Fabienne McMillan on mon 13 sep 10
Hi,
I'm digging deep into the archives and with this message, I am at a dead
end. The original discussion prompting this message a month later was
entitled _Crystalline glazes_ and in May of 2007. Just wondering if
anything else came out of this intriguing issue. TIA
I recently overheard a crystalline potter at a local fair say that her wa=
=3D
re
was food safe because it was glass and my jaw dropped to the floor. I'm
still trying to pick it back up ;)
Fabienne
"We never touch people so lightly that we do not leave a trace." ~ Peggy
Tabor Millin
---- quoting original message below -----
Dear=3D20
Ron, John and Others with an interest in "The Knowledge"
Travelling Library turned up trumps last week and brought me a copy of
"Ceramics Glaze Technology" by Taylor and Bull that you have recommended
several times.
Great text that summarises what Glazing is all about. It also has some
interesting tables in the back as well. Lots of basic information. Seems =
=3D
to
be the sort of text that would be used by the Ceramics Institute as a bas=
=3D
is
for their examinations for membership. To get real value it would be
necessary to follow through and read some of the papers from which they d=
=3D
raw
their conclusions.
One piece that caught my eye was in the description of Zinc in the Raw
Materials section. They say "Several countries include Zinc in their
regulations controlling toxic metals release" but do not give references =
=3D
for
this statement.
Do you have additional information ?
Glad that the system is restored.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
ivor and olive lewis on tue 14 sep 10
Fabienne,
Lead Crystal is a glass, but I would not suggest to anyone that it was food
safe because of its material condition. I understand there are warnings not
to store alcoholic beverages in Lead Crystal Decanters.
Would anyone actually make crystalline glazed domestic ware ?
Our glazes are vitreous. Their compositions are very different, as a group
from the Glass group.
Cannot recall the conclusion of that episode in 2007. In many situation,
Zinc and its compounds can be a health hazard.
Regards,
Ivor
----- Original Message -----
From: "Fabienne McMillan"
To: ;
Cc: "Fabienne McMillan"
Sent: Tuesday, September 14, 2010 9:06 AM
Subject: Re: Glaze Durability.
Hi,
I'm digging deep into the archives and with this message, I am at a dead
end. The original discussion prompting this message a month later was
entitled _Crystalline glazes_ and in May of 2007. Just wondering if
anything else came out of this intriguing issue. TIA
I recently overheard a crystalline potter at a local fair say that her ware
was food safe because it was glass and my jaw dropped to the floor. I'm
still trying to pick it back up ;)
Fabienne
"We never touch people so lightly that we do not leave a trace." ~ Peggy
Tabor Millin
---- quoting original message below -----
Dear
Ron, John and Others with an interest in "The Knowledge"
Travelling Library turned up trumps last week and brought me a copy of
"Ceramics Glaze Technology" by Taylor and Bull that you have recommended
several times.
Great text that summarises what Glazing is all about. It also has some
interesting tables in the back as well. Lots of basic information. Seems to
be the sort of text that would be used by the Ceramics Institute as a basis
for their examinations for membership. To get real value it would be
necessary to follow through and read some of the papers from which they dra=
w
their conclusions.
One piece that caught my eye was in the description of Zinc in the Raw
Materials section. They say "Several countries include Zinc in their
regulations controlling toxic metals release" but do not give references fo=
r
this statement.
Do you have additional information ?
Glad that the system is restored.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
William & Susan Schran User on tue 14 sep 10
On 9/13/10 7:36 PM, "Fabienne McMillan" wrote:
> I recently overheard a crystalline potter at a local fair say that her wa=
re
> was food safe because it was glass and my jaw dropped to the floor. I'm
> still trying to pick it back up ;)
I do ^6 crystalline glazes and the glaze formula for ^6 is not much
different than for ^10.
The one thing needed in a functional and stable glaze is a certain
percentage of alumina. ALL macro-crystalline glazes are severely deficient
in alumina, so they will be a rather "soft" glaze and may more easily leach
metallic oxides.
In the case of crystalline potter thinking these are functional glazes -
well, ignorance is not always bliss...
