Chris Schafale on sun 15 nov 98
Questions about iron reds:
I found a couple of similar recipes in the archives -- one called
Rust Red and one called Ketchup Red. Both have lots of red iron
oxide -- 15% in one case, 18% in the other. When I run them through
Insight, I note that they both have low silica, very low alumina and
Si:Al ratios that are super high (17, for instance). My questions:
Why don't these glazes run completely off the pot, with so little
alumina? Does the iron somehow make up for the lack of alumina? Is
the low alumina necessary for the development of the red color? What
would happen if you revised the recipe to bring the silica and
alumina up into the suggested limits? Also, both recipes contain
talc, which I don't happen to have at the moment. I can sub
dolomite in order to get equivalent magnesium, but that increases the
calcium quite a bit (though well within limits). Is the low calcium
essential to the color? What would be likely to happen if the
calcium were higher?
Here's the Ketchup Red recipe, in case anyone wants to play with it:
Ketchup Red, ^5
EPK 5
Flint 30
Custer Feldspar 20
Talc 5
Gerstley Borate 32
Add Red Iron Oxide 15
Here's what Insight had to say:
CaO .18 Si:Al 17.55
MgO .22 SiB:Al 22.13
K2O .04 Expan 6.21
Na20 .11
Al2O3 .10
B203 .45
SiO2 1.73
(B2O3 included in unity, iron not included in formula --
should it be??)
If I don't include B203 as a flux, then I get:
CaO .33
MgO .40
K2O .08
Na20 .20
Al2O3 .18
B203 .82
SiO2 3.15
Thanks for any help in understanding all this. I plan to do the
tests to actually see the what-if's but would like to have some idea
what to expect.
Chris
Light One Candle Pottery
Fuquay-Varina, NC
candle@nuteknet.com
Laurel Carey on tue 17 nov 98
Ron Roy wrote:
>
> ----------------------------Original message----------------------------
> Hi Chris,
>
> When I calculate your glaze I get a completely different result and I am
> wondering if I have the right recipe - note: it only totals 107 including
> the 15% red iron.
>
i just posted that i used the ketchup red recipe as advertised. my
assumption was wrong. i assumed it because of the name. the version i
used has more talc:
epk 5
flint 30
custer 20
talc 14
g.b. 32
r.i.o. 15
any comments about this version?
oh, and while i have the attention of great glaze people (and my
notebook inside), i wondered about another glaze i have. this glaze was
the mainstay of the university ceramic department i attended. the
teacher promoted it because it had a wide firing range and was
"dependable".
oil spot black
cone 5-9
(i fired it to ^6)
epk 9.5
custer 152.6
whiting 10.6
g.b. 20.8
copper carb. 8
manganese 8
cobalt carb 4
thanks (in advance) for your help!
laurel
lacarey@ioa.com
asheville, nc, usa
Chris Schafale on tue 17 nov 98
Oops. Sorry Ron. I mis-typed the talc. Should be 14 instead of 5.
Chris
> ----------------------------Original message----------------------------
> Hi Chris,
>
> When I calculate your glaze I get a completely different result and I am
> wondering if I have the right recipe - note: it only totals 107 including
> the 15% red iron.
>
> I don't think it is necessary to have the iron in the formula if you are
> not firing in reduction - but you can have it - all you need to do is find
> it in the materials table.
