MarjB on sat 22 may 04
Morning Ivor - would the answer be on page 54/55 ?
" One special technique is that of the black slip used in painting =
Classical Greek wares. Its blackness depends upon a special firing =
technique. Oxide is applied to an iron slip deliberatelty decayed to a =
colloidal fineness, perhaps derived from BLOOD (my emphasis), so that =
its particles will instantly lose their oxygen to a reduced atmosphere." =
=20
Wondering - or maybe I just do not understand the question or Rawson's =
statement . MarjB
Ivor and Olive Lewis on mon 24 may 04
Dear MarjB,
Thank you for your consideration.
I think Rawson draws a clear line between Slip and Glaze.
His note about Greek Red and Black Ware is as you quote. By what does he =
mean by "Deliberately decay to colloidal fineness" Does he tell us that =
clay undergoes some sort of decomposition or should we interpret this as =
a reference to preparing Terra Sigillata. Why does he bring Blood into =
the discussion? He can be read as saying blood changes into clay.
It is in the following paragraph that he gives an analysis of a glaze =
and the materials which comprise the batch. I am wondering if there is =
an error when he tells us that Lead Oxide is an opacifier, grouping it =
with Tin. But it is at the end of that paragraph which introduces the =
concept of adding organic materials for the Carbon they contain.=20
What I wish to know is, (1) Are there any examples of glaze recipes =
where this known to be the case (2) Who has used, or is using them. (3) =
Where would we see concrete examples?.
By the way, years ago I was given a sample of Brick Earth from the Clare =
valley. I thought I had got rid of the very coarse material but while =
throwing some I chanced upon a flake of white quartz. People who have =
met this material will appreciate the razor sharp edge which sliced into =
the tip of my right middle finger. The clay spun round and I pulled up =
leaving a final spiral of blood on the pot. Beyond that I recall there =
was a slight brown stain after bisquing, but the iron was adsorbed into =
the glaze and I do no recall that it made a significant contribution to =
the aesthetics of the pot.
Rawson has wonderful ideas but they are not backed up by examples of his =
own work, nor by references or a bibliography.
Thanks for getting in touch. I will CC this to clayart
Best regards,
Ivor Lewis. Redhill, South Australia
Potters Council Member
Morning Ivor - would the answer be on page 54/55 ?
=20
" One special technique is that of the black slip used in painting =
Classical Greek wares. Its blackness depends upon a special firing =
technique. Oxide is applied to an iron slip deliberatelty decayed to a =
colloidal fineness, perhaps derived from BLOOD (my emphasis), so that =
its particles will instantly lose their oxygen to a reduced atmosphere." =
=20
=20
Wondering - or maybe I just do not understand the question or Rawson's =
statement . MarjB
Ivor and Olive Lewis on tue 25 may 04
Dear MarjB,
I think our posts have crossed in space. If you read this paragraph
carefully you will come to appreciate that it is a jumble of ideas.
Whether this is a consequence of poor editing or inadequate proof
reading, or if it is a question of writing style I would not know.
But think of it this way <> <of Blood instantly decompose when heated in a reducing atmosphere>> <<
slip is of colloidal fineness>> <colour to the slip>>
The idea of "Decay" is interesting. But I think the chapter would
benefit from being edited by a qualified ceramic arts technician.
Notice also that he does not say which oxide is "perhaps" derived from
blood. As informed readers, we make an assumption that it is from
Iron. If the blood came from a crustacean, it could be Copper ! !
MarjB wrote and quotes the relevnat passage.
<" One special technique is that of the black slip used in painting
Classical Greek wares. Its blackness depends upon a special firing
technique. Oxide is applied to an iron slip deliberatelty decayed to
a colloidal fineness, perhaps derived from BLOOD (my emphasis), so
that its particles will instantly lose their oxygen to a reduced
atmosphere."
Wondering - or maybe I just do not understand the question or Rawson's
statement . MarjB>>
Best regards,
Ivor Lewis. Redhill, South Australia
Potters Council Member
Wes Rolley on tue 25 may 04
Dear Ivor Lewis,
After reading the posts on this subject, my instinct says the Rawson is=20
maybe more poetic than scientific in his reasoning. However, it did make=20
me start to think about something that I had observed in firing one of my=20
glazes.
It is a very simple glaze: 2 parts (by weight) Redart Clay, 1 part Oak Ash=
=20
+ 7% Red Iron Oxide. I single fire this glaze to Cone 8 in an electric. I=
=20
get my Oak Ash from a neighbor's fireplace, screen it through 80 mesh, dry=
=20
mix it into my big plastic storage bin of ash, and do NOT wash it. I=20
screen the glaze one time through the 80 mesh to disperse the Redart.=20
Therefore, I have considerable fine grained residual charcoal in the glaze.
