Gracedart on thu 26 mar 98
To all you glaze chemists out there...would someone explain the vinegar test
for leaching...i poured straight white vinegar into a bowl with metallic
black glaze and i do see a lightening from black to a deep blue in some
areas... i did this after my husband had said in one of the bowls the tomato
sauce had blackened...
definitate signs of leaching ????
Metallic Black:
kaolin 190
potash feldspar 3052 !!???
whiting 212
gert borate 416
copper carb 160
manganese di 160
cobalt dioxide 80
Gavin Stairs on fri 27 mar 98
Hi Grace,
Definite signs of leaching. Tomato sauce is one of the most agressive
leaching solutions. The FDA test requires leaching for 24 hours in 4%
acetic acid (essentially vinegar). Acetic acid is rather nice for this
type of testing because of its lab properties, and because it is a common
cooking acid. However, there are more agressive acids, and tomato sauce
can be one of them. This is definitely a wake up call. Especially if your
glaze contains any of the really bad guys, like lead or cobalt (super bad
guy list), or manganese, barium, chromium, vanadium, etc., or even copper,
or iron. Basically, you don't want your glaze dissolving into your food.
So fix it. You don't say what cone this is for. Insight give the
following unity formula for your glaze:
Your glaze 3-7 Limit 8-10 Limit
CaO 0.36 0.3 - 0.6 0.35 - 0.7
KNaO 0.64 0.2 - 0.5 0.2 - 0.45
Al2O3 0.66 0.2 - 0.35 0.3 - 0.55
SiO2 3.63 2.5 - 3.5 3 - 5
B2O3 0.33 0.3 - 0.5 0 - 0.3
ZnO 0.1 - 0.25 0 - 0.3
BaO 0.1 - 0.3 0 - 0.3
MgO 0.1 - 0.2 0 - 0.35
As you can see, your glaze looks a lot like a ^10 glaze with a bit too much
flux. If you are firing below cone 8, this glaze will probably not mature.
At cone 10, it has rather too little CaO. Perhaps the B2O3 is trying to
compensate. If you are using this glaze at the lower cones, you need to
reformulate.
Gavin
At 07:45 AM 3/26/98 EST, Grace Dart wrote:
.... i did this after my husband had said in one of the bowls the tomato
>sauce had blackened...
>definitate signs of leaching ????
....
Gavin Stairs
Toronto, Canada
Gavin Stairs on sat 28 mar 98
Oops. You're right. I meant to write Cadmium, but it came out Cobalt.
Sorry. Gavin
At 06:24 AM 3/27/98 -0800, Brad Sondahl wrote:
>Since when did cobalt get on the super bad guy list next to lead?
>Cadmium I can understand (both being proscribed at the federal level),
>but why cobalt? I personally would have put Vanadium, Chrome, and
>Manganese as more dangerous than Cobalt, ...
Gracedart on sat 28 mar 98
Gavin,
this was a glaze used by a studio firing at cone 6....i don't believe it was
considered a ^10...As I asked "TomBuck." ; what is a good source to read to
learn what you so easily report to me ?
thanks for your help...wish there as many workshops on ceramic chemistry as
there is in throwing...
grace
Ron Roy on thu 2 apr 98
I tried look at this from the molecular standpoint but - there is no info
about firing temp and I need to know what kind of spar and kaolin. I am
also curious about the !!??? after the spar amount. Do I assume the cobalt
is the oxide?
Anyway the answer is - yes - the glaze is leaching. The vinegar test is a
standard one for ascertaining if a glaze is durable when in contact with
acids. Many foods are acid - If a glaze has enough silica (but not too
much) it will be able to withstand acid attack well.
I do think it is difficult to formulate glazes that have these amounts of
colouring oxides that will not leach - there has been a lot of discussion
on this list recently about the subject.
Keep in mind - the metallic black glazes have a layer of oxides on the
surface that cannot be held in solution in the glaze as it cools. These
uncombined oxides ( cobalt, copper and manganese in this case) are right
there on the surface waiting for something to dissolve them. If you must
use a black glaze it would be much better to make it in such a way as to
have the colouring oxides remain dissolved in the glass - in other words
shiny. This is still no guarantee they will remain there however.
Alumina in a glaze (in the proper amount) is what gives glazes resistance
to alkalies - some foods are alkaline and so is soap and dish washing
detergent.
I would like to try and draw some conclusions about this glaze so if I
could have the exact recipe I will be happy to pass on my opinion.
>----------------------------Original message----------------------------
>To all you glaze chemists out there...would someone explain the vinegar test
>for leaching...i poured straight white vinegar into a bowl with metallic
>black glaze and i do see a lightening from black to a deep blue in some
>areas... i did this after my husband had said in one of the bowls the tomato
>sauce had blackened...
>definitate signs of leaching ????
>
>Metallic Black:
>kaolin 190
>potash feldspar 3052 !!???
>whiting 212
>gert borate 416
>copper carb 160
>manganese di 160
>cobalt dioxide 80
Ron Roy
93 Pegasus trail
Scarborough Otario
Canada M1G 3N8
Phone: 416-439-2621
Fax: 416-438-7849
Web page: Home page http://digitalfire.com/education/people/ronroy.htm
Claire Hasselbeck on thu 26 nov 98
If vinegar is left for 48 hours on a cone 6 glazed pot and the color
changes, is the glaze then considered unsafe for food, etc. What does
the color change mean and if unsafe, can the glaze be adjusted in
anyway? This is happening to our classroom glaze and we want to be
sure. Thanks for any input.
John Fazzino on fri 27 nov 98
In a message dated 11/26/98 10:34:14 AM, clairemac@erols.com writes:
<changes, is the glaze then considered unsafe for food, etc. What does
the color change mean and if unsafe, can the glaze be adjusted in
anyway? This is happening to our classroom glaze and we want to be
sure. Thanks for any input.>>
Hi, I'm at Providence College and we've been having our science dept run somes
tests on barium leaching in cone ten glazes. Apparently they use an acidic
substance in the pot and then test that. He said that he ran the tests
several times and each time got a lower reading, less barium. His thought was
that, leaving this substance,(we could use vinegar), in the pot would leach
the toxins out and it would be safe. Although inconclusive, (he's running
another set of tests), it would seem that this is plausible.
John Fazzino
John Hesselberth on fri 27 nov 98
Claire,
I don't think anyone can answer your question with certainty; however I
would say it is certainly an unsatisfactory glaze for functional work.
It clearly is not durable for use with things like orange juice, tomato
sauce and other acid foods. Whether or not it is unsafe depends on the
exact metal oxides that are leaching and in what quantity. I would not
use it for food simply because I don't want to expose my customers to a
significant level of metal ions that have leached from my pots, nor do I
want my pots to change color over time.
Can it be improved? Yes, but maybe not without changing the surface or
color. Post the recipe and someone will undoubtedly be willing to
recommend changes for you to try that will make it more stable. John
Hesselberth
Claire Hasselbeck wrote:
>----------------------------Original message----------------------------
>If vinegar is left for 48 hours on a cone 6 glazed pot and the color
>changes, is the glaze then considered unsafe for food, etc. What does
>the color change mean and if unsafe, can the glaze be adjusted in
>anyway? This is happening to our classroom glaze and we want to be
>sure. Thanks for any input.
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"The whole aim of practical politics is to keep the populace alarmed, and
hence clamorous to be led to safety, by menacing it with an endless
series of hobgoblins, all of them imaginary." H.L. Mencken, 1925
Leona Stonebridge Arthen on sun 29 nov 98
Could the fact that glaze leaches be due to it not firing to its maturity?
In my case, my studio is so far from the transformer, the voltage is too
low for me to reach the cone 9 maturing temperature. Until the very slow
telephone and electric companies finally finish their promised job of
installing primary line and a new transformer, I don't expect anything to
be fired correctly. If I save the glazes that leach and test them
(eventually) at cone 9, are they more likely to not leach?
Leona
---
Leona Stonebridge Arthen
leona@javanet.com
Worthington, Massachusetts/USA
John Hesselberth on mon 30 nov 98
Hi Leona,
Glazes will definitely leach more if not fired to maturity. I have a
cone 6 glaze containing copper (3%) and cobalt (1.5%). While the cobalt
leaching is minimal, copper leaches at 9 ppm at Cone 5 and only 2 ppm at
Cone 6. Neither number is particularly high for copper which is tough to
keep from leaching, but it does illustrate that you can get a 4-5 fold
difference with only a single cone difference in firing temperature.
That is why I recommend you place a cone pack right beside samples you
are going to have tested for leaching. If you have variable temperatures
in your kiln you should place leaching test samples in the coolest spot.
Leona Stonebridge Arthen wrote:
>----------------------------Original message----------------------------
>Could the fact that glaze leaches be due to it not firing to its maturity?
>In my case, my studio is so far from the transformer, the voltage is too
>low for me to reach the cone 9 maturing temperature. Until the very slow
>telephone and electric companies finally finish their promised job of
>installing primary line and a new transformer, I don't expect anything to
>be fired correctly. If I save the glazes that leach and test them
>(eventually) at cone 9, are they more likely to not leach?
>
>Leona
>
>
>---
>Leona Stonebridge Arthen
>leona@javanet.com
>Worthington, Massachusetts/USA
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"The whole aim of practical politics is to keep the populace alarmed, and
hence clamorous to be led to safety, by menacing it with an endless
series of hobgoblins, all of them imaginary." H.L. Mencken, 1925
Ron Roy on wed 2 dec 98
Glazes can be adjusted to fire at different temperatures.
Firing a glaze at a lower temperature than recommended can make a glaze
more durable - just as overfiring can do the same thing. Sounds all very
complicated I am sure - but when you start to look at glazes in
relationship to firing temperature and amounts of silica and alumina it
becomes quite clear.
RR
>----------------------------Original message----------------------------
>Could the fact that glaze leaches be due to it not firing to its maturity?
>In my case, my studio is so far from the transformer, the voltage is too
>low for me to reach the cone 9 maturing temperature. Until the very slow
>telephone and electric companies finally finish their promised job of
>installing primary line and a new transformer, I don't expect anything to
>be fired correctly. If I save the glazes that leach and test them
>(eventually) at cone 9, are they more likely to not leach?
>
>Leona
>
>
>---
>Leona Stonebridge Arthen
>leona@javanet.com
>Worthington, Massachusetts/USA
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Dorothy Weber on wed 14 apr 99
Here is another question to all of you knowledgeable folks. We have recently
started using an oribe glaze, that produces some very interesting results. We
fire to cone 10 reduction. If used on a bowl it produces a translucent green
glaze, turn metallic where thin and where thick will have red/cranberry
areas, on lidded vessel the interior will turn cranberry red. we did a leach
test with the vinegar in both. The cup/bowl did change to a lighter shade
(left a line) after 3 days. Interesting it was still clear when poured into a
glass. Then the lidded vessel after one week never altered the interior.
What's happening? Can anyone tell me about these chemicals/changes? I welcome
any info
recipe:
custer feldspar 80
whiting 20
bentonite 2
copper carbonate 5
Thanks!
Dorothy
Manakin-Sabot, Va.
75 today and gorgeous, too bad I have to go to my day job
John Baymore on sun 23 apr 00
------------------
=3Csnip=3E
The reason that any of us with any scientific literacy at all look
dubiously upon your =22research=22 is due to such comments as this:
=3ECan anyone out there help me? There have been a number of orange juice
=3Eincidents, but I seem to remember that one happened in the 1970s or =
1980s
in
=3ECanada. I can't find my file on this and would appreciate anyone who
could
=3Epoint me to a record of this incident. As near as I can remember, a =
mother
=3Ekept orange juice in a jug in the frig. I don't think it was Mexican
ware.
=3EWhen her twin boys came down with symptoms of the flu which were actually
=3Elead poisoning symptoms she doubled up on the orange juice to get more
fluids
=3Einto them. I think one twin died or was brain damaged.
This is NOT data, it is a story--a not a very believeable one the way you
have told it. =22I seem to remember....=22, =22I can't find my file on =
this...=22
=22As near as I remember....=22 =22I don't think it was Mexican =
ware...=22
(meaning you don't know), =22I think one twin died...=22
You obviously don't really know what you are talking about.
=3Cclip=3E
Interesting response...........
I took Monona's post not as a scare story, but as Monona honestly
requesting some HELP from those of us on CLAYART in locating some more data
in order to continue her research as to the validity of the concerns being
expressed by many about leaching issues. Yes...... it IS a STORY
................. that is being relayed in order to narrow down the search
to a specific incident about which someone might have real hard FACTS.