>
> My calculation of your recipe - boron in unity
>
> Untitled Recipe 2
> -----------------
> EPK................. 5.00
> SILICA.............. 30.00
> CUSTER SPAR......... 20.00
> TALC................ 5.00
> Gers Borate 9/97.... 32.00
> RED IRON OX......... 15.00
> ----------
> Total 107.00
> FORMULA & ANALYSIS
> ------------------
> *CaO........ .32 6.93%
> MnO2....... .00 .01%
> *MgO........ .19 2.97%
> *K2O........ .06 2.24%
> *Na2O....... .09 2.09%
> Fe2O3...... .25 15.80%
> TIO2....... .00 .02%
> *B2O3....... .34 9.33%
> AL2O3...... .15 6.09%
> SiO2....... 2.34 54.51%
> P2O5....... .00 .01%
>
> RATIO 15.20
> EXPAN 440.05
> WEIGHT 257.11
>
>
>
>
>
> 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
>
>
Light One Candle Pottery
Fuquay-Varina, NC
candle@nuteknet.com
rballou@mnsinc.com on tue 17 nov 98
Hey Craig,
What about an increased likelihood of pyroxene crystals from the added
magnesium in talc? Low alumina will help, too and the extra silica in the
talc may give the crystals seed points. I don't fully understand the role
of boron in the development or prevention of crystals or color with iron,
so maybe not. There's quite a bit of boron in this glaze and I'd like to
have a better picture of how it contributes to the glass matrix. Also, if
I've read Hamer correctly, it's likely that some of the iron is
participating in the glass network, too. I don't think any glaze
calculation program takes this into account and I don't recall it being
discussed before. Will it contribute to glaze durability?
Ruth Ballou
Silver Spring, MD
>----------------------------Original message----------------------------
>Hi:
>
>Just a word about calcium in iron reds. Calcium will cause iron to dissolve
>and not form crystals. You need the iron crystals to get the red color. A
>higher amount of alumina will also help this. Some calcium in this glaze
>is fine but there's a point where the CaO will negate the red.
>
>I don't have Insight loaded into a new computer so I can't retrieve some of
>my info to give you a better picture of this. I'd buy some Talc and use
>that instead of upping the CaO.
>
>regards, Craig Martell in Oregon
Paul Lewing on tue 17 nov 98
Chris,
I've been testing ^5 oxidation iron reds for years now, including the
two that you mention. In fact, Rick Malmgren published one of my
recipes (Towy Red, named after my dog) in the June issue of CM. I've
since changed it to make it a bit redder, but it's not far off what's
there. I've probably tested 300 variations on these things over the
last five years.
These are very frustrating glazes. You are correct in thinking that
with that high a Si:AL ratio, they would run right off a vertical
surface. They do. And if you bring them into line with any set of
limit formulas, you lose the red color. My experience was that the
ratio needed to be at least 15:1, and 20:1 was better. You also
needed a lot of calcium, and a lot of magnesium, but not more than the
calcium. And you needed really a lot of iron- at least 15%- 18%,
which makes them sort of matt, even though the ratio is so high. And
then there's the firing problems. These glazes need to be THICK to be
red, which compounds the running problem. And a slow firing, and
especially a slow cooling, help a lot. They are also very sensitive
to the clay body they're put on, and to what kind of iron you use.
I've tested maybe ten different forms of iron oxide (black, yellow,
Spanish, crocus martis, fritted, and many forms of red). You want the
iron that is the most orange before firing.
In fact, I've given up on Towy Red for now. I've invented a better,
redder, more consistent iron red. It's pretty nice, but it's (GASP!)
a lead glaze.
Keep on testing, though, and if you ever get a consistent, reliable
really red recipe from this kind of stuff, let me know.
Paul Lewing, Seattle
Paul Lewing on wed 18 nov 98
Chris,
I wanted to add something to my post on this subject from yesterday.
I've seen some iron red recipes that have no clay in them at all, and
some with no clay and no feldspar either. In that case, any alumina in
the formula would be coming from the gerstley borate. So the Si:AL
ratio you get then really depends on the analysis you're using for GB.
I've seen analyses that give that ratio as over 100:1, and in a
database that lists the formula of GB as just calcium borate and
nothing else, the ratio is infinite! (Or zero?)
Anyway, these glazes all have very little alumina, so be aware of
where your program says it's coming from. The ratio may be a bit
arbitrary.
Paul Lewing, seattle
Marty Anderson on wed 18 nov 98
Paul, here is the ^6 iron red we use. It does go on thick but don't find
that it runs. I fire my kiln a straight med, no soaks and it cools down on
it own. I have been know to open the kiln at 200 degrees to finish cooling
when in a hurry.
Gerstley Borate 45
EPK 10
Talc 15
Flint 30
Red Iron Oxide 20
I have attached a sample. Is not a great jpg . The pitcher is a very dark
brown clay body. The bowl is a light brown clay body.
Marty
attached is a sample of the red
Marty
-----Original Message-----
From: Paul Lewing
To: CLAYART@LSV.UKY.EDU
Date: Tuesday, November 17, 1998 9:15 PM
Subject: Re: iron reds and glaze calc
>----------------------------Original message----------------------------
>Chris,
>
>I've been testing ^5 oxidation iron reds for years now, including the
>two that you mention. In fact, Rick Malmgren published one of my
>recipes (Towy Red, named after my dog) in the June issue of CM. I've
>since changed it to make it a bit redder, but it's not far off what's
>there. I've probably tested 300 variations on these things over the
>last five years.
>
>These are very frustrating glazes. You are correct in thinking that
>with that high a Si:AL ratio, they would run right off a vertical
>surface. They do. And if you bring them into line with any set of
>limit formulas, you lose the red color. My experience was that the
>ratio needed to be at least 15:1, and 20:1 was better. You also
>needed a lot of calcium, and a lot of magnesium, but not more than the
>calcium. And you needed really a lot of iron- at least 15%- 18%,
>which makes them sort of matt, even though the ratio is so high. And
>then there's the firing problems. These glazes need to be THICK to be
>red, which compounds the running problem. And a slow firing, and
>especially a slow cooling, help a lot. They are also very sensitive
>to the clay body they're put on, and to what kind of iron you use.
>I've tested maybe ten different forms of iron oxide (black, yellow,
>Spanish, crocus martis, fritted, and many forms of red). You want the
>iron that is the most orange before firing.
>
>In fact, I've given up on Towy Red for now. I've invented a better,
>redder, more consistent iron red. It's pretty nice, but it's (GASP!)
>a lead glaze.
>
>Keep on testing, though, and if you ever get a consistent, reliable
>really red recipe from this kind of stuff, let me know.
>
>Paul Lewing, Seattle
>
Craig Martell on fri 20 nov 98
----------------------------Original message----------------------------
Ruth Ballou wrote:
What about an increased likelihood of pyroxene crystals from the added
magnesium in talc? Low alumina will help, too and the extra silica in the
talc may give the crystals seed points. I don't fully understand the role
of boron in the development or prevention of crystals or color with iron,
so maybe not.
----------------------------------------------------------------------------
-----------
Hi Ruth:
What's pyroxene? It's really hard to find a good definition of the
pyroxene mineral group but it seems to be a widely varied bunch of stuff. I
looked at some pyroxene minerals in my mineralogy text and the composition
can be quite different. Anyway, the simplest and purest form of the
pyroxene mineral group is enstatite which is magnesium silicate and forms
crystals in glazes with enough magnesia. I have a white matt that I use and
I get some really nice small enstatite "floaters" in a glassy matrix. Look
up enstatite in Hamer, if you haven't already.
Perhaps boron aids iron reds by developing calcium-borate crystals. I know
that this will happen and it seems that if you prevent calcium from
dissolving the iron, by forming crystals, the red is more likely to happen.
An undisclosed source told me that a student was working with Gibby's Wild
Rose Tenmoku, a lithium-nepy sy fluxed cone 6 iron red, and got the glaze to
do a really brilliant red by line blending in some gerstley borate. As I
recall, the CaO may have been low in this one. So it would seem that boron
is a player in iron reds. At least in oxidation.
I have no idea at all about iron contributing to glass formation or
durability. I looked through a few texts, including Hamer, but I came up
with "bupkis"!
I read Paul Lewing's post about iron reds he's done (Hi Paul!) and noted
that he uses a very high % of iron. Up to 18% which is saturating the glaze
and allowing some to precipitate on cooling to give the red. Anyway, I was
wondering if this % could be lowered if the calcium was cut back, or crystal
development could be enhanced by boron or some other oxide blend that would
encourage this. Of course, why change the glaze if it works? I was just
wondering....................................?
later, Craig Martell in Oregon
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