Dana Henson on sat 17 oct 98
Has anyone ever seen vertical cracks in a coil pot? I've seen the typical
horizontal cracks that appear when coils aren't joined very well, but small
vertical cracks? The pot in question was smoothed inside and out.
Puzzled and not very experienced with coiling...
Dana Henson
Claudia O Driscoll on sun 18 oