Steve Slatin on sun 17 jul 05
Forgive me, for I ramble.
The first thing to do is to take heart -- mixed
results for a first firing is a GREAT outcome. Lots
of folks get all bad.
Second, don't be distressed that a glaze that gives
one result in reduction gives another in oxidation.
This is fairly common. Is the glaze thicker where
it's milkier? If so, possibly that's the issue. I'm
not entirely sure I understand what you mean by
'splotchy' though. If they are random patches of
partial opacity in an otherwise clear glaze, it might
just be a case of not-quite-perfectly blended
ingredients in the mix -- especially if you mixed
without a suspending agent and let it settle and then
re-mixed but didn't sieve.
Third, are you using a stoneware body that you don't
have prior experience with? (I get unpredictible
results every time I change clay bodies. And
manufacturers don't always deliver clay as well-mixed
as you'd hope.) Also, some bodies might be better in
reduction than in oxidation.
As to the question on food-safe ware -- absolutely,
you don't need to go to cone 10. Painted china is
low-fire ware, and it's (if properly formulated)
perfectly safe. Likewise mid-fired ware.
And no, there isn't very much difference between the
final temperature of a cone 6 and a cone 10 firing.
But the glazes and clay bodies are different.
(Similar in materials, different in concentrations.)
That cone 6 to cone 10 temperature difference (maybe
150 F) is the last and hardest range for your kiln to
get to. It's like getting to 90 on a car; 20-30 takes
a few seconds, 80-90 takes quite a bit. And burns
lots of fuel. My impression of kiln element wear
(this is not science here, this is impression) is that
the formation of crystalline granularities in the
heating wire is a greater part of the deterioration of
the element than simple oxidation. The extended
high-heat and high-voltage exposure of a higher cone
firing encourages this rapid deterioration. In any
event, folks who keep records and fire to different
temps do show elements last much longer if you cut 100
degrees or so off the top.
I've met china doll creators who have NEVER changed
If you want to understand the nature of glaze
formulation and the safety issues I'd recommend
reading Mastering Cone 6 Glazes, by Hesselberth and
Roy. It is not a glaze recipe book*, though it has
some really good glazes in it, it's a book of
methodology, safety, glaze formulation and firing
techniques. The safety testing procedures are
especially well laid out, and, if you're like me,
you'll feel better using a "belt and suspenders"
approach -- avoid the more dangerous chemicals in the
first place and make sure your glazes don't leach as
well. This also increases the probability of
durability over time and with extensive use.
This gets us up to the what's-a-cone-5-clay-doing
under-a-cone-6 glaze question. When a manufacturer
rates a clay as cone 5 that's usually the minimum
heat-work at which the clay assumes completed (no
longer porous, fully dense packing of clay particles)
form. Many clay manufacturers give a range of
acceptable results, though, and there's rarely a clay
rated at one cone that won't perform pretty well at
one cone over the rated heat-work. So, generally
speaking, it's no big deal to fire a cone 5 clay to
cone 6. But check with the manufacturer first; if
there's a risk your use will end up with a lump melted
into a shelf you don't want it, and they don't want
you telling your friends ... so they let on what the
range is in which a clay will work. See also Big
Ceramic Store's page on this issue;
Good luck, and keep experimenting.
-- Steve Slatin
*If you do want a recipe book for cone 6, the Bailey
book is recommended by many folks, and I like it quite
a bit. It has small but useful pictures of literally
hundreds of variations on his recipes.
If you are going to stick with cone 10,
John Britt has a book devoted to that; I haven't
bought it yet, but his research and writing are
excellent (and he has totally mastered oil-spot
glazes), I can't believe it's not excellent. Consider
also getting Val Cushing's Handbook, which is a riot
of information, and has *gobs* of recipes.
--- Mary/Adams wrote:
> Help! I'm new at this stuff. Hope you all might
Steve Slatin --
Frail my heart apart and play me little Shady Grove
Ring the bells of Rhymney till they ring inside my head forever
Start your day with Yahoo! - make it your home page
Mary/Adams on sun 17 jul 05
Help! I'm new at this stuff. Hope you all might remember being here
yourselves and provide lots of good info? I just got my new Bailey electric
kiln and just opened my first Cone 10 glaze firing and have mixed results.
I have some really good test tiles having tested a
bunch of colors( and so that is a good thing) and some really pretty
porcelain pieces. But, most stone ware was awful and mostly due to the
clear glaze I used. I had used this glaze with much success in gas kiln
firings. I used a clear base glaze over them and it turned out milky and
splotchy. It was a base glaze that I had used for reduction and it was
listed for both reduction and oxidation. I am a newbie at all this
and so would so much appreciate your response and answers to the following
1. I'm of course very concerned about the cost of firing to cone 10
alot. I've read that it wears the kiln out quicker (seems weird because
there is not much difference in degrees from cone 5 and 6 to cone 10)
I'm interested in making tableware. Can you make food-safe tableware
at a lower cone? And can you get pretty colors? Would that make a
difference in cost and wear and tear on the kiln?
