John K. Dellow on fri 15 may 98
Has anyone experenced the inability of getting red when firing in wet
weather ?. If so is there a cure ?.
Also has anyone noticed the glaze crasing when the firing has not porduced
red & no crasing with good red colour ?.
thanks in advance
John ( jacka ) Dellow
Blue Mtns Pottery
Gold Coast
Australia
dellow@usa.net
David Hendley on sat 16 may 98
Funny John, wet, humid weather PROMOTES good
copper reds for me. In fact, if I don't have an urgent need for
the pots, I'll wait for a good drizzly day to fire ( it also a lot
cooler, a big plus with a wood kiln).
I think that if you see crazing when there is no red color that
you really also have crazing when there is a red color.
You just can't see it.
Try looking very closely at your red glaze with a hand magnifier
and I'll bet you see crazing lines.
David Hendley
Maydelle, Texas
At 05:40 PM 5/15/98 EDT, you wrote:
>----------------------------Original message----------------------------
>
>
>Has anyone experenced the inability of getting red when firing in wet
>weather ?. If so is there a cure ?.
>Also has anyone noticed the glaze crasing when the firing has not porduced
>red & no crasing with good red colour ?.
> thanks in advance
>John ( jacka ) Dellow
>Blue Mtns Pottery
>Gold Coast
>Australia
>dellow@usa.net
>
Dana Henson on sat 16 may 98
Hello,
When I was trying for copper red using a Peter Lane recipe, I would get reds
with no crazing, but when the red color was absent, I did experience crazing.
I had never thought about it until I saw your post. I haven't the faintest
idea as to why this is, but just wanted to let you know that it happened to me
also.
Dana Henson
G_Henson@venus.twu.edu near Denton, TX
Marcia Selsor on sun 17 may 98
I have noticed that rainy days are good for reduction gas firings in general.
(They are pretty rare in Montana) I think the low barametric pressure has
something to do with it. The firings are easy to control
and very even reduction throughout the chamber (both of our stoneware gas
kilns being about 36 Cu.ft. and 56 Cu. ft.)
Marcia in Montana
David Hendley wrote:
>
> ----------------------------Original message----------------------------
> Funny John, wet, humid weather PROMOTES good
> copper reds for me. In fact, if I don't have an urgent need for
> the pots, I'll wait for a good drizzly day to fire ( it also a lot
> cooler, a big plus with a wood kiln).
>
> I think that if you see crazing when there is no red color that
> you really also have crazing when there is a red color.
> You just can't see it.
> Try looking very closely at your red glaze with a hand magnifier
> and I'll bet you see crazing lines.
>
> David Hendley
> Maydelle, Texas
>
> At 05:40 PM 5/15/98 EDT, you wrote:
> >----------------------------Original message----------------------------
> >
> >
> >Has anyone experenced the inability of getting red when firing in wet
> >weather ?. If so is there a cure ?.
> >Also has anyone noticed the glaze crasing when the firing has not porduced
> >red & no crasing with good red colour ?.
> > thanks in advance
> >John ( jacka ) Dellow
> >Blue Mtns Pottery
> >Gold Coast
> >Australia
> >dellow@usa.net
> >
Tom Buck on sun 17 may 98
Jacka: FYI, there is a commercial process called "partial oxidation of
hydrocarbons for production of hydrogen" that is, a hydrocarbon fuel is
burned with insufficient air (oxygen) causing some carbon monoxide (CO)
and hydrogen (H2) form, and this is followed by a "shift reaction" where
CO reacts with H2O (steam) to form more H2 (& CO2). This process is widely
used at fertilizer plants to produce hydrogen for subsequent reaction with
nitrogen (from air) to make ammonia. So the process has been studied in
detail.
A pottery kiln using a hydrocarbon fuel (natural gas, propane,
fuel oil) can BY CHANCE become a "hydrogen producer" under certain
conditions, and clearly the presence of a small amount of moisture in the
combustion air is one of the factors that will increase the amount of
hydrogen in the combustion gases inside the kiln .... all these rather
unusual chemical reactions occur quickly (a second or two) just after the
fuel and the air leave the burner tip.
Why is hydrogen significant in a copper red reduction process?
Because Hydrogen is by far the most active and efficient reducing agent,
superior to CO and Carbon itself. Hydrogen will enhance the change of CuO
(black oxide) to Cu2O (red oxide) and to Cu (metal).
If you, Jacka, are not getting better reds during a rainy firing,
then you are using too much "secondary" air and not allowing the "partial
oxidation" reactions to occur. You should check on your burner system and
see if you can reduce the air-to-fuel ratio.
For those who say their Cu red glaze crazes if no "red" is formed,
they may be noting a change in overall thermal expansion of the glaze. A
typical red mix uses some copper compound, plus either iron oxide or tin
oxide, as the components of a red glaze upon appropriate firing. Most
(all?) glaze analysis programs (computer software) ignore the contribution
of colourant oxides to a fired glaze's overall Coefficient of Thermal
Expansion. Such oxides DO change the glaze's COE but in what way etc. is
not generally reported so data is lacking.
Potters should recognize that "my claybody" doesn't match "your
claybody" and therefore my non-crazing glaze may not work on your
claybody. It is possible a body/glaze combo are on the "cusp" if Cu2O is
formed (a red glaze) but that the glaze contracts slightly more if the
copper remains as CuO (a green glaze) and results in crazing. If one
observes this, the glaze recipe needs adjusting to lower the
expansion/contraction somewhat.
Til later.
Tom Buck ) tel: 905-389-2339 & snailmail: 373 East
43rd St. Hamilton ON L8T 3E1 Canada (westend Lake Ontario, province of
Ontario, Canada).
On Fri, 15 May 1998, John K. Dellow wrote:
> ----------------------------Original message----------------------------
>
>
> Has anyone experenced the inability of getting red when firing in wet
> weather ?. If so is there a cure ?.
> Also has anyone noticed the glaze crasing when the firing has not porduced
> red & no crasing with good red colour ?.
> thanks in advance
> John ( jacka ) Dellow
> Blue Mtns Pottery
> Gold Coast
> Australia
> dellow@usa.net
>
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