Hank Murrow on mon 26 feb 07
On Feb 26, 2007, at 5:48 PM, Bob Johnson wrote:
> Hi, Stephani--
>
> My understanding is that obsidian is essentially the same as pumice
> or volcanic ash, which is readily available to potters.
Obsidian is chemically very like volcanic ash, but it was formed
differently from ash.
I once took a
> geology course, but I can't remember the precise distinction between
> pumice, rhyolite, and some other volcanic products. As I recall, they
> are all various forms of basalt.
Actually, there is a continuum from Rhyolite to Basalt, with Rhyolite
having mostly alkaline content, low alumina, and little to no iron;
while basalt is the highest in iron, magnesia and alumina....... no
alkaline content. Or....white to black if you will.
>
> I've done some experiments with pumice collected from various
> volcanoes in the Northwest, and I've found different colors. None
> were very striking---but I am intrigued by the possibility of using
> native/local materials in glazes, particularly ash from Mt. Mazama
> (Crater Lake) near where I live.
I have made deeply colored shinos from Mazama ash. Jim Robinson of
Phoenix, OR remains the best reference for the rocks useful to potters
in OR and CA. Suggest you get in touch with him.
Cheers, Hank
www.murrow.biz/hank
Bob Johnson on mon 26 feb 07
Hi, Stephani--
My understanding is that obsidian is essentially the same as pumice
or volcanic ash, which is readily available to potters. I once took a
geology course, but I can't remember the precise distinction between
pumice, rhyolite, and some other volcanic products. As I recall, they
are all various forms of basalt.
I've done some experiments with pumice collected from various
volcanoes in the Northwest, and I've found different colors. None
were very striking---but I am intrigued by the possibility of using
native/local materials in glazes, particularly ash from Mt. Mazama
(Crater Lake) near where I live.
A few years ago, many NW potters played around with St. Helens ash,
but many of the glazes were heavily doctored with colorants and other
additives, so they weren't true to their label.
Does anyone have a recipe, in the ^6-^10 range, for an interesting
volcanic ash glaze? I would be happy to trade for a good ^6 R wood ash glaze.
Bob - in Southern Oregon
At 2/26/2007 01:22 PM, you wrote:
>Thinking about obsidian, a naturally occurring glass
>(r hardness of 5 to 5.5, density 2.6.)
>rich in silica, and usually iron and some manganese
>too....
>
>is ground or crushed obsidian ever used in formulation
>of glazes or as an additive to a clay body?
>
>i'm wondering if it would melt easier than it's
>relatives: granite or rhyolite, because it is already
>in a glassy state ( i.e. like ground frit )
>
>have no idea if this would be true and i wonder if
>anyone has heard of it's use?
>
>or perhaps it occcurs too rarely or is too problematic
> to be feasible
>for industry...
>
>I remember an alkaline lake bed in N.Nevada, strewn
>with what looked like white pebbles. the pebbles and
>rocks turned out to be black obsidian, with a white
>alkaline crust...and of course the beautiful Glass
>buttes in oregon. Though of course , you leave the
>obsidian in place when you visit...
>
>never seen reference to it in ceramics...
>just curious
>
>Stephani Stephenson
>
>
>
>
>____________________________________________________________________________________
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>(and love to hate): Yahoo! TV's Guilty Pleasures list.
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>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
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>
>Moderator of the list is Mel Jacobson who may be reached at
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Pamela Regentin on mon 26 feb 07
Years ago I was doing a geology homeschool project with my son and we crushed basalt and I fired the result in little unglazed stoneware bowls to ^10 oxidation. The result were pools of a beautiful brown glaze. Still have the tests somewhere...
Pam
In the shadow of Mt. Hood, Oregon
Bob Johnson wrote: Hi, Stephani--
My understanding is that obsidian is essentially the same as pumice
or volcanic ash, which is readily available to potters. I once took a
geology course, but I can't remember the precise distinction between
pumice, rhyolite, and some other volcanic products. As I recall, they
are all various forms of basalt.
I've done some experiments with pumice collected from various
volcanoes in the Northwest, and I've found different colors. None
were very striking---but I am intrigued by the possibility of using
native/local materials in glazes, particularly ash from Mt. Mazama
(Crater Lake) near where I live.
A few years ago, many NW potters played around with St. Helens ash,
but many of the glazes were heavily doctored with colorants and other
additives, so they weren't true to their label.
Does anyone have a recipe, in the ^6-^10 range, for an interesting
volcanic ash glaze? I would be happy to trade for a good ^6 R wood ash glaze.
Bob - in Southern Oregon
At 2/26/2007 01:22 PM, you wrote:
>Thinking about obsidian, a naturally occurring glass
>(r hardness of 5 to 5.5, density 2.6.)
