Lili Krakowski on thu 28 feb 08
So when Anne D. wrote in the other day about her troubles with her Floating
Blue pinholing, and sent in both recipe and analysis, my reaction was that
the alumina and silica were sky high over the published "limits" and that
that might be a cause of poor healing etc.
The I started poking about and found a long list of floating
blue comments in the Archives, many plaints about the floating blue turning
out awful, etc.
All I read went back to Chappell's as the original source.
Comments: Date: Tue, 23 Apr 1996 18:04:54 EDT
Reply-To: Ceramic Arts Discussion List
Sender: Ceramic Arts Discussion List
From: CRoeder1@aol.com
Subject: Re: cone 6 glaze test results-floating blue
Floating Blue Glaze Cone 6
Neph Sy...............47.3
Gerstley borate......27.0
Silica................... 20.3
Kaolin (EPK)........ 5.4
Red Iron Ox......... 2.0
Cobalt Oxide....... 1.0 (I substitute Cobalt Carb 2.0)
Rutile (milled)...... 4.0
C.M.C. ............. 1.0
And I had from Lee Center:
Now that recipes is :
Recipe Name: Chappell floating blue
Cone: Color:
Firing: Surface:
Amount Ingredient
43.8 Nepheline Syenite
25 Gerstley Borate--9/97
18.8 Silica
5 Kaolin--EPK
1.9 Iron Oxide--Red
1.9 Cobalt Carbonate
3.7 Rutile
100.1 Total
Which is close enough for me to use ITS formula:
Unity Oxide
.404 Na2O
.101 K2O
.105 MgO
.391 CaO
1.000 Total
.558 Al2O3
.465 B2O3
.059 Fe2O3
3.821 SiO2
.203 TiO2
Now possibly the use of cobalt carbonate hampers
best results, as the oxide
tends to make speckles and that may be what gives the desired look.
But that really is not my point.
Which is that for c. 6, alumina and silica are very high, which in this
case give a
glaze that is likely to craze! (COE almost 78) Most puzzling.
Boron also is high according to the
limits Rhodes indicates.
I am under the impression that Rhodes was the originator of
limit formulas...which became very popular indicators.
Still, a lot of materials are not given: magnesium, strontium
for instance.
It all makes me wonder, in two directions. Direction One: Are limit
formulas
of greater use thanlooking at the formula of glazes one has in the same
family?
Direction Two: Could the B2O3 which is so mysteriously like
alumina but then also like silica, be "setting up shop" on its own,
creating a parallel glaze universe to whatever glass is being formed
by the alumina/silica mix, both vying for the attention of the
fluxes, and creating some sort of layering? In other words is this
glaze when it works a sort of marble cake effect?
This is really puzzling....Ideas?
Lili Krakowski
Be of good courage
Taylor Hendrix on fri 29 feb 08
Dear Mama,
When I was very new to the list, I took it upon myself to read as much
published material from list personalities as I could. I am still
behind, but I did find an article or two written by John H. regarding
your question about limit formulas. I think this article was the start
of his and Ron's great cone 6 book. In the article, if my memory
doesn't fail me, John discusses the many different limit formula
schemes and how they came about. The short answer is No, Rhodes was
not the originator of limit formulas.
My feeling after reading and rereading many of the sources is that
these formula limits are more like guidelines really and they vary
widely depending on who you follow. If you ever find a limit set that
proves to be useful to you in your cone whatever firing, keep it and
use it to help you understand those glazes you wish to examine. For
myself, I do not have the experience with such things so I take Ron
and John's limits as my own and use them accordingly (when I do).
I remember John's article was very helpful in explaining many things
related to glaze formulation using Segar etc. I'll try to remember
which article that was and post it here on this thread (remembering is
going to be the tricky part).
I too wonder about Boron and how it makes glass beside silica. Many of
my 5 20 glazes have what I consider yucky blotches of what I think of
as boron glass. I'm probably wrong, but there you are. Somewhere
floats up the idea that boron glass and silica glass do not mix. True?
always question eveything!
Taylor, in Rockport TX
On 2/28/08, Lili Krakowski wrote:
...