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
Ron Roy on tue 14 sep 10
Hi Ivor,
Not sure what you are asking about zinc - Ceram - my latest data on
toxic metal release is not current (2006) but - Austria has a release
of 3mg/L of zinc.
Fabienne - did I reply to your question re durability of crystalline
glazes - crystals take silica from adjacent glaze and low alumina etc?
If not I can do it again.
RR
Quoting ivor and olive lewis :
> Fabienne,
> Lead Crystal is a glass, but I would not suggest to anyone that it was fo=
od
> safe because of its material condition. I understand there are warnings n=
ot
> to store alcoholic beverages in Lead Crystal Decanters.
>
> Would anyone actually make crystalline glazed domestic ware ?
>
> Our glazes are vitreous. Their compositions are very different, as a grou=
p
> from the Glass group.
>
> Cannot recall the conclusion of that episode in 2007. In many situation,
> Zinc and its compounds can be a health hazard.
>
> Regards,
>
> Ivor
>
>
>
> ----- Original Message -----
> From: "Fabienne McMillan"
> To: ;
> Cc: "Fabienne McMillan"
> Sent: Tuesday, September 14, 2010 9:06 AM
> Subject: Re: Glaze Durability.
>
>
> Hi,
>
> I'm digging deep into the archives and with this message, I am at a dead
> end. The original discussion prompting this message a month later was
> entitled _Crystalline glazes_ and in May of 2007. Just wondering if
> anything else came out of this intriguing issue. TIA
>
> I recently overheard a crystalline potter at a local fair say that her wa=
re
> was food safe because it was glass and my jaw dropped to the floor. I'm
> still trying to pick it back up ;)
>
> Fabienne
>
> "We never touch people so lightly that we do not leave a trace." ~ Peggy
> Tabor Millin
>
>
> ---- quoting original message below -----
>
> Dear
>
> Ron, John and Others with an interest in "The Knowledge"
>
> Travelling Library turned up trumps last week and brought me a copy of
> "Ceramics Glaze Technology" by Taylor and Bull that you have recommended
> several times.
>
> Great text that summarises what Glazing is all about. It also has some
> interesting tables in the back as well. Lots of basic information. Seems =
to
> be the sort of text that would be used by the Ceramics Institute as a bas=
is
> for their examinations for membership. To get real value it would be
> necessary to follow through and read some of the papers from which they d=
raw
> their conclusions.
>
> One piece that caught my eye was in the description of Zinc in the Raw
> Materials section. They say "Several countries include Zinc in their
> regulations controlling toxic metals release" but do not give references =
for
> this statement.
>
> Do you have additional information ?
>
> Glad that the system is restored.
>
> Best regards,
>
> Ivor Lewis.
> Redhill,
> South Australia.
>
Fabienne McMillan on tue 14 sep 10
Hi Ron,
I read the exchange between you and Ivor in 2007. You mention the
silica being robbed by crystals so they can actually form thus
undermining the glazes functionality. I'm also aware of the low
alumina content which is a must if one is to have crystal formation
and as Bill says the Al:Si ratio is off as far as stability is
concerned. I also understand what Ivor was saying in terms of the
alumina proportion being raised outside the crystal formation since
they do rob their surrounding from silica; however, whether that is
sufficient or not to make the glaze stable around the crystals is
another story. I'm also wondering if the glaze around the crystals
would be homogeneous in terms of silica content or if there would be
progressively less as one gets closer to the crystal; I haven't gone
in depth into the glaze flow chemistry when in the kiln - not sure
it's even really that useful to me at this point. I am a firm believer
that crystalline glazes are NOT functional just on the basis of the
low Al:Si ration and, based on my past experience, the glaze I made
(c6, c8 and c9) were super soft and can be easily scratched with metal
not to mention crazed (unsanitary). I think that summarizes it.
Let me know if I've missed anything :)
Thanks,
Fabienne
"We never touch people so lightly that we do not leave a trace." ~
Peggy Tabor Millin
ivor and olive lewis on wed 15 sep 10
Hello again Ron,
I was responding to Fabienne's statement..
