iandol on wed 24 oct 01
The notion that some of the zinc can remain in a glaze seems tenable to =
me, as does the opinion that during the processes which lead to =
vitrification zinc may have some influence through assisting in the =
breakdown of silicate minerals. However, at high temperatures with =
strong reduction provided by a carbon rich atmosphere, zinc oxide is =
reduced to a metallic state. It is the boiling point of Zinc metal which =
causes the problem. It has a boiling point of 910 deg Celsius (1670 F ). =
The higher the temperature and the longer the duration of the reduction, =
the greater will be the proportion of zinc which will boil from the =
maturing glaze. Evidence of this can often be seen on the outer wall of =
a kiln above a spy hole where gas seeps out. An incrustation or bloom =
which is yellowish when hot and white when cold indicates volatilisation =
of zinc, which reoxidises on contact with cold air.
One thing which is not clear in the literature is the nature of the =
compound which causes high levels of zinc to contribute to the opacity =
of a glaze. Is this due to a widespread precipitation of Zinc silicate =
crystals or that the oxide has not dissolved in the silicate melt?
Ivor Lewis. Redhill, South Australia.
Earl Brunner on thu 25 oct 01
We all know some materials that we use have individual characteristics
that are different then those that we observe in a mix. Some fluxes, by
themselves have very high melting temperatures but serve as melters when
mixed with other ingredients. So why would the boiling point of zinc be
any different? Maybe I'm being too simplistic here, but zinc in a glaze
is widely dispersed, not sitting in a container by itself, so just
because by itself it will boil at 1670F, why do we assume that it will
vaporize out of a glaze if widely dispersed?
iandol wrote:
> The notion that some of the zinc can remain in a glaze seems tenable to me, as does the opinion that during the processes which lead to vitrification zinc may have some influence through assisting in the breakdown of silicate minerals. However, at high temperatures with strong reduction provided by a carbon rich atmosphere, zinc oxide is reduced to a metallic state. It is the boiling point of Zinc metal which causes the problem. It has a boiling point of 910 deg Celsius (1670 F ). The higher the temperature and the longer the duration of the reduction, the greater will be the proportion of zinc which will boil from the maturing glaze. Evidence of this can often be seen on the outer wall of a kiln above a spy hole where gas seeps out. An incrustation or bloom which is yellowish when hot and white when cold indicates volatilisation of zinc, which reoxidises on contact with cold air.
> One thing which is not clear in the literature is the nature of the compound which causes high levels of zinc to contribute to the opacity of a glaze. Is this due to a widespread precipitation of Zinc silicate crystals or that the oxide has not dissolved in the silicate melt?
> Ivor Lewis. Redhill, South Australia.
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
--
Earl Brunner
http://coyote.accessnv.com/bruec/
bruec@anv.net
iandol on fri 26 oct 01
Dear Earl,
You say <but serve as melters when mixed with other ingredients.>>
Would you please explain what this means and describe the physical =
processes which are involved. Where did this notion originate? Why are =
refractory substances described as "Fluxes"?. These notions seem very =
confusing.
By the way, WE put ZINC OXIDE into our glazes as this is the most =
convenient material at a reasonable price. We do not put Metallic zinc =
into our mixtures. When reduction creates the metal from the oxide it =
will be above its vaporisation temperature. That metallic zinc at the =
surface will evaporate into the kiln atmosphere. This creates a =
concentration gradient. Deeper held zinc will diffuse to areas of lower =
concentration, which is the surface. Diffusion is time dependent. Long =
periods of intense reduction at higher temperatures will assist in =
discharging greater quantities of metal as a gas which reoxidises on =
contact with air to becoome an aerosol. Breathing such a dispersion of =
white powder may cause Metal Fume Fever which can be a frightening =
experience, thought seldom fatal. =20
Best regards,
Ivor Lewis, Redhill, South Australia.
Ron Roy on fri 26 oct 01
David Hendley did a series of tests on copper reds - wrote an article for
CM about it - if I remember correctly - none of them need the zinc they
started with - glazes looked exactly the same with and without.
I wish someone would do some more testing in this area. The anawers must be
in how soon the glaze melts, how much reduction before it melts and the
level of reduction throughout the firing.
Could it be - glazes that have a lot of boron - seal over before reduction
and protect the zink from being reduced?