Being a scientific semi-literate person =3Cg=3E, I take Monona's =
information
the same way I take anyone's. She has academic credentials in both
ceramics and industrial hygiene. For what she currently does with respect
to health hazards in the ceramics field, that is a reasonable background to
say that she at least =22has a clue=22. Does she know absolutely =
everything?
No. But no one does. We must not expect perfection from anyone.
And the old addage about not shooting the messanger is a good one to
remember.
respectfully,
.............................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752 (s)
800-900-1110 (s)
JBaymore=40compuserve.com
John.Baymore=40GSD-CO.com
=22Earth, Water, and Fire Noborigama Woodfiring Workshop August 18-27,
2000=22
Ron Roy on tue 25 apr 00
I must say I agree completely with John here - as I often do - in fact
can't remember when I didn't.
I don't have as much faith in those who never ask as those that do. I have
always had respect for Monona - sometimes I don't like the way she talks to
people - but many of us don't talk very nice to her either.
I see the lead thing is upon us again - I think her mention of this is
appropriate - if you are going to mess with lead you better know what you
are doing - cause if you don't - you risk doing harm to your customers your
workers and yourself.
And if you get caught it's gona cost us all - again!
Why anyone would consider doing liner glazes with lead is completely beyond
my understanding.
RR
>----------------------------Original message----------------------------
>
>
>The reason that any of us with any scientific literacy at all look
>dubiously upon your "research" is due to such comments as this:
>
>>Can anyone out there help me? There have been a number of orange juice
>>incidents, but I seem to remember that one happened in the 1970s or 1980s
>in
>>Canada. I can't find my file on this and would appreciate anyone who
>could
>>point me to a record of this incident. As near as I can remember, a mother
>>kept orange juice in a jug in the frig. I don't think it was Mexican
>ware.
>>When her twin boys came down with symptoms of the flu which were actually
>>lead poisoning symptoms she doubled up on the orange juice to get more
>fluids
>>into them. I think one twin died or was brain damaged.
>
>This is NOT data, it is a story--a not a very believeable one the way you
>have told it. "I seem to remember....", "I can't find my file on this..."
>"As near as I remember...." "I don't think it was Mexican ware..."
>(meaning you don't know), "I think one twin died..."
>
>You obviously don't really know what you are talking about.
>
>
>
>
>Interesting response...........
>
>I took Monona's post not as a scare story, but as Monona honestly
>requesting some HELP from those of us on CLAYART in locating some more data
>in order to continue her research as to the validity of the concerns being
>expressed by many about leaching issues. Yes...... it IS a STORY
>................ that is being relayed in order to narrow down the search
>to a specific incident about which someone might have real hard FACTS.
>
>Being a scientific semi-literate person , I take Monona's information
>the same way I take anyone's. She has academic credentials in both
>ceramics and industrial hygiene. For what she currently does with respect
>to health hazards in the ceramics field, that is a reasonable background to
>say that she at least "has a clue". Does she know absolutely everything?
>No. But no one does. We must not expect perfection from anyone.
>
>And the old addage about not shooting the messanger is a good one to
>remember.
>
>
>respectfully,
>
>.............................john
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Tom Sawyer on tue 20 apr 04
I know that at one time there was consideration to encase in glass
radioactive materials because of the resistance of glass to leaching. I know
I must be missing something here but remember guys/gals I'm just an amateur.
My question is why don't glass artists have problems with leaching? I don't
ever remember hearing that they do nor do I remember in my experience any
glasses losing color. If glass is silica + flux and glazes are silica + flux
+ alumina, does the alumina promote leaching? What's the cause??
Tom Sawyer
tsawyer@cfl.rr.com
John Hesselberth on tue 20 apr 04
Hi Tom,
There is glass and there is glass. All glass does not have the same
composition and hence varies in its resistance to leaching or attack by
acids or bases.
Regards,
John
On Tuesday, April 20, 2004, at 04:21 PM, Tom Sawyer wrote:
> I know that at one time there was consideration to encase in glass
> radioactive materials because of the resistance of glass to leaching.
> I know
> I must be missing something here but remember guys/gals I'm just an
> amateur.
> My question is why don't glass artists have problems with leaching?
http://www.frogpondpottery.com
http://www.masteringglazes.com
Steve Slatin on tue 20 apr 04
Tom --
Yes, and they decided not to rely on vitrification for highly concentrated waste because it wasn't stable. Glass is not stable, in fact, some argue it's not technically a solid.