I have been single firing this glaze on an iron rich body and slow cool=20
with a 30 minute hold at 1980 F and fird down at 150 deg./hr to 1550. If I=
=20
fire the piece, it has a very predictable surface and a rather textured=20
gold color. If I were to place one of the pieces back into the next=20
firing, it would emerge with a very different surface, especially the color=
=20
range, becoming black, grey, with some redish streaks and more satin than=20
glossy. Again, very predictably so. Areas where I had applied additional=
=20
amounts of the original glaze are generally the surface and color of the=20
first firing. The original observation came from a refire to correct a=20
glaze fault...crawling, probably induced by dust. I have repeated it.
I am not quite sure what is going on but one possible explanation might be=
=20
that the presence of significant quantities of charcoal in the raw glaze=20
contribute to the color and surface of the first firing.
I have not tried to measure the LOI of the ash to get a feel for the amount=
=20
of charcoal. I probably should. Lacking the analysis of the ash, it would=
=20
not be easy to set up a test in which I could produce the same glaze=20
without the charcoal. The closest that I can come to it would be to use the=
=20
chart in Phil Rogers's book Ash Glazes, and take the figures for "English=20
Oak". The similarity is that English Oak ash is high in P2O5 and I have=20
noted the opalescent effects of P2O5 in other glazes that I have made using=
=20
my California Oak ash.
Wes
"I find I have a great lot to learn =96 or unlearn. I seem to know far too=
=20
much and this knowledge obscures the really significant facts, but I am=20
getting on." -- Charles Rennie Mackintosh
Wesley C. Rolley
17211 Quail Court
Morgan Hill, CA 95037
(408)778-3024
Ivor and Olive Lewis on wed 26 may 04
Dear Wes Rolley,
The processes you describe are complex but the critical stage, if
there is a significant proportion of charcoal in your ash is during
the heating phase. At this point you have to: 1 assume that there is
insufficient air in the kiln to burn up the charcoal.
2 heat the mixture fast enough to seal the surface of the glaze before
the charcoal is oxidised.
3 Assume residual charcoal is in intimate association with your Iron
Oxide.
Your post Glost Firing characteristics depend on:
1 a feature in the reheat curve called the Glass Transition Point. At
this point the glass in the glaze becomes less viscous and diffusion
takes place into and through the glaze.
2 the presence of absence of relicts of Carbon suspended in the glaze.
If they exist they might excite additional reduction. Colour would
change because optically the black of the carbon is deactivated and
reduction of Iron Oxide has a different colour (black)
3 precipitation of new insoluble compounds formed by chemical
reactions.
There may be other factors to consider.
4 rate of cooling. Retarding the rate of cooling tends to promote
crystal growth.
Spread a weighed quantity of your raw ash as a thin layer on a kiln
shelf and fire to about 700=BA C with a current of air flowing. The mass
difference will give you a value for the proportion of charcoal.
Enjoy your research
Best regards,
Ivor Lewis. Redhill, South Australia
Potters Council Member
Wes Rolley on wed 26 may 04
At 04:12 PM 5/26/04 +0930, you wrote:
>The processes you describe are complex but the critical stage, if
>there is a significant proportion of charcoal in your ash is during
>the heating phase. At this point you have to:
>1 assume that there is insufficient air in the kiln to burn up the=
charcoal.
>2 heat the mixture fast enough to seal the surface of the glaze before
>the charcoal is oxidised.
It would seem that neither of these first two assumptions is valid. I heat=
=20
with a slow ramp schedule in the electric kiln. The heating is only 150=20
deg. / hr to 900 F (482 C.) and then 100 deg. hr to 1100 F (593 C.) at=20
which point the lid that had been propped open, is closed. Even beyond=20
this, I only ramp up at 155 deg. / hr.
>3 Assume residual charcoal is in intimate association with your Iron
>Oxide.
This would likely be true, due to the dispersion screening.
So, then you are saying that absolutely no reduction interaction can=20
possibly be taking place, even deep within a glaze that is applied very=20
thickly.
"I find I have a great lot to learn =96 or unlearn. I seem to know far too=
=20
much and this knowledge obscures the really significant facts, but I am=20
getting on." -- Charles Rennie Mackintosh
Wesley C. Rolley
17211 Quail Court
Morgan Hill, CA 95037
(408)778-3024
Ivor and Olive Lewis on thu 27 may 04
Dear Wes,
No, I am not.
Assumptions are assumptions, not facts. Consider what facts are known,
then search for unknown facts which are not yet recognised.
If the facts contradict the assumptions abandon the assumptions. But
be sure that your facts are facts and not artefacts.
Best regards,
Ivor Lewis. Redhill, South Australia
Potters Council Member
.
Wes Rolley on thu 27 may 04
At 03:49 PM 5/27/04 +0930, you wrote:
Dear Ivor Lewis,
>Assumptions are assumptions, not facts. Consider what facts are known,
>then search for unknown facts which are not yet recognised.
Here is part of the problem. It is not clear to me what the total set of=20
relevant facts really is.
Take for example a question concerning iron. If I ask "what are the causes=
=20
of color in iron laden glazes?" there are a number of answers and oxidation=
=20
/ reduction is only one factor. The behavior of iron on the surface of=20
traditional (Japanese) shino glazes is a commonly documented, including the=
=20
increased redness if the glaze is refired in a bisque firing=20
(oxidation). Similarly, the behavior of iron in celadon glazes is well=20
documented, including the facts that a green (rather than an amber) celadon=
=20
depends on reduction and there are a number of different descriptions of=20
how much reduction and when the reduction should start to give the best=20
response.
It appears to be too simplistic to say that the appearance of black, grey=20
and green colors in an iron rich glaze are caused by reduction and the=20
appearance of gold, amber, red, orange hues is caused by oxidation. And=20
what is the case when these colors are mixed on the surface of the same=20
glaze?
In my glaze example multiple processes are obviously at work:
1. The local ash in the glaze contains some magnesium, (local soils are=20
sometimes described as serpentine [Magnesium Iron Silicate Hydroxide])=20
which will encourage the formation of pyroxene crystals under the slow=20
cooling process that I use.
2. Local oak ash has a comparatively high level of P2O5 and that may=20
contribute other optical effects by introducing a quantity of phosphate=20
glass. It does produce opalescent effects in otherwise clear glazes.
3. Some volatiles (sodium from the unwashed ash?) are being produced as the=
=20
ITC sprayed kiln lid has developed a pinkish tone near the kiln walls where=
=20
vapors are depositing on the surface. However, this is only in a ring just=
=20
inside the walls, so that the slight temperature difference at that=20
location (the lid does not sit tightly) may contribute to the appearance=20
only in one place and at temperatures above that at which I close the lid.=
=20
Maybe I should use a venting system...but I don't.
4. Phase separation is a component of the process.
5. Other chemical combinations can be forming of which I know nothing.
There are several courses of action that I might follow. One of which is=20
to conclude that it is all too complex and not search further for an=20
explanation of the processes at work; simply use the observed results in my=
=20
work. I currently use it deliberately to impart a particular feeling to=20
specific pieces. It would appear that many, if not most, potters work in=20
this manner to some extent. This is what we do when we take a glaze we=20
like and tweak it for our clay body and firing conditions.
I can gain more insight by asking some questions such as "What happens when=
=20
I increase the amount of magnesium in the glaze?" I might add dolomite, do=
=20
a line blend or grid, and then, if the results are pleasing, make use of=20
it. This relies more on aesthetic judgement than on scientific reasoning.
Then again, if I did truly understood what is going on, I might be able to=
=20
gain more control and have more predictability in what I am doing. I=20
contribute increased variability anyway by using the jet of air from the=20
spray gun to push the wet glaze over the surface of the pot, pooling it in=
=20
some places and thinning it in others.
Wes
"I find I have a great lot to learn =96 or unlearn. I seem to know far too=
=20
much and this knowledge obscures the really significant facts, but I am=20
getting on." -- Charles Rennie Mackintosh
Wesley C. Rolley
17211 Quail Court
Morgan Hill, CA 95037
(408)778-3024
Ivor and Olive Lewis on fri 28 may 04
Dear Wes,
Reads as though you have the whole process well under control.
As facts are revealed you have to first question their validity as
facts, basically asking if there is consistency. Then you consider the
pathway which brought them into being. You are well aware of several
of the processes. By this I mean the Phase Changes, Precipitation's,
Immiscibility, Rates of Volatility, Evaporation, Diffusion and
Decomposition. One thing which is neither clearly understood nor
exposed to discussion through presentation in recent and past media
are Chemical Reactions, when they happen and the influence they have
on the attainment of maturity. Does current Glaze Dogma allow for the
presence of Solid State Reactions?
I do not think your questions about the influence of variations in
specific chemicals are rhetorical. You may recall I asked a similar
question about Barium Carbonate. The knowledge base told me I would
progressively opacify a glaze if I overload it. My tests showed this
to be untrue. My samples were mostly transparent even for very high
quantities, like 40%, of Barium Carbonate. I am told that Zinc Oxide
is a strong melting flux. If this is so it should change Silica into a
liquid below the melting point of Silica. Who has tested blends of
Zinc Oxide and Quartz and found this to be a fact?
By all means continue to spray glazes for a variety of effects. But if
you are experimenting be guided by the Canon "Examine one Variable at
a Time"
Best regards,
Ivor Lewis. Redhill, South Australia
Potters Council Member
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