iandol on sat 12 apr 03
Dear Jim Murphy,
Yes, you give pretty good values for the oxide content of the 1170 =
Equilibrium Eutectic or the CaO-Al2O3-SiO2 System. But if you prepare =
the obvious mixture of those oxides from Burnt Lime, Quartz and Alumina =
and heat to even to cone 8 down it will not melt. In fact, it does not =
even sinter.
Lawrence Ewing helped me out three years ago with that by confirming =
good values of the appropriate minerals to use to activate that =
particular part of the system and it does give a fair glass fired to =
Cone 8+, much higher than the theoretical value.=20
This was all the evidence I needed to put one of several nails in the =
notion that Eutectics are responsible for vitrification and melting. =
Wollastonite and Quartz give what Ian Currie referes to as a "Eutectic =
Valley", that is a series of mixtures of these two minerals which do =
melt at lower temperatues. But I suspect this is predicated the presence =
of Aluminium Oxide in Solid solution in both minerals. This is highly =
unlikely.
You speak of the K2O-CaO-SiO2 System. In this situaltion things are even =
more complex. To prepare this example could start with Pearl Ash, Quartz =
and Quick Lime. Now the definition of Eutectic is that this is where the =
system melts at the lowest temperature possible and you offer 950 deg C. =
Logically, this means that Potassium Oxide melts at a higher =
temperature. But the fact is that this Potassium Monoxide is thermallly =
unstable, decomposing at 350 deg C. Note also, Potassium Carbonate melts =
at 891 deg C which also makes the proposition illogical. To get the =
Eutectic you have given you would need to prepare your mixture of =
compounds from beta CaO.SiO2, 4K2O.CaO.10SiO2 and K2O.CaO.SiO2. Never =
read that any of these are in the chemical suppliers lists.
Now you say "I believe a fair share of glaze chemistry and material =
selection is based,in part, on these eutectics." Well, that does seem to =
be the case
The problem with the way you have given the information from the Ceramic =
Material handbook is that what you describe in only in terms of single =
molecular oxides. If you were to ask the authors if they add Quick Lime, =
CaO, Potassium Oxide or Sodium Oxide to either glaze of clay body =
mixtures they would tell you that would be idiocy. Ask at your ceramics =
supplier for a Kilo of Sodium or Potassium oxide.. The neares you would =
get from a chemical supplier would be Soda-Lime
Jim Murphy on sat 12 apr 03
Hi Ivor,
In reply to your last two posts, yeah - I know - I threw alot of "pieces"
out there (from different sources) in my original message.
(I hope I've got this right now) - I believe Phase Equilibrium Diagrams have
pure oxides located at the vertices which helps provides a framework - a
tool - to understanding/comprehending how ceramic materials can work
together when used in different proportions, at different temperatures, etc.
It is understood not to be absolute though.
I read once that racecar drivers - the good ones at least - are trained to
"look and steer in the direction they want to go" when they're about to
collide into a wall. Focus, I think, is the key.
Perhaps, the outer limits of the Phase Equilibrium Diagrams, bound by the
vertices, is like the wall around the racetrack. We don't WANT to ride or
hit that wall. Clay body development is "inside" the wall. While this is not
about winning a race though, I must not get too caught-up in the science of
pure oxides either.
In my original post, I wondered how Wollastonite, a source of CaO, along
with Potash Feldspar (high K2O) may be used in ceramic bodies to reduce
maturation temperature and improve vitrification. Specifically, in midfire
clay bodies.
Why midfire bodies ? To me - a newbie reading pottery books, literature,
etc. - there seems to be some sort of fork-in-the-road mentality for
developing these midfire clay bodies. The majority appear to take the path
of relying on Soda Feldspar, Neph Sy, or combinations thereof with Potash
Feldspar for midfire clay body maturation and vitrification.
I'm particularly interested in midfire casting bodies, so, 50% plastics
(clay) and 50% nonplastics along with water and defloculant(s) is a normal
starting point. Those rascally high Na oxide-bearing Spars and Neph Sy can
present some unwanted clay body behavior such as high expansion, solubility
issues, warping, etc.
Perhaps, "the road less traveled by" for midfire clay bodies may be built
around lime causing a sharp increase in the fusibility of potash feldspar.
Think about it, Herman Seger's pyrometric cone compositions are built - not
around BaO, MgO, or Na2O - but rather, CaO and K2O.
Although some sources may differ slightly in the Al2O3 and SiO2 levels, the
unity formula for cone 5 through 10 compositions is something like this
below:
Cone 5 - K2O 0.3, CaO 0.7, Al2O3 0.5, SiO2 5
Cone 6 - K2O 0.3, CaO 0.7, Al2O3 0.6, SiO2 6
Cone 7 - K2O 0.3, CaO 0.7, Al2O3 0.7, SiO2 7
Cone 8 - K2O 0.3, CaO 0.7, Al2O3 0.8, SiO2 8
Cone 9 - K2O 0.3, CaO 0.7, Al2O3 0.9, SiO2 9
Cone 10 - K2O 0.3, CaO 0.7, Al2O3 1.0, SiO2 10
In his book "Revealing Glazes", Ian Currie also explains in practical terms
(quite masterfully I may add) the 0.3 K2O-0.7 CaO flux unity relationship
for his 0.7 Limestone Set. Out of respect for Mr. Currie and his book
copyright, I won't go into his specifics, nor will I reveal his recipe for
"The 0.7 Limestone Loaf Cake" which is also based on 0.3 K2O - 0.7 CaO.
(Quite tasty though and not too fattening when eaten in moderation !)
For clay body development, perhaps the 0.3 K2O - 0.7 CaO relationship is
something to keep in mind. Pyrometric cones bend and melt. Like the
racetrack wall, you don't want to go there. It's good to comprehend what an
outer limit for Cao-K2O may be though.
Again it is understood that these are pure oxide formula. I suspect Herman
Seger developed his cones from his unity formula rather than the other way
around. Formula first, then choose raw materials wisely.
Best wishes,
Jim Murphy
iandol on sun 13 apr 03
Dear Jim,
I look up to the wall above our computer at four ternary Phase =
Equilibrium Diagrams. They hang there like religious icons. I ponder on =
their creation, recalling many hours spent collecting evidence to =
construct similar diagrams for alloy steels. My experience is that =
whatever the origins of the materials chosen to obtain the thermal =
analysis and physical information used in their construction, what you =
see represents an abstract expression of what happens when samples are =
cooled slowly from a uniform melt. The best way to express this =
abstraction is to represent compositions as assemblies of molecular =
oxides rather than structural entities.
To someone concocting clay bodies the race tracks you are looking for =
are the Isotherms. Choose a composition on the high side of the glazing =
temperature you are intending to use and for the glaze a composition on =
the low side of that isotherm. Since Mullite is one of the high =
temperature components in a mature clay ceramic you might look inside =
the Mullite field for an answer. Beyond that it is difficult to explain =
unless you possess the Icons for Systems CaO.Al2O3.SiO2 and =
K2O.Al2O3.SiO2.
Why should any one wish to use a mid fire clay body. Ask them. Perhaps =
something was suggested and it worked. Why substitute Sodium minerals =
for Potassium minerals. Perhaps expediency. But if you are wanting to =
return to first principles you need to clearly distinguish between the =
fundamental compositions and properties of mature glazes and ceramics =
based on clay, silica and a glass forming flux. If you have access to a =
calcs program, working out the unity equations for these contrasting =
systems may be a starting point.
I think your quest for alternative casting bodies may only be solved as =
it was in the past, by making an array of mixtures based on mineral =
ingredients and firing to the temperature you intend working at. Then =
you have to examine your samples, determine apparent density, =
absorbency, CoE and so forth. Make cones and fire them.
If you are serious about this, perhaps investing in Plates 1, 3, 4 and 5 =
Phase Equilibrium Diagrams of Oxide Systems. You can order direct from =
ACS via their on line site.
Best regards, Ivor Lewis. Redhill, South Australia=20
With the pyro at 990 and another five hours to go, then a day to wait to =
find out what my 340 glaze samples achieved!!!
Jim Murphy on tue 15 apr 03
Hello Ron,
Nice to hear from you again.
> I work with a lot of cone 6 bodies and most are fluxed with combinations of
> low fire red clays, neph sy, and spar - one with talc.
> First it does not follow that alkalies produce high expansion bodies ...
Increased body expansion is a good
> anti craze up to a point - with low expansion glazes - well you don't want
> to wind up there.
Actually, I'm developing a casting body that will fire with Jasperware-like
qualities. That is to say, the body is colored throughout - no glazing
involved for this particular body. Looking for that nice colored-clay
surface character, etc. So, expansion issues related to glazing won't apply.
Solubility - from high NaO - is still of concern.
Control of clay body expansion/contraction is still extremely crucial for my
artwork though since slumping, warping and/or cracking would be totally
unacceptable.
> CaO/MgO are not wanted at higher temps - you want the KNaO to gobble up
> that cristobalite as it is forming.
One could conceive of developing a midfire test recipe - at least a good
starting point - built around an appropriate amount of Potash Feldspar(s)
(G-200, Custer, etc.) to source plenty of good KNaO with a "l-i-t-t-l-e bit"
of a CaO source (like Wollastonite) to help "tweak" things along.
Other midfire and highfire recipes (including Jasperware) along with an
understanding of what the different body elements (Clays, Spar, Flint, etc.)
add to the mix - before and after firing - provide hints as to starting
point proportions of plastics-to-nonplastics, ratio of BallClays-to-Kaolins,
amount of spar (I know - Wedgwood Jasperware used Barytes like BaSO4),
amount of Flint (Silica), etc.