Lead crystal isn't stable either; see http://vm.cfsan.fda.gov/~dms/fdalead.html
Tom Sawyer wrote:
I know that at one time there was consideration to encase in glass
radioactive materials because of the resistance of glass to leaching. I know
I must be missing something here but remember guys/gals I'm just an amateur.
My question is why don't glass artists have problems with leaching?
-- Steve Slatin -- Entry-level potter, journeyman loafer, master obfuscator
No website, no sales room, no scheduled hours
All talk, no action
Sequim, Washington, USA
48.0937°N, 123.1465°W or thereabouts
---------------------------------
Do you Yahoo!?
Yahoo! Photos: High-quality 4x6 digital prints for 25¢
Ivor and Olive Lewis on wed 21 apr 04
Dear Tom Sawyer,
The majority of glass products are based on Silicon Dioxide which,
because of its high melting point has to be fluxed. Common fluxes are
Na2O, K2O and PbO, augmented by CaO, MgO and ZnO to give chemical
stability. Al2O3 is added to suppress devitrification and increase
viscosity.
Over time, glass buried in the earth begins to decompose under the
influence of wet acid conditions and when dried is found to have a
form of optical lustre which gives an iridescence. I think this is due
to the leaching of the Alkali Metal ions.
Charles Bray, "Dictionary of Glass. Materials and Techniques" ISBN
976-6910-89-5. 1995, tells us that Na+ and K+ ions are very mobile and
can be leached with water.
Such information leads me to believe that our Ceramic Glazes, which
contain high proportions of Alumina as well as Alkali Earth elements
and are usually low in Alkali metal elements would have a greater
degree of chemical stability than most of the glass used by Glass
Artists.
Perhaps someone else could find confirmation of my opinion in the
ACerS archives.
Best regards,
Ivor Lewis. Redhill, South Australia
Lee Love on wed 21 apr 04
Tom Sawyer wrote:
>My question is why don't glass artists have problems with leaching?
>
Hi Tom,
I think they share the problem of leaching. Lead crystal is
probably the most common item discussed.
John H. mentioned that the original problem with copper was
that it assisted lead in glazes to leach. John also shared a
paragraph from Cardew on glaze calculation. I was looking at the
archives and found something Janet Kaiser wrote about Cardew and his
shift from lead glazes to high fire that had qualities of the old lead
glazes. You can read the whole post here:
http://www.potters.org/subject41494.htm (Thank you Janet):
Janet Kaiser on tue 22 may 01
(janet@THE-CHAPEL-OF-ART.FREESERVE.CO.UK
) wrote:
> At the conclusion of this essay, Cardew writes:
>
> "Most of the Abuga stoneware was biscuit-fired before
> glazing, but I also developed an interesting
> "slip-glaze" containing grass-ash as well as the
> wood-ash slag. This fired a clear, transparent, warm
> brown on the local clay, an effect which was somewhat
> similar to the golden browns of the slipware we used to
> make at Winchcombe during the 1920s and 30s. I used it
> on the plain dark body, scratching the decoration
> through the glaze to the raw clay but owing to the lack
> of reliable white slip I was unable to develop its full
> potential for sgraffito decoration.
>
> A glaze of the same type, containing grass-ash, local
> wood-ash slag and a high proportion of clay, is the
> main one used here since my return to Cornwall in 1965.
> Over a white slip the golden brown is lighter in tone,
> and where the dark body is exposed it tens towards a
> black tenmoku. Thus the 'post-West-African' stoneware
> made at Wenford Bridge represents, after many years of
> searching, a kind of return to the beginnings, and to
> the idea, conceived during the 1930s that slipware
> techniques, and even the warmth of slipware colours,
> could be translated into the new dimension of
> stoneware."
--
Lee
in Mashiko, Japan http://mashiko.org
Tom Sawyer on sun 25 apr 04
If glass artist have the same problems with leaching - perhaps even more as
you suggest, this begs the question if we should be concerned about leaching
of various chemicals from the glasses we use about the home. As I stated in
my earlier post, I don't ever remember colored glazes that I purchased
changing color or showing signs of leaching; admittedly, I didn't look. I
remember many years ago buying a bunch of glasses in Mexico and then finding
the same in Pier One and replacing broken ones; they are all gone now but I
do wonder what third world artist might use and whether such items are safe.