My own experience and knowledge of Zinc Toxicity comes from its
metallurgical use rather than its ceramic use. Without adequate ventilation=
,
fume rising from the metal vats during galvanisation or while welding
galvanised metal can. lead to a pulmonary condition often referred to as
"Metal Fume Fever". Not contagious but rather unpleasant while it lasts
Zinc orthosilicate, the compound responsible for dramatic, decorative
ceramic crystals which grow in Zinc rich silicate glazes is noted in the CR=
C
Compound Tables as being insoluble in water but soluble in dilute acids.
Best regards,
Ivor
----- Original Message -----
From:
To: "ivor and olive lewis"
Cc:
Sent: Wednesday, September 15, 2010 11:01 AM
Subject: Re: Glaze Durability.
> Hi Ivor,
>
> Not sure what you are asking about zinc - Ceram - my latest data on toxi=
c
> metal release is not current (2006) but - Austria has a release of 3mg/L
> of zinc.
>
> Fabienne - did I reply to your question re durability of crystalline
> glazes - crystals take silica from adjacent glaze and low alumina etc? I=
f
> not I can do it again.
>
> RR
>
>
> Quoting ivor and olive lewis :
>
>> Fabienne,
>> Lead Crystal is a glass, but I would not suggest to anyone that it was
>> food
>> safe because of its material condition. I understand there are warnings
>> not
>> to store alcoholic beverages in Lead Crystal Decanters.
>>
>> Would anyone actually make crystalline glazed domestic ware ?
>>
>> Our glazes are vitreous. Their compositions are very different, as a
>> group
>> from the Glass group.
>>
>> Cannot recall the conclusion of that episode in 2007. In many situation,
>> Zinc and its compounds can be a health hazard.
>>
>> Regards,
>>
>> Ivor
>>
>>
>>
>> ----- Original Message -----
>> From: "Fabienne McMillan"
>> To: ;
>> Cc: "Fabienne McMillan"
>> Sent: Tuesday, September 14, 2010 9:06 AM
>> Subject: Re: Glaze Durability.
>>
>>
>> Hi,
>>
>> I'm digging deep into the archives and with this message, I am at a dead
>> end. The original discussion prompting this message a month later was
>> entitled _Crystalline glazes_ and in May of 2007. Just wondering if
>> anything else came out of this intriguing issue. TIA
>>
>> I recently overheard a crystalline potter at a local fair say that her
>> ware
>> was food safe because it was glass and my jaw dropped to the floor. I'm
>> still trying to pick it back up ;)
>>
>> Fabienne
>>
>> "We never touch people so lightly that we do not leave a trace." ~ Peggy
>> Tabor Millin
>>
>>
>> ---- quoting original message below -----
>>
>> Dear
>>
>> Ron, John and Others with an interest in "The Knowledge"
>>
>> Travelling Library turned up trumps last week and brought me a copy of
>> "Ceramics Glaze Technology" by Taylor and Bull that you have recommended
>> several times.
>>
>> Great text that summarises what Glazing is all about. It also has some
>> interesting tables in the back as well. Lots of basic information. Seems
>> to
>> be the sort of text that would be used by the Ceramics Institute as a
>> basis
>> for their examinations for membership. To get real value it would be
>> necessary to follow through and read some of the papers from which they
>> draw
>> their conclusions.
>>
>> One piece that caught my eye was in the description of Zinc in the Raw
>> Materials section. They say "Several countries include Zinc in their
>> regulations controlling toxic metals release" but do not give references
>> for
>> this statement.
>>
>> Do you have additional information ?
>>
>> Glad that the system is restored.
>>
>> Best regards,
>>
>> Ivor Lewis.
>> Redhill,
>> South Australia.
>>
>
>
>
>
Fabienne McMillan on wed 15 sep 10
Ivor,
I want to make sure I understand what you are saying. The crystals
themselves can be attacked by dilute acids, i.e. vinegar, OJ (?);
wouldn't that make it unsuited for ceramic use since they can be
colored by metals like cobalt and copper, for example? The concern
here would be leaching. Also not considering the surrounding glaze in
this case, just the crystal formations.