> I am under the impression that Rhodes was the originator of
> limit formulas...which became very popular indicators.
> Still, a lot of materials are not given: magnesium, strontium
> for instance.
>
> It all makes me wonder, in two directions. Direction One: Are limit
> formulas
> of greater use thanlooking at the formula of glazes one has in the same
> family?
> Direction Two: Could the B2O3 which is so mysteriously like
> alumina but then also like silica, be "setting up shop" on its own,
> creating a parallel glaze universe to whatever glass is being formed
> by the alumina/silica mix, both vying for the attention of the
> fluxes, and creating some sort of layering? In other words is this
> glaze when it works a sort of marble cake effect?
..
Steve Slatin on fri 29 feb 08
Lili --
My turn to rant! (Rattling noise as I get
on my soapbox.)
I have many ideas about this, but the first
is that the unity formula doesn't work as
well as many people think it does, the second
is that limit formulas are at best rough guides,
and the third is that boron is a .
Now you may be wondering what a
is.
It's a constituent of a material that has
the effect of encouraging melting, and
under Seger is not a flux. (Some people
object to the use of the word flux. Some
object to the use of the word melter.
Others spell it 'meltier' and still
object to it.) Seger users identify
the word flux most often to identify
ions that act within a glaze melt as
a network modifier. A
encourages melting, like a flux, but
does something different in the
structure of a glass -- in the case
of boron, it's a because
it's a glass-former, not a network
modifier.
The fact is, though, that the users of
the traditional FB recipe often find that
it melts satisfactorily at ^5 or between
^5 and ^6, even though, as you point
out, seger shows too much silica and
alumina in it, both of which can be hard
to melt at ^6 in the unity formula
representation of this recipe.
If you look at this glaze in terms of
the concentration as a molar percentage
you see something different --
Silica 64.84
Alumina 9.99
Which are very much 'normal' molar
percentage numbers for ^6 glazes.
So why does it look normal with
molar percentage and out of whack
with seger? It's because seger
compares moles of silica to moles
of seger-defined fluxes (lithium,
sodium, potassium, magnesium,
calcium, strontium, barium, zinc,
lead).
If you want to, for yucks and jollies,
you can add the boron in this
glaze (6.78 percent molar) to the
seger-fluxes (14.23 percent molar)
and you get 21.01 -- at the lower
edge of normal, but within the normal
range. (My experience is that this
isn't the best way to evaluate a
potential glaze, but it's a handy
rule of thumb.)
I will, here, with no shame
reiterate my two other points
(previously made in this venue)
-- the Chapelle recipe isn't the
only floating blue. It relies
heavily on the cobalt to make
a bluish background, and the
rutile gives brown or yellow,
and cobalt blue plus rutile
yellow = snot green. second,
it does so particularly when
overfired, and a bit longer a
firing or a slow cooling will
often cool pinholing.
The traditional recipe is
touchy, and hard to generate
reliably results with, because
it walks a fine line between
pinholing and snot-greening.
(Rattling noise as soapbox is
put away.)
Best wishes -- Steve Slatin
Lili Krakowski wrote:
So when Anne D. wrote in the other day about her troubles with her Floating
Blue pinholing, and sent in both recipe and analysis, my reaction was that
the alumina and silica were sky high over the published "limits" and that
that might be a cause of poor healing etc.
Steve Slatin --
History teaches us that there have been but few infringements of personal liberty by the state which have not been justified ...
in the name of righteousness and the public good, and few which
have not been directed ... at politically helpless minorities.
-- Harlan Fiske Stone
---------------------------------
Looking for last minute shopping deals? Find them fast with Yahoo! Search.
John Hesselberth on fri 29 feb 08
On Feb 28, 2008, at 4:07 PM, Lili Krakowski wrote:
> I am under the impression that Rhodes was the originator of
> limit formulas...which became very popular indicators.
> Still, a lot of materials are not given: magnesium, strontium
> for instance.
Hi Lili,
I got curious about limit formulas and did a fairly extensive
literature search about 10 years ago. At the time I was searching for
whether or not there was any data backing them up. If you have
insomnia some evening go to
http://www.frogpondpottery.com/glazestab.html
and read the article summarizing my literature search. I give credit
to Hermann Seger for inventing limit formulas although he did not use
exactly that name--and my article is an instant cure for insomnia.