Tom Sawyer
tsawyer@cfl.rr.com
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Ivor and Olive
Lewis
Sent: Wednesday, April 21, 2004 2:50 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Leaching
Dear Tom Sawyer,
The majority of glass products are based on Silicon Dioxide which, because
of its high melting point has to be fluxed. Common fluxes are Na2O, K2O and
PbO, augmented by CaO, MgO and ZnO to give chemical stability. Al2O3 is
added to suppress devitrification and increase viscosity.
Over time, glass buried in the earth begins to decompose under the influence
of wet acid conditions and when dried is found to have a form of optical
lustre which gives an iridescence. I think this is due to the leaching of
the Alkali Metal ions.
Charles Bray, "Dictionary of Glass. Materials and Techniques" ISBN
976-6910-89-5. 1995, tells us that Na+ and K+ ions are very mobile and can
be leached with water.
Such information leads me to believe that our Ceramic Glazes, which contain
high proportions of Alumina as well as Alkali Earth elements and are usually
low in Alkali metal elements would have a greater degree of chemical
stability than most of the glass used by Glass Artists.
Perhaps someone else could find confirmation of my opinion in the ACerS
archives.
Best regards,
Ivor Lewis. Redhill, South Australia
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Ivor and Olive Lewis on mon 26 apr 04
Dear Tom,
I think I pointed out in another post that it is a question of degrees
of Mass and Time.
A shot of Bourbon in a lead crystal glass might convey a few parts per
billion PbO over the time between serving and imbibing. The same
Bourbon stored in a Lead Crystal Decanter for a few years and then
imbibed might give many parts per million.
Most storage glass vessels in which beverages are sold are Soda Lime
Silicates. What dissolves during storage is of little consequence.
If you wish to know more about Glass Batch Recipes, consult Charles
Bray's Book or one of the other major text books cited in his
bibliography.
Best regards,
Ivor Lewis. Redhill, South Australia
Ron Roy on tue 27 apr 04
We all have old glasses that have been through the dish washer a lot - they
tend to get cloudy - that is alkaline etching. The key ingredient in glazes
to prevent alkali etching is alumina.
If the ware gets dulled inside and out - thats from soap - if it is only
dulled on the inside then it's from acidic food. If it's dulled more on the
inside than the outside - then from both acid and alkaline.
I remember - a radio consumer program - long time ago - reported traces of
lead in wine in a crystal glass - after only a couple of minutes.
Maybe it has to do with how much you pay for glass - the more silica in a
soda silica glass - the more durable but the higher temperature needed to
melt it.
RR
>If glass artist have the same problems with leaching - perhaps even more as
>you suggest, this begs the question if we should be concerned about leaching
>of various chemicals from the glasses we use about the home. As I stated in
>my earlier post, I don't ever remember colored glazes that I purchased
>changing color or showing signs of leaching; admittedly, I didn't look. I
>remember many years ago buying a bunch of glasses in Mexico and then finding
>the same in Pier One and replacing broken ones; they are all gone now but I
>do wonder what third world artist might use and whether such items are safe.
>
>Tom Sawyer
>tsawyer@cfl.rr.com
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Hank Murrow on tue 27 apr 04
Dear Ron;
Now that is a clear, concise post. Anybody could figure their problem
out from that. I noticed that one of my copper glazes was scumming up
when used in a dishwasher, even though it had been tested in a lab. I
found out that Calgonite is much more caustic than hand-type
soaps.....probably to make up for the lack of 'hands' in an automatic
washer. since I know it is not leaching into the food, I tell folks to
wash that glaze by hand. No problems there.
Thanks, Hank in Eugene
On Apr 27, 2004, at 8:29 AM, Ron Roy wrote:
> We all have old glasses that have been through the dish washer a lot -
> they
> tend to get cloudy - that is alkaline etching. The key ingredient in
> glazes
> to prevent alkali etching is alumina.
>
> If the ware gets dulled inside and out - thats from soap - if it is
> only
> dulled on the inside then it's from acidic food. If it's dulled more
> on the
> inside than the outside - then from both acid and alkaline.
>
> I remember - a radio consumer program - long time ago - reported
> traces of
> lead in wine in a crystal glass - after only a couple of minutes.
>
> Maybe it has to do with how much you pay for glass - the more silica
> in a
> soda silica glass - the more durable but the higher temperature needed
> to
> melt it.