RR
>We all know some materials that we use have individual characteristics
>that are different then those that we observe in a mix. Some fluxes, by
>themselves have very high melting temperatures but serve as melters when
>mixed with other ingredients. So why would the boiling point of zinc be
>any different? Maybe I'm being too simplistic here, but zinc in a glaze
>is widely dispersed, not sitting in a container by itself, so just
>because by itself it will boil at 1670F, why do we assume that it will
>vaporize out of a glaze if widely dispersed?
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Ron Roy on fri 26 oct 01
It is easy to tell if you are not getting any effect from the zinc in your
glazes - make the glaze up without and with - see if they look the same
when fired beside each other.
RR
>Does this mean that zinc in high temp reduction glazes is wasted? I've seen
>the yellow you're talking about, thought is was just heat. Does a
>percentage stay behind, at what amount does it become a waste???? chris
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Edouard Bastarache on fri 26 oct 01
Hello Ivor,
metal fume fever, a bening condition, is a flu-like disease that lasts
24 to 48 hours and disappears by itself; treatment is solely
symptomatic.
Differential diagnosis is made by observation, if after 24-48 hours
symptoms have disappeared, then it is metal fume fever, otherwise
it is the flu.
It often occurs among welders working on galvanized steel.
I have seen many cases at our local shipyard. In Quebec french,
welders say: "j'ai attrap=E9 le galvanis=E9".
Later,
Edouard Bastarache
Irreductible Quebecois
Indomitable Quebeker
Sorel-Tracy
Quebec
edouardb@sorel-tracy.qc.ca
http://sorel-tracy.qc.ca/~edouardb/
http://www.absolutearts.com/portfolios/e/edouardb/
----- Original Message -----
From: iandol
To:
Sent: Friday, October 26, 2001 2:49 AM
Subject: Zinc problems
Dear Earl,
You say <but
serve as melters when mixed with other ingredients.>>
Would you please explain what this means and describe the physical proces=
ses
which are involved. Where did this notion originate? Why are refractory
substances described as "Fluxes"?. These notions seem very confusing.
By the way, WE put ZINC OXIDE into our glazes as this is the most
convenient material at a reasonable price. We do not put Metallic zinc in=
to
our mixtures. When reduction creates the metal from the oxide it will be
above its vaporisation temperature. That metallic zinc at the surface wil=
l
evaporate into the kiln atmosphere. This creates a concentration gradient.
Deeper held zinc will diffuse to areas of lower concentration, which is t=
he
surface. Diffusion is time dependent. Long periods of intense reduction a=
t
higher temperatures will assist in discharging greater quantities of meta=
l
as a gas which reoxidises on contact with air to becoome an aerosol.
Breathing such a dispersion of white powder may cause Metal Fume Fever wh=
ich
can be a frightening experience, thought seldom fatal.
Best regards,
Ivor Lewis, Redhill, South Australia.
_________________________________________________________________________=
___
__
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
chris clarke on fri 26 oct 01
Does this mean that zinc in high temp reduction glazes is wasted? I've seen
the yellow you're talking about, thought is was just heat. Does a
percentage stay behind, at what amount does it become a waste???? chris
temecula, california
chris@ccpots.com
www.ccpots.com
Khaimraj Seepersad on tue 30 oct 01
Hello to All ,
Ivor ,
[ where is Earl's response , been waiting ]
I don't know if this is what your looking for but -
See Ti02 , B203 or Alkali sans Al203 in a Frit or
[ a glaze at probably above 1150 deg .C. ]
The Ti02 helps keep the Si02 mobile.
Works in glazes with 70 % SiO2 glazes -no B203
and as low as 40.9 % Si02 + 29.0 Ti02 [ frit or
glaze ] no B203 .
Or 25 % B203 , 25 % Si02 ,plus up to 10% Ti02.
[ All are Ultra Gloss Glass ]
Also possible with Zr02 , but more ticklish , higher
B203 needed.
There are also the amazing mixes of Borates -
SrO , Mg0 and Ca0 [ to a lesser extent ].
By the way , beautiful pot ----- uggh tree .
Bonfire /Guy Fawkes night , good fuel .
[From your letter with the bonsai pot ]
It is a pity your so far away , I have a few trees
that might be worthy of your talented hands.
Apologies , I am way behind in mail and the
painting is keeping me fully occupied.
