Hightower Pottery
unique porcelain and stoneware
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For me, the draw of crystalline pottery comes
from the unique nature of each piece. Even when
throwing similar forms, once fired, each piece takes
on it's own character. This is due to the crystalline
formations within the glaze. Crystalline glazes really
have a mind of their own. Unfortunately this
unpredictability means the failure rate is high. Even
after all the extra work that is required, only half
make it to the end.
The first step in crystalline glazes is of coarse
acquiring materials. The raw chemicals used in the
macro-crystalline process are a bit pricey. The most
expensive component to these glazes is Zinc Oxide.
Zinc Oxide seems to have doubled in price in the last
few years. Also required in my particular glaze what
is called a frit. A frit is a glass that has been mixed
from raw powdered minerals and then melted. Once
cooled rapidly in water it is ground into a fine powder.
There are many frits available for a variety of
purposes. One purpose of fritting materials is to
reduce their melting temperature and improve melt
predictability. Although I fire my crystalline between
cone 9 and cone 10, this powerful fluxing ability is
ideal as crystalline glazes must become quite fluid.
Also, there is very little alumina in the frit that I use.
from the unique nature of each piece. Even when
throwing similar forms, once fired, each piece takes
on it's own character. This is due to the crystalline
formations within the glaze. Crystalline glazes really
have a mind of their own. Unfortunately this
unpredictability means the failure rate is high. Even
after all the extra work that is required, only half
make it to the end.
The first step in crystalline glazes is of coarse
acquiring materials. The raw chemicals used in the
macro-crystalline process are a bit pricey. The most
expensive component to these glazes is Zinc Oxide.
Zinc Oxide seems to have doubled in price in the last
few years. Also required in my particular glaze what
is called a frit. A frit is a glass that has been mixed
from raw powdered minerals and then melted. Once
cooled rapidly in water it is ground into a fine powder.
There are many frits available for a variety of
purposes. One purpose of fritting materials is to
reduce their melting temperature and improve melt
predictability. Although I fire my crystalline between
cone 9 and cone 10, this powerful fluxing ability is
ideal as crystalline glazes must become quite fluid.
Also, there is very little alumina in the frit that I use.
| Crystalline Glazes, the technical stuff. |
Alumina stiffens a glaze and makes it impossible for crystals to grow. Eliminating it from a glaze that is already
being fired well past it's normal mature point makes for an even runnier glaze. Finally there is silica which is
used in my crystalline glazes as a filler and to adjust expansion. That is a very basic start to putting together a
simple zinc-silicate crystalline glaze. There are some additional chemicals I use in my glaze for fluxing and
application purposes. The best thing to is start experimenting.
being fired well past it's normal mature point makes for an even runnier glaze. Finally there is silica which is
used in my crystalline glazes as a filler and to adjust expansion. That is a very basic start to putting together a
simple zinc-silicate crystalline glaze. There are some additional chemicals I use in my glaze for fluxing and
application purposes. The best thing to is start experimenting.
As I mentioned, crystalline glazes are quite
runny. If fact, if you don't fire crystalline properly you
WILL ruin your shelves and probably be repairing (or
rebuilding) your kiln floor. Did I mention crystalline
glazes were expensive? The best way to keep this
from happening is to throw or hand-build crystalline
glaze catchers. These are stoneware or porcelain
saucers that go under the pot like a drip pan. Of
course if you just place the pottery on the crystalline
glaze catcher, you will end up with a pot fused to the
catcher by glaze. There are several methods for
elevating the pot from the catcher. The easiest is to
use a cut piece of firebrick. I have never tried this as
crystalline glazes will eat right through firebrick that is
not properly kiln washed. When I started working in
crystalline glazes, I took an old trick I learned from
salt firing. Using wads of silica and kaolin, I was able
to easily separate the catcher. Actually, the
crystalline pot would just pull right away from it. The
main problem with this method was the excessive
grinding required to finish the bottom of the pot. For
me, the Cadillac of methods would be the porcelain
ring. A porcelain ring is thrown to match the bottom of
the pot. The pot and ring are set on the crystalline
glaze catcher and the whole thing is loaded into the
kiln. During the firing the crystalline glaze runs down
the pottery, over the foot, and into the catcher. After
firing, a few taps of the hammer and maybe a little
torch action around the ring and you have a freed
piece of crystalline pottery ready for just a slight bit of
grinding around the edge. That is provided you did it
right. If you did not, don't put away that hammer just
yet. It has one more task.
runny. If fact, if you don't fire crystalline properly you
WILL ruin your shelves and probably be repairing (or
rebuilding) your kiln floor. Did I mention crystalline
glazes were expensive? The best way to keep this
from happening is to throw or hand-build crystalline
glaze catchers. These are stoneware or porcelain
saucers that go under the pot like a drip pan. Of
course if you just place the pottery on the crystalline
glaze catcher, you will end up with a pot fused to the
catcher by glaze. There are several methods for
elevating the pot from the catcher. The easiest is to
use a cut piece of firebrick. I have never tried this as
crystalline glazes will eat right through firebrick that is
not properly kiln washed. When I started working in
crystalline glazes, I took an old trick I learned from
salt firing. Using wads of silica and kaolin, I was able
to easily separate the catcher. Actually, the
crystalline pot would just pull right away from it. The
main problem with this method was the excessive
grinding required to finish the bottom of the pot. For
me, the Cadillac of methods would be the porcelain
ring. A porcelain ring is thrown to match the bottom of
the pot. The pot and ring are set on the crystalline
glaze catcher and the whole thing is loaded into the
kiln. During the firing the crystalline glaze runs down
the pottery, over the foot, and into the catcher. After
firing, a few taps of the hammer and maybe a little
torch action around the ring and you have a freed
piece of crystalline pottery ready for just a slight bit of
grinding around the edge. That is provided you did it
right. If you did not, don't put away that hammer just
yet. It has one more task.
More to come.