>
> RR
>
>
>> If glass artist have the same problems with leaching - perhaps even
>> more as
>> you suggest, this begs the question if we should be concerned about
>> leaching
>> of various chemicals from the glasses we use about the home. As I
>> stated in
>> my earlier post, I don't ever remember colored glazes that I purchased
>> changing color or showing signs of leaching; admittedly, I didn't
>> look. I
>> remember many years ago buying a bunch of glasses in Mexico and then
>> finding
>> the same in Pier One and replacing broken ones; they are all gone now
>> but I
>> do wonder what third world artist might use and whether such items
>> are safe.
>>
>> Tom Sawyer
>> tsawyer@cfl.rr.com
>
> Ron Roy
> RR#4
> 15084 Little Lake Road
> Brighton, Ontario
> Canada
> K0K 1H0
> Phone: 613-475-9544
> Fax: 613-475-3513
>
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Ivor and Olive Lewis on sun 7 jan 07
Ivor,
The point is - you can reduce the level of copper leaching to a minimal
level by formulating a durable glaze. It is the durability of the glaze
that is important - not the state of the copper in it.
Dear Ron,
I agree with your thinking here. But that presupposes none of the other =
ingredients of the vitreous solid are taken by the acid. If the glaze is =
stable they remain in situ and shall we say "anchor Copper ions within =
the silicate framework". So it seems to me that Stability is about a =
relationship between group 1 and group 2 elements and the influence of =
Boron,
A good thing we can have these discussions.
Best regards,=20
Ivor
Ron Roy on thu 11 jan 07
Hello Ivor,
What are group 1 and 2 elements?
Boron is not necessary for a stable glaze - at least not between cone 6 and
up. The 4 criteria are well documented in our book by the way - enough
alumina, enough silica, a good melt and not oversupplied with colouring
additions.
You might be tempted to say a balanced glaze (fluxing oxides balanced)
would do best - and that is so sometimes - but there are two examples of
high calcium glazes in our book that are very stable as well.
We potters seem to want to think in generalities but it seems to me that
exceptions are the norm in this business.
RR
>Ivor,
>
>The point is - you can reduce the level of copper leaching to a minimal
>level by formulating a durable glaze. It is the durability of the glaze
>that is important - not the state of the copper in it.
>
>
>Dear Ron,
>
>I agree with your thinking here. But that presupposes none of the other
>ingredients of the vitreous solid are taken by the acid. If the glaze is
>stable they remain in situ and shall we say "anchor Copper ions within the
>silicate framework". So it seems to me that Stability is about a
>relationship between group 1 and group 2 elements and the influence of
>Boron,
>
>A good thing we can have these discussions.
>
>Best regards,
>
>Ivor
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
claystevslat on fri 12 jan 07
Ron -
I believe Ivor's talking about the first two columns on
the left hand side of a conventionally displayed periodic
table of the elements.
The group one elements that we use in pottery are lithium
sodium, and potassium -- the metals that in their pure
state react explosively to water. These are also our key
strong fluxes. All +1 valence.
The group two elements we use are magnesium, calcium, and
strontium. (Barium and beryllium are also a group two, but
I know nothing about useing them in pottery.) Generally less
strong fluxes than the group I's, and, of course, with +2
valence each ion accommodate/use up/require twice as many
ions of oxygen or silicon or whatever as each group I ion.
Ivor suggests that stability comes from a balance of the
group 1 and group 2 metals (and boron) in a glaze. I have
so little experience of glazes that concentrate on just group 1 or
group 2 melters that I can't really comment -- it's my impression
that a glaze that had only calcium and magnesium in it wouldn't
melt well enough, at least at ^6 to be a proper glaze. (And I'm
not interested in Strontium and Barium's of dubious safety, so
those are my only two options.) I could always do a test with
just silica, a bit of alumina and gobs of whiting just to see if I
could get it to melt, I suppose ... it's no doubt easier to
experiment on the other side, with lithium carb and soda ash or
some similar combo to get an all-group-I glaze.
It could be interesting to try ...
Best wishes -- Steve Slatin
--- In clayart@yahoogroups.com, Ron Roy wrote:
>
> Hello Ivor,
>
> What are group 1 and 2 elements?
>
> Boron is not necessary for a stable glaze - at least not between
cone 6 and
> up. The 4 criteria are well documented in our book by the way -
enough
> alumina, enough silica, a good melt and not oversupplied with
colouring
> additions.