>rich in silica, and usually iron and some manganese
>too....
>
>is ground or crushed obsidian ever used in formulation
>of glazes or as an additive to a clay body?
>
>i'm wondering if it would melt easier than it's
>relatives: granite or rhyolite, because it is already
>in a glassy state ( i.e. like ground frit )
>
>have no idea if this would be true and i wonder if
>anyone has heard of it's use?
>
>or perhaps it occcurs too rarely or is too problematic
> to be feasible
>for industry...
>
>I remember an alkaline lake bed in N.Nevada, strewn
>with what looked like white pebbles. the pebbles and
>rocks turned out to be black obsidian, with a white
>alkaline crust...and of course the beautiful Glass
>buttes in oregon. Though of course , you leave the
>obsidian in place when you visit...
>
>never seen reference to it in ceramics...
>just curious
>
>Stephani Stephenson
>
>
>
>
>____________________________________________________________________________________
>We won't tell. Get more on shows you hate to love
>(and love to hate): Yahoo! TV's Guilty Pleasures list.
>http://tv.yahoo.com/collections/265
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
>melpots@pclink.com.
______________________________________________________________________________
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
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Michael McDowell on tue 27 feb 07
Stephanie and others have been speculating on the relative merits of
obsidian and volcanic ash as a glaze material. I feel the need to join in
the response since I have been using ash from the 1980 eruption of Mt. St.
Helens in Washington State for decades now. So has Michael Wendt and many
others. It makes a nice glaze at cone 10 with some reduction by itself, or
maybe a couple or three percent bentonite addition to help bind the glaze
coat and float the glaze slurry. The chemical analysis of Volcanic Ash can
vary widely around the globe. Out here on the Pacific Rim of the North
American Continent we are "blessed" with volcanoes of the high silica
variety. These are the type of volcano that can produce obsidian, but also
explosions. As I understand it, the difference between say volcanic ash,
pumice, and obsidian from one of these high silica volcanoes is strictly
structural, not chemical. Mostly associated with the quantity of gasses
trying to escape from the volcano simultaneously with the molten rock. It
seems to make a certain sense then to choose the form that will require the
least effort to make it ready to apply to our pots. So I would always be
biased toward the ash from a good distance away so the larger particles have
had a chance to settle out. Of course if you have the crushing and grinding
equipment laying around to make powder out of obsidian... I like thinking
I'm doing just the opposite.
Michael McDowell
Whatcom County, WA, USA
michael@mcdowellpottery.com
http://www.McDowellPottery.com
Timothy Joko-Veltman on tue 27 feb 07
On 2/26/07, Bob Johnson wrote:
> Does anyone have a recipe, in the ^6-^10 range, for an interesting
> volcanic ash glaze? I would be happy to trade for a good ^6 R wood ash glaze.
Bob,
I'll attach a couple of interesting possibilities below, but it might
be most interesting to do a line-blend or triaxial. Especially since,
as you know, not all ashes are alike - not to mention feldspars and
kaolins.
Regards,
Tim
Supereasy Ash Glaze
56 Feldspar (Soda or Potash)
40 Ash
4 Bentonite
Easy High-Fire Ash Glaze
30 Feldspar
30 Ash
25 Quartz
15 Kaolin
sacredclay on tue 27 feb 07
Hi Bob, Don't know if this one has been used, but it's called Ryoko
Cream ^6 ox Recipe follows as :
Volcanic Ash------63.47
Gerstley Borate---35.55
Bentonite---------.98
Add
RIO---------------1.5
Tin Oxide---------3.5
This is a very forgiving glaze, meaning that just about anything works
with it. It very closely resembles a rust breaaking cream. It's warm
brown, light RR's Nutmeg, and cream where thick. You can rub some of
the glaze off and it'll be browner. Textures show up wonderfully under
it. Iron wash is great under it, copper wash bleeds under it. Contrasts
nicely with other glazes. Hope you like it. It's a fave of mine, up
there with Nutmeg. I'm curious to know what Ron says about it.
(hint,hint, Ron!) No need to trade. In the words of a well-known imp in
the mountains near me, pass on the love! Warmly Kathryn in NC
> Does anyone have a recipe, in the ^6-^10 range, for an interesting
> volcanic ash glaze? I would be happy to trade for a good ^6 R wood
ash glaze.
>
> Bob - in Southern Oregon
>
>
>
Bob Johnson on tue 27 feb 07
Thanks! I'll give it a try---in reduction and probably without the
RIO and tin the first time, to get a sense of the base color. I'll
let you know what I find.