2. Can I get pretty bright colors at the lower cones.
3. Why do I find a lot about cone 6 firing and glazing but less about cone
5 when in fact I've found a cone 5 for tableware and it looks like cone 6
clay is for modelling?
3. Can anyone suggest a clear base glaze for oxidation in Cone 10 and
and 6? I want to make my own glazes.
Would appreciate any help I can get.
Snail Scott on mon 18 jul 05
At 02:27 PM 7/17/2005 -0700, you wrote:
>1. I'm of course very concerned about the cost of firing to cone 10
> alot. I've read that it wears the kiln out quicker (seems weird because
>there is not much difference in degrees from cone 5 and 6 to cone 10)
It does seem like a small difference in temperature,
but it's getting very close to the design limitations
of the kiln. A lot of it has to do with insulation.
The hottest temperature of the elements on 'high' is
constant; what makes the kiln get hotter and hotter
is the amount of heat retained in the insulating brick
shell. The higher the temperature you want, the longer
you have to pump that heat into the kiln. Meanwhile a
lot of it is radiating out. Also, the closer you get
to the temperature of the elements, the less change in
temperature you get for a given span of time. So, it
takes much longer raise the temperature from ^8 to ^10
than it does to raise it from ^010 to ^08. And it's
the time spent firing that wears out the elements.
>I'm interested in making tableware. Can you make food-safe tableware
> at a lower cone? And can you get pretty colors? Would that make a
>difference in cost and wear and tear on the kiln?
You cam make food-safe tableware at earthenware
temperatures, but if you don't want to go that low,
the mid-range stoneware temperatures (^4-6) are
gaining rapidly in popularity, in part because of
the reduced wear and tear on electric kilns. An
effect of this increased popularity is a newly-
expanded literature on that temperature range, and
many more glaze recipes, both published and on-line,
intended for that range as well.
Since fewer things melt at mid-range temperatures,
mid-range glazes tend to rely more in frits and
have slightly fewer options in materials choice,
but I find the shorter firings, reduced energy bills,
and the reduced stress on the equipment to be well
worth it. I don't miss ^10 at all since I gave it
up (around 1993). But, if you really want to stick
with ^10, and your reasons are good, then do it. The
added costs will simply be part of a higher overhead,
just as it would you were using a pricey grolleg
clay, or using glazes with lots of cobalt. Your
As for color, mid-range actually allows for a
greater range of color, since many stains which
would burn out at ^10 are viable at ^5.
>3. Why do I find a lot about cone 6 firing and glazing but less about cone
>5 when in fact I've found a cone 5 for tableware and it looks like cone 6
>clay is for modelling?
No reason at all. Once upon a time, mid-range stoneware
was much less popular, and ^10 was the dominant cone
for 'real' ceramics. So, there was no real consensus
as to what the 'usual' mid-range cone should be. So,
each manufacturer tended to pick what someone on their
staff preferred, or what a major customer asked for.
So, some had ^4 clay, or ^5, or ^6, and some just called
their clay ^4-^10 as though underfiring their ^10 clay
would work just as well. (It won't.) Lately, ^6 has
sort of settled out as the consensus, but that's
relatively recent, and many manufacturers are still
making their old recipes for ^5 or ^4 or whatever.
It's got absolutely nothing to do with throwing versus
handbuilding, or pottery versus sculpture. You're just
seeing some manufacuter's recipe that happens to be ^5
and also happens to be designed for pottery, and
another recipe that happens to be ^6 and for sculpture.
There are just as many ^6 clays for pottery as for
sculpture - you just haven't run across them. Perhaps
you local supplier just doesn't stock them. Look further
afield - they're out there.
(Also, many commericially-made glazes are now available
for the mid-range. Making your own has many benefits,
but it's also nice to be able to dip into the pool of
manufacutred products for this-and-that.)
That said, when someone calls a clay ^6 or ^5 or some
such, that may not meet what you would consider to be
its best temperature. The same goes for glazes. It
may say ^5, but you like it better at ^6, etc. With
any new clay or glaze, you're going to have to test,
so use those ratings as a starting point, not an
>3. Can anyone suggest a clear base glaze for oxidation in Cone 10 and
> and 6?
For ^5-6, try Tony Hansen's 5x20 as a start. It's in
the archives. Very easy, and easy to modify.
Your old ^10 reduction glaze will probably work just
fine in oxidation. I would bet that the reason it's
not working now is that electric kilns are (as I
mentioned) quite poorly insulated compared with most
fuel-firing (reduction) kilns, and when you shut them
off, they basically crash-cool. You may have been
relying on the thermal mass and insulation of your
old reduction kiln to give a little extra 'cooking
time', before. So, just fire with a long soak at the
top, to make up for it. I'll bet you can get it to
work pretty much like it used to.
Bear in mind that reduction causes iron to flux,
and a glaze over a high-iron body in reduction will
melt a bit more than the same glaze over the same
clay in oxidation.