> If you want 50% none plastics and you get more melting you may find
> yourself in a jam - certainly - as you add MgO/CaO you will need to cut
> back on the spar to control melting - you will have to replace that none
> plastic with another.
50% clay to 50% nonplastics is not mandatory. There's room to play on either
side when deciding what the test "starting point" will be. Also, perhaps the
added CaO will be more forgiving at Cone 6 than MgO (from Talc).
Ron, do you have any idea or guesstimate for how much CaO in a Cone 6
vitrified or semi-vitrified body may be too much ?
For his cones - midfire and up - Herman Seger kept CaO/K2O constant and
incrementily varied Al2O3 and SiO2. For casting body test samples, perhaps a
relatively small number of test recipes may be formulated and tested by
holding KNaO, Al2O3 and SiO2 "somewhat" constant with variations of CaO.
Now, the following is NOT my planned starting point. I understand, in a clay
body, it's not about the "pure-oxides" which is why I said "'somewhat'
constant" above. It's more about clay body physics, particle diversity,
Potash Feldspar viscosity during the "melt", etc.
Again, this is only an example: To simplify, IF I were talking about a
triaxial body starting point, the recipe total % of clay, flux (Pot spar
with or w/o Wollastonite) and flint would remain constant, e.g. 50% Clay,
30% Flux, 20% Flint. The 30% Flux would be made-up of different proportions
of Potash Feldspar and Wollastonite in different test samples. Only the
proportion of Pot spar to Wollastonite changes. Firing below, at, and above
the intended working temperature would be necessary to properly evaluate the
test samples.
> I have a dilatometer - I would be willing to share that if you are willing
> to share the chemistry.
I'll have to give that some more thought. Thanks for the offer.
Best wishes,
Jim Murphy
Jim Murphy on sat 19 apr 03
on 4/18/03 11:55 PM, Ron Roy at ronroy@TOTAL.NET wrote:
> If you want to tell me what materials you plan to use I can give you some
> idea based on the experience I have with talc - and tell me what amounts of
> non plastics you would like to start with.
Sure Ron,
For discussion purposes, feel free to comment on this "totally imaginery"
(hasn't been tested) casting body recipe below - targeted to mature around
Cone 6 - using 50% Clay, 30% Flux (K-spar + Wollastonite) and 20% Flint:
FC-340 Ball Clay......... 25.00
TILE #6 KAOLIN........... 15.00
VELVACAST KAOLIN......... 10.00
G-200 FELDSPAR........... 28.00
WOLLASTONITE............. 2.00
SILICA................... 20.00
=========
100.00
CaO 0.11 1.38%w 1.65%m
MgO 0.02 0.21%w 0.34%m
K2O 0.17 3.53%w 2.51%m
Na2O 0.07 0.94%w 1.02%m
TiO2 0.04 0.68%w 0.57%m
Al2O3 1.00* 22.04%w 14.52%m
SiO2 5.45 70.84%w 79.22%m
Fe2O3 0.01 0.39%w 0.16%m
Thanks in advance,
Jim Murphy
Jim Murphy on tue 22 apr 03
Hello Ron,
> I don't have an analysis for the FC 340 ball so I used C&C - I can give a
> better answer maybe - if I have that analysis.
FC-340 does have more silica than C & C. I had entered the following FC-340
Ball Clay analysis from Old Hickory's website into Insight (Version 5.3.11
for Mac):
> Spar is just not a good melter at cone 6 and the amount of wollastonite is
> small so I am guessing you will need more melt - the neph sy will be a
> quick way to get it. I am guessing that you will need to increase the
> wollastonite a lot to get the kind of melting you expect.
I'll try some Neph Sy substitutions in test samples as well for comparison
to help evaluate what happens to the color in a stained-body, solubilitywise
(after firing).
I'll need to run a series of tests to find the right proportion of K-spar to
Wollastonite. Normally, the fiberous form of Wollastonite is used in
low-fire ware (tile bodies, etc.). By using the powdered-form of
Wollastonite as an auxiliary body flux, a little bit could go a long way at
Cone 6 although more than 2% may still be necessary. Wollastonite has that
volume-thing going for it too due to its relatively low density. Only
testing will tell.
> I compared your molecular formula with what I get and everything is close
> except the SiO2 - perhaps I do need the analysis for that ball clay - it
> might be high in silica. If not then I suspect another material definition
> is way out.
See above link for FC-340 Ball Clay analysis. I also had entered the
following Velvacast (kaolin) analysis into Insight:
Velvacast Analysis
SiO2 45.4%
Al2O3 38.7%
Fe2O3 0.3%
TiO2 1.4%
K2O 0.0%
Na2O 0.1%
MgO 0.2%
CaO 0.1%
LOI 13.8%
Well it's late and my brain is still swollen from reading "Intro To
Ceramics" by Kingery over the weekend. (LOL)
Best wishes,
Jim Murphy
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