Bob
At 2/26/2007 09:13 PM, you wrote:
>Hi Bob, Don't know if this one has been used, but it's called Ryoko
>Cream ^6 ox Recipe follows as :
>Volcanic Ash------63.47
>Gerstley Borate---35.55
>Bentonite---------.98
>Add
>RIO---------------1.5
>Tin Oxide---------3.5
>
>This is a very forgiving glaze, meaning that just about anything works
>with it. It very closely resembles a rust breaaking cream. It's warm
>brown, light RR's Nutmeg, and cream where thick. You can rub some of
>the glaze off and it'll be browner. Textures show up wonderfully under
>it. Iron wash is great under it, copper wash bleeds under it. Contrasts
>nicely with other glazes. Hope you like it. It's a fave of mine, up
>there with Nutmeg. I'm curious to know what Ron says about it.
>(hint,hint, Ron!) No need to trade. In the words of a well-known imp in
>the mountains near me, pass on the love! Warmly Kathryn in NC
>
>
> > Does anyone have a recipe, in the ^6-^10 range, for an interesting
> > volcanic ash glaze? I would be happy to trade for a good ^6 R wood
>ash glaze.
> >
> > Bob - in Southern Oregon
> >
> >
> >
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
>melpots@pclink.com.
Joseph Herbert on sun 4 mar 07
"Obsidian is chemically very like volcanic ash, but it was formed
differently from ash.
I once took a > geology course, but I can't remember the
precise distinction between > pumice, rhyolite, and some other volcanic
products. As I recall, they > are all various forms of basalt.
Actually, there is a continuum from Rhyolite to Basalt, with
Rhyolite having mostly alkaline content, low alumina, and little to no iron;
while basalt is the highest in iron, magnesia and alumina....... no alkaline
"
Hank,
True. But... There is a more important continuum between Rhyolite and
Basalt - The total amount of Quartz (SiO2) present. Large in Rhyolite (70%)
small in Basalt (Less than 50%)
Rhyolite is the fine grained (aphanitic) equivalent of Granite. Basalt is
the equivalent of Gabbro. Same composition, different grain size. Obsidian
has no grain size (except for the snow flakes in the tears, actually
crystals) because it is glass. As we know from other discussions, lots of
silicon dioxide is required to make a good glass (read glaze), and the more
"granitic" composition of rock has lots more SiO2. Rapidly cooled samples
of molten rock of a high silica composition will form a nice stable glass.
Basaltic composition molten material, not so much.
Pumice and scoria, porous, foamy rocks, are erupted as liquid with high gas
content causing the foam formation. Pumice is usually lighter in color
(grey) and "glassier" than scoria which tends to be red and dull. Pumice
tends to have composition on the high silica end of the range while scoria
is higher iron, lower silica.
One of the other aspects of the High silica - Low silica continuum is molten
rock viscosity. High silica magmas (molten bodies of rock) tend to be very
viscous. Low silica magmas are not. Hence we see the spectacle of flowing
rivers of molten rock in Hawaii - the magma is of basaltic, low silica,
composition and flows easily. Easily is a relative term here but the liquid
also has a density of nearly 3 making gravity work extra hard. In the case
of granitic "lavas" the viscosity is very high. There is a recorded
occurrence of a "Volcanic Spine" in which a column of "molten" rock was
extruded vertically some 1300 feet. It glowed, it flowed (some, straight
up), it was hotter than the melting point of the rock., it just wouldn't
fall over.
More commonly, high silica rock compositions are explosive Mt. St. Helens
type eruptions. The rock is melted but it also contains lots of volatiles,
like water or carbon dioxide. These contribute significantly to lowering
the viscosity of the melt, allowing it to move more easily in the
sub-surface. As the rock melt nears the surface in an eruption, the
pressure is reduced and the gases start to exsolve and form bubbles. Since
the gassy materials were significant in keeping the rock more fluid, the
process of bubble formation also stiffens the material (glass) in the bubble
skin. Further expansion of the bubble requires breaking the bubble. Hence,
"Volcanic Ash" actually the glassy shards of the broken bubbles of high
silica molten rock.
There is more about volcanic ash composition on Tony Hansen's Digitalfire
web site. In the glaze materials section, I think.
Joseph Herbert
PS. I left a previous post about volumetric mixing of glazes incomplete.
My final point was that the glazes we use are glass and as such are
forgiving in terms of formulation. If there is sufficient silica present,
there can be pretty wide variations in the amounts of the other materials
and a glass will still form. If the materials in the glaze are all about
the same density, a volumetric method of measurement will come close to
weighing. Mel pointed out he weighs colorants. Well, they generally have
higher densities than most glaze components. A volumetric measurement would
put too much into the mix.
Glasses are forgiving, but there are limits. By using volumetric measures,
you add one more variable to the already considerable list we deal with.
When trying to figure out what went wrong sometime, that extra variable may
be just that much too much.
JH
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