Thanks,
Fabienne
"We never touch people so lightly that we do not leave a trace." ~
Peggy Tabor Millin
On Sep 14, 2010, at 11:25 PM, ivor and olive lewis wrote:
>
> Zinc orthosilicate, the compound responsible for dramatic, decorative
> ceramic crystals which grow in Zinc rich silicate glazes is noted in
> the CRC
> Compound Tables as being insoluble in water but soluble in dilute
> acids.
> Best regards,
> Ivor
Fabienne McMillan on wed 15 sep 10
I stand corrected all by myself lol I just read some literature on the
glaze at oxide level in the kiln and it was an eye opener. Pretty
cool stuff. it addressed the crystals only though.
Fabienne
"We never touch people so lightly that we do not leave a trace." ~
Peggy Tabor Millin
On Sep 14, 2010, at 10:13 PM, Fabienne McMillan wrote:
> I haven't gone
> in depth into the glaze flow chemistry when in the kiln - not sure
> it's even really that useful to me at this point.
>
> Fabienne
>
> "We never touch people so lightly that we do not leave a trace." ~
> Peggy Tabor Millin
Fabienne McMillan on thu 16 sep 10
Ivor,
I am not considering them for functional ware unless, as you point
out, they won't come in contact with the food. However, I believe
they would never survive a dishwasher and the crazing plus crystals
being 3-D doesn't help.
I appreciate your taking the time to answer. This thread has
definitely reinforce my belief they are not functional glazes. Now I
just wish I could put my hands on my notes in storage from my glaze
tests and firings a few years ago. Which box though? :(
Thank you,
Fabienne
"We never touch people so lightly that we do not leave a trace." ~
Peggy Tabor Millin
On Sep 15, 2010, at 11:41 PM, ivor and olive lewis wrote:
> Dear Fabienne,
> To have any certainty about these things samples would have to be
> professionally analysed by a reputable public laboratory. My
> information is sparse but sufficient to give cause for note in the
> face of uncertainty. Introducing colouring agents just compounds
> the uncertainty.
> Have you considered this as a design issue if a product has to
> contain food or beverage, limiting suspect decorative glazes to
> exterior surfaces that would never be in contact with corrosive
> substances.
> Regards
> Ivor Lewis,
> REDHILL,
> South Australia
>
> ----- Original Message ----- From: "Fabienne McMillan" > >
> To: "ivor and olive lewis"
> Cc:
> Sent: Thursday, September 16, 2010 2:21 AM
> Subject: Re: Glaze Durability.
>
>
>> Ivor,
>>
>> I want to make sure I understand what you are saying. The crystals
>> themselves can be attacked by dilute acids, i.e. vinegar, OJ (?);
>> wouldn't that make it unsuited for ceramic use since they can be
>> colored by metals like cobalt and copper, for example? The
>> concern here would be leaching. Also not considering the
>> surrounding glaze in this case, just the crystal formations.
>>
>> Thanks,
>>
>> Fabienne
>>
>> "We never touch people so lightly that we do not leave a trace." ~
>> Peggy Tabor Millin
>>
>> On Sep 14, 2010, at 11:25 PM, ivor and olive lewis wrote:
>>>
>>> Zinc orthosilicate, the compound responsible for dramatic,
>>> decorative
>>> ceramic crystals which grow in Zinc rich silicate glazes is noted
>>> in the CRC
>>> Compound Tables as being insoluble in water but soluble in dilute
>>> acids.
>>> Best regards,
>>> Ivor
>
ivor and olive lewis on thu 16 sep 10
Dear Fabienne,
To have any certainty about these things samples would have to be
professionally analysed by a reputable public laboratory. My information is
sparse but sufficient to give cause for note in the face of uncertainty.
Introducing colouring agents just compounds the uncertainty.
Have you considered this as a design issue if a product has to contain food
or beverage, limiting suspect decorative glazes to exterior surfaces that
would never be in contact with corrosive substances.
Regards
Ivor Lewis,
REDHILL,
South Australia
----- Original Message -----
From: "Fabienne McMillan"
To: "ivor and olive lewis"
Cc:
Sent: Thursday, September 16, 2010 2:21 AM
Subject: Re: Glaze Durability.
