Rafael Molina on mon 23 aug 99
------------------
Clayarters:
On Saturday August 14 I participated in my final wood-firing at CVC. The =
kiln
is the catenary arch cross draft described in Jack's =22Woodfire...=22 book =
and a
Studio Potter publication. I built this kiln over a period of months in '97=
and
'98. This was the eighth time in the last 18 months we have fired it.
Four of us worked 17 hours to reach cone 13. The temp was even top and =
bottom,
good reduction throughout, and the best natural ash accumulation we've ever
achieved. Hence, my query. The ash is brown. I mean =22Tobacco Spit=22 =
brown (to
borrow a phrase from the folk potters of the Southeast). Why is this? Why =
not
green?
I doubt it is the species of wood. We use old pallet wood in the early part=
of
the firing which is mostly pine. We also use some cedar splits (with bark)
early on. We later change to scrap from a local cabinet door manufacturer.
That wood is ash, oak and maple.
Can it be the firing cycle? We try to keep the rise in temp between 150 and=
200
degrees an hour. We reduce no later than cone 010 to be sure and get the =
color
development in the Shino glazes. Reduction lasts approximately an hour. =
After
reducing we fire up to cone 10 in about six hours followed by cone 13 three
hours later.
Any comments will be greatly appreciated. TIA.
Rafael
Gavin Stairs on wed 25 aug 99
At 02:08 PM 23/08/99 , Rafael Molina wrote:
>... Hence, my query. The ash is brown. I mean "Tobacco Spit" brown (to
>borrow a phrase from the folk potters of the Southeast). Why is
>this? Why not
>green?
...
Hi Rafael,
Iron green in glazes is from olivine and chlorite groups of minerals, I
think. Other more authoritative voices can correct me if I'm
wrong. Here's a generalized chlorite:
(Mg,Fe,Al)3(Si,Al)4O10(OH)2*(Mg,Fe,Al)3(OH)6. And here's an olivine:
(Fe,Mg)2SiO4. As you can pretty well see chlorite is a hydrated
olivine. Similar to Fe3O4 mixed up with a lot of MgO, SiO2 and Al2O3. So,
at a guess, maybe not enough MgO? Also, full reduction to the Fe2O3
phase. I don't know if the chlorite can form at all, but the olivine form can.
I'm pretty green at this iron colour game, if you'll pardon the pun. I
know that, in principle, (Fe,Mg) complexes can make greens and blues,
tending all the way down to browns and reds on the green end, and blacks up
on the other end. If you're getting brown, I would guess that you are
reoxidizing, and maybe missing some Mg. I'd like to test this conjecture
out, but I haven't gotten to it yet. Maybe soon.
Gavin
Gavin Stairs on thu 26 aug 99
Addendum: Chris Trabka reminded me off list that Manganese can produce
brown ash glazes.
"Robert Tichane reports that browns come from Manganese in his book "Ash
Glazes". Manganese can vary from 0.3% to 28.5% in the ash."
Thanks, Chris.
Gavin
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