Regards,
John
John Hesselberth
http://www.masteringglazes.com
http://www.frogpondpottery.com
"I love everything that's old: old friends, old times, old manners,
old books, old wines." Oliver Goldsmith, "She Stoops to Conquer" (1773)
May Luk on sat 1 mar 08
Dear Lili;
When you go buy panty hose, you look at the chart to see what size you are according to your weight and height, but the chart doesn't tell you whether you would look like the model on the front of the package. I see Seger Formulas as that chart. Within the parameter suggested, I would find the best melt area - *a* glaze. It doesn't guarantee visual effects.
Floating blue and waterfall glaze might sound good in a sales brochure, they are just rutile break up glazes. If the glaze is looked at from this angle, the solutions are easier to find. If you understand the role of boric oxide over a high-iron oxide slip or claybody (blue purple without cobalt!), as well as how titanites grow, then you know what everything else in the recipe is doing. From there on, it's a shorter step away to fixing the glaze defect.
Regards
May
Kings County
Ivor and Olive Lewis on sat 1 mar 08
Dear Lili Krakowski,=20
Concerning Silicon Dioxide and Boric Oxide.
Both are glass formers. Boric oxide has a very low melting point and =
requires no additional chemicals to encourage it to form a lower melting =
point vitreous fluid that will freeze as a glass. Silica does need other =
chemicals to encourage it to melt at a reasonable low temperature and it =
is common for us to use compounds that have a lower melting point which =
already contains a good quantity of Silicon dioxide to assist this =
operation.
Kingery, Bowen and Uhlmann write " The addition of modifier oxides to =
the two most important glass forming oxides, SiO2 and B2O3, often leads =
to liquid-liquid immiscibility."
As far as I am aware, High Boron-Silicate glazes cool to give phase =
separation and form what can only be looked upon as an Emulsion, a =
dispersion of fine globules of one glass within a second, continuous =
phase which make it a Colloid. If the globules are of the right size and =
of the best density in the continuous phase, light is affected. Viewed =
by reflected light the colour shows as blue and the transmitted light =
shows as a dull orange.
It is my opinion that adding Cobalt in any form will mask this optical =
effect since it stains both fused borates and fused silicates.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ivor and Olive Lewis on sun 2 mar 08
I would agree with May Luk,=20
Having a dark background gives the visual contrast necessary for us to =
see some of these optical effects. Nor is Cobalt necessary to give the =
colour. This effect can be tested by looking at tobacco smoke using =
reflected and transmitted light.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
John Sankey on sun 2 mar 08
A study of one 'limit' formula:
Studies of copper leaching from glazes by Hesselberth&Roy (MC6G)
suggest that a stable cone 6 glaze should have a silica Seger
ratio above 3.0 and an alumina ratio of 0.25-0.45. H&R exclude
boron and all materials they consider to be colorants from the
Seger ratios on which they based their recommendations, i.e. they
include solely SiO2, K2O, Na2O, Li2O, MgO, SrO, ZnO, BaO, CaO,
Al2O3.
I've analysed the 211 successful gloss/semigloss cone 6 glazes in
the glaze database using their criteria.
The average SiO2 is 2.62; 143 (68%) are below 3. Most glazes in
the database that are considered to be successful by testers have
much less silica than a glaze should have by H&R criteria.
The average Al2O3 is 0.31; 60 (28%) are below 0.25, 23 (11%) are
above 0.45, so a significant number have alumina outside their
limits as well.
Potters seem to be saying that the criteria of Hesselberth&Roy
are stricter than necessary. It's true that H&R used 5% copper
carbonate, a larger than normal amount of one of the most
leachable colorants. Lower amounts of colorant leach much less.
However, for centuries potters used large quantities of lead in
glazes, even cadmium and uranium; all are now known to be
dangerous and are legally prohibited in many countries. So,
perhaps H&R are the voice of the future.
The glaze database: http://sankey.ws/glazedata.html
Analysis details at http://sankey.ws/glazeanalysis.html
John Sankey
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