> Ivor,
>
> I want to make sure I understand what you are saying. The crystals
> themselves can be attacked by dilute acids, i.e. vinegar, OJ (?);
> wouldn't that make it unsuited for ceramic use since they can be colored
> by metals like cobalt and copper, for example? The concern here would b=
e
> leaching. Also not considering the surrounding glaze in this case, just
> the crystal formations.
>
> Thanks,
>
> Fabienne
>
> "We never touch people so lightly that we do not leave a trace." ~ Peggy
> Tabor Millin
>
> On Sep 14, 2010, at 11:25 PM, ivor and olive lewis wrote:
>>
>> Zinc orthosilicate, the compound responsible for dramatic, decorative
>> ceramic crystals which grow in Zinc rich silicate glazes is noted in th=
e
>> CRC
>> Compound Tables as being insoluble in water but soluble in dilute acids=
.
>> Best regards,
>> Ivor
>
Neon-Cat on thu 16 sep 10
Hi Fabienne and all,
for over a hundred years material and ceramic scientists have referred
to glaze as a glass. Although we do things a little differently than
glass makers we can expand our knowledge and thought processes by
thinking of glazes correctly. The type of sintering occurring during
the firing of our clay bodies is termed vitrification (where a liquid
phase fills up pores).
I recently read a journal article from the American Ceramic Society
from the turn of the last century in which a cone 10 crystalline glaze
is detailed from every available angle at that time. The glaze was for
commercial functional ware. The recipe does contain aluminum along
with whiting for greater glaze durability (the recipe also contains
magnesium and barium) and the willemite crystals were reliably
reproducible but only measured 3/16th of an inch in diameter. For a
little fun I hope to try this glaze soon for myself. The article gives
firing schedules and amounts of oxides to use for various glaze matrix
colors and companion crystal colors. Like more modern works it nails
just how various oxides preferentially partition (segregate or
diffuse) within the glaze matrix or crystals.
The article I recently posted on crystalline glazes mentions
dinnerware produced by a few potteries in the not too distant past.
Typically in a zinc-based crystal glaze the crystalline phases that
may be present are willemite (a-Zn2SiO4 or as 2ZnO.SiO2), gahnite
(ZnAl2O4) and rutile crystals (TiO2). The gahnite and rutile crystal
are not usually visible to us. While the density of the glaze matrix
is usually lower than that of willemite, the willemite crystals in no
way "rob" the glaze glass of silica. Studies show that the average
composition of willemite contains far less silica than the glaze
matrix and that glaze glass is interspersed amid the wee needle-like
willemite growths within each visible crystal.
Most of our ceramic materials are soluble to some extent in water or
acids. Many reactions are very slow to a human sense of time.
Willemite has a hardness of 5 1/2 (specific gravity/density of 3.89 -
4.19 g/cm3), gahnite is listed as having a hardness of 7 1/2 - 8
(density of 4.62 g/cm3), and rutile crystals come in at 6 - 6 1/2
(density of 4.23- 4.25 g/cm3). Since crystals are a surface phenomenon
over-crystallization can lead to mattness or roughness which would
impact durability (because there's more surface area for possible
reactions). While not all low-aluminum art macro-crystalline glazes
may be suitable for functional ware, only testing would give you the
peace of mind and security you need to offer crystalline glazed ware
as food safe. Jeff Zamek in his 2002 book "Safety in the Ceramics
Studio" has zinc listed under those things tested for at a secondary
maximum contamination level of 5.0 mg/L (over 16 times the suggested
maximum contamination level of 0.3 for iron). Since I don't normally
do functional ware and haven't checked for myself lately current
standards or guidelines you'd have to check current US standards for
yourself. Without knowing all about the other potter's work I'd have
to stick with "never say never" with regard to functionality. But her
saying her ware is food safe 'because it is glass' is just not enough.
Marian
www.neon-cat.com
(back to remodeling around the house...there's light at the end of
this adventure and I'm pleased and glad because I sure have been
missing wet clay work)
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