "same" ash artificially applied
before the firing.
If the ash results from a cooler burn, the loss will be less, possibly
insignificant if we are talking of around red heat. This is just a guess.
Anyway it will give us something to argue the toss about when we get
together again.... :)
>If we are going to use woodash in glazes, it's important to realize that
>what you have in your hands is not going to be the same thing you see on
>paper. Woodash requires a great deal of hands on, empirical testing and
>constant test fires to insure you are going to get the result you
>expect. I save burned ash from the stoves over a period of perhaps six to
>seven months and blend it, and then test in my ash glazes. I will then
>have a large supply of something that I can trust to yield "fairly"
>predictable results. If you want consistency on the molecular level, use
>something other than woodash. If you want a great natural material that's
>a bit of work to use but will often give wonderful surprises and results,
This is really good advice. See also Hank Murrow's posting on the same date
(03 Aug 2001) in the archive. A general principle we are using here with
these difficult but interesting natural materials, including rock materials,
is to collect enough material for a heap of work, make sure it is a
uniformly mixed batch, run trials with it and then use it. Even if we don't
know what is in it, we can use it and often come up with something quite new
and interesting. When you run out of your bag or hopper or silo of the
material, you repeat the process.
One safety warning here... I would not use wood-ash from timber preserved
with that green stuff. I seem to remember from a previous life as a
forester a long time ago it used to be called Tanalith. It contains
copper/chrome/arsenic (all poisonous) and is SUPPOSED to be stable in the
timber, but is almost certainly unstable and present in the ash once the
timber is burned.
Anyone interested in my books can see summaries and purchase them at my web
Craig Martell on sun 5 aug 01
Thanks for commenting on my woodash post and making the points about volatiles.
My main point, although I do go off on tangents, was that an efficient burn
vs an inefficient burn will not reduce the amount of fluxing oxides in the
resulting woodash. I don't think it matters if they are soluble or
not. But once the ash is placed in a glaze and subjected to hellish
temperatures there is, as you have pointed out, a greater opportunity for
loss due to fluxing oxides becoming volatile. With some, such as zinc, the
atmosphere in the kiln will determine what is or isn't going to be fugitive.
After I read your post I went out to the shop and hauled out some of my
limestone sets and looked at the flashing on the underside of some of the
tiles. In all instances the flashing occurred aroung the "c", or high flux
corner. This is exactly as you have described. One possibility that I
thought of for this happening was the high proportion of flux in these
glazes. As one moves away from the "c" corner to areas of the grid that
contain less flux and more alumina and silica it may be just the lower
proportion of flux to the other components that lessens or eliminates the
flashing from volatiles and not the fact that the fluxes are chemically
"bound" by the alumina and silica. Some of the potash and soda is still
becoming volatile but it may not be enough to leave a footprint. The
reason that I'm thinking about this is that even though there is more Al2O3
and SiO2 in areas away from the "c" corner, all of this dissociates under
heat and the fluxes are not bound by the other stuff at high temperatures,
or are they?
Also, it's hard to tell just WHAT IS volatilizing from a potash spar
because they contain some sodium too. Custer and G-200 Potash Spars both
contian about 3.0% sodium to about 10.0% potassium. In Frank Hamer's
Dictionary, there is a graph that shows the effective range of fluxes. It
suggests that soda starts volatilizing much earlier than potassium and
becomes more actively volatile. So, I'm wondering too if the volatile soda
will have an affect on it's neighbor, potassium, and cause it to volatilize
earlier than expected were it not associated with the sodium.
Anyway, those are some of the things playing pinball with my brain in
regard to this stuff. Perhaps we should discuss this over a pint of Tree
Frog Ale while gazing across the valley at Mt. Hood?
regards, Craig in Oregon