Best to you ,
Khaimraj
-----Original Message-----
From: iandol
To: CLAYART@LSV.CERAMICS.ORG
Date: 27 October 2001 2:20
Subject: Zinc problems
Dear Earl,
You say <serve as melters when mixed with other ingredients.>>
Would you please explain what this means and describe the physical processes
which are involved. Where did this notion originate? Why are refractory
substances described as "Fluxes"?. These notions seem very confusing.
< snip >
Best regards,
Ivor Lewis, Redhill, South Australia.
Earl Brunner on tue 30 oct 01
Earl's been trying to think where he got this idea from as well.
So I did some looking.
In Glen Nelson's book "Ceramics, a Potter's Handbook" I found a chart
that listed the melting points of various materials. According to Nelson,
Barium Carbonate melts by itself at 2480 F, it is considered a strong
stoneware flux.
Dolomite 4658-5072 F
Calcium Oxide is listed as melting at 4658 F.
Whiting (dissociates) at 1517 F. (how that is physically different than
melting I don't know)
I was taught (rightly or wrongly) that some glaze materials when
combined, formed a eutectic (melted) at a lower temperature than either
of them would melt by themselves.
Nelson defines " A eutectic is the lowest point at which two or more
chemicals will combine and melt. It is much lower than the melting
points of any of the individual chemicals." For me, this and much of my
understanding of glazes is based I will admit, more on theory than
practice, I have not done a lot of glaze research.
Perhaps Ivor, if either Nelson or I have this wrong, you would be so
kind as to set us straight? I'm sure that you have a fairly good idea
of the general processes involved. Perhaps you could explain how Barium
(an apparently highly refractory material) could act as a flux?
> -----Original Message-----
> From: iandol
> To: CLAYART@LSV.CERAMICS.ORG
> Date: 27 October 2001 2:20
> Subject: Zinc problems
>
>
> Dear Earl,
>
> You say <> serve as melters when mixed with other ingredients.>>
>
> Would you please explain what this means and describe the physical processes
> which are involved. Where did this notion originate? Why are refractory
> substances described as "Fluxes"?. These notions seem very confusing.
>
> < snip >
>
> Best regards,
>
> Ivor Lewis, Redhill, South Australia.
>
--
Earl Brunner
http://coyote.accessnv.com/bruec/
bruec@anv.net
Earl Brunner on tue 30 oct 01
A little further research in Hamer & Hamer Fourth edition, page 381,
Table 30, there is a MP notation and a D notation equaling melting
temperatures and decomposition respectively, the note at the top of the
table says "which give some indication of behavior but are often lowered
by interaction." (which I think is what I was talking about)
From Hamer I would add to my previous list Bone Ash, said to melt at
3038 F
BTW-Hamer shows a lower (significantly) melting point for Dolomite -
1472 F
Iron Oxide 2588-2849 F , said to act as an active flux in reduction.
Spodumene 2552 F (Lithium feldspathic material)
I tried to focus on materials that clearly melted by themselves above
the cone 10 temperature, yet are often used as "fluxes"
In Hamer, there are also numerous materials that melt at, or near, the
cone 3-10 range, (many of the feldspathic materials for example). They
are considered fluxing agents by most of the potters that I know, yet
serve to lower the melting points of both the silica, 3110 F and the
alumina in the various kaolinite materials used in glazes. I know that
they are usually compounds of potassium and soda (both active fluxes)
but how do you explain this fluxing action of these materials, if for
example you combine a potash feldspar 2192 F (barely melting by itself
at cone 10), with some Silica 3110 F and some Kaolin 3218 F. How do you
get a cone 10 glaze (2360 F) with out eutectics?
If on the other hand Ivor, you simply wanted me to say the word
"eutectic", perhaps in the future you could not play dumb and just spell
it out.
> -----Original Message-----
> From: iandol
> To: CLAYART@LSV.CERAMICS.ORG
> Date: 27 October 2001 2:20
> Subject: Zinc problems
>
>
> Dear Earl,
>
> You say <> serve as melters when mixed with other ingredients.>>
>
> Would you please explain what this means and describe the physical processes
> which are involved. Where did this notion originate? Why are refractory
> substances described as "Fluxes"?. These notions seem very confusing.
>
> < snip >
>
> Best regards,
>
> Ivor Lewis, Redhill, South Australia.
>
>
--
Earl Brunner
http://coyote.accessnv.com/bruec/
bruec@anv.net
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