>
> You might be tempted to say a balanced glaze (fluxing oxides
balanced)
> would do best - and that is so sometimes - but there are two
examples of
> high calcium glazes in our book that are very stable as well.
>
> We potters seem to want to think in generalities but it seems to
me that
> exceptions are the norm in this business.
Ivor and Olive Lewis on sat 13 jan 07
Dear Ron Roy=20
You ask "What are group 1 and 2 elements?"
Group 1 elements are those in the first column of the Periodic Table of =
Chemical Elements. Group 2 elements are those the second column of that =
resource.
Frank Hamer makes use of this "Table". Two versions are given on page =
348 of the first Ed of his dictionary. You can download pretty versions =
with all the details from the web.
I just find it an easy way of generalising. Something I learned to do in =
High School. Thought it was known universal knowledge.
Enjoy the day.
Ivor
Ron Roy on sat 13 jan 07
Thanks Steve,
So the alkaline fluxes and the alkaline earths then?
The other interesting point is Zinc Oxide - you can melt a cone 6 glaze
with just zinc oxide - and it's way over in the 11B column.
Does Zinc have a valence of 2 - the same as the alkaline earths - is that a
connection?
Any words of wisdom on that?
RR
>Ron -
>
>I believe Ivor's talking about the first two columns on
>the left hand side of a conventionally displayed periodic
>table of the elements.
>
>The group one elements that we use in pottery are lithium
>sodium, and potassium -- the metals that in their pure
>state react explosively to water. These are also our key
>strong fluxes. All +1 valence.
>
>The group two elements we use are magnesium, calcium, and
>strontium. (Barium and beryllium are also a group two, but
>I know nothing about useing them in pottery.) Generally less
>strong fluxes than the group I's, and, of course, with +2
>valence each ion accommodate/use up/require twice as many
>ions of oxygen or silicon or whatever as each group I ion.
>
>Ivor suggests that stability comes from a balance of the
>group 1 and group 2 metals (and boron) in a glaze. I have
>so little experience of glazes that concentrate on just group 1 or
>group 2 melters that I can't really comment -- it's my impression
>that a glaze that had only calcium and magnesium in it wouldn't
>melt well enough, at least at ^6 to be a proper glaze. (And I'm
>not interested in Strontium and Barium's of dubious safety, so
>those are my only two options.) I could always do a test with
>just silica, a bit of alumina and gobs of whiting just to see if I
>could get it to melt, I suppose ... it's no doubt easier to
>experiment on the other side, with lithium carb and soda ash or
>some similar combo to get an all-group-I glaze.
>
>It could be interesting to try ...
>
>Best wishes -- Steve Slatin
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Ivor and Olive Lewis on sun 14 jan 07
Dear Steve Slatin,
Yes, that is what I was speaking of. The Periodic Table of Chemical =
Elements. People who are not aware of this tool will find two versions =
in Frank Hamer's dictionary. See the appendix.
But I seem to be the only person making pottery glazes who regards =
oxides of Group 2, whose melting points are all in the region of 2000 to =
3000 Deg Celsius, as non melting fluxes. I believe their function is to =
alter the properties of the first fluids that form as temperature rises. =
They change or modify surface tension, viscosity and possibly adhesive =
strength, aided by responding to a Solvent + Solute =3D Solution =
reaction (Solvation)
In a New Year post I made the suggestion, albeit with a little more =
precision than you, to test a mixture of Silica, Kaolin and Whiting. In =
fact I suggested that the recipe given by Michael Cardew in "Pioneer =
Pottery" should be used and that it should only be fired too, but not =
beyond cone 6 . I know this mixture will fuse above cone 8 though a =
mixture of Silica, Alumina and Whiting compounded to the same oxide =
percentage formula will not. Michael Cardew discuses why on pages =
50-51. I tested that in my last firing.
The recipe given by Mr Cardew is 33.66% Calcite (Whiting), 30.39% =
Kaolin, 35.95% quartz(silica) (P 51)
As for avoiding ingredients that supply Group 2 Elements and restricting =
a glaze batch to Group 1 Elements. That would guarantee the production =
of a glaze that had little resistance to corrosion.
A prosperous New Year to you and thanks for joining the discussion. =
looking forward to learning of your results.
Best regards,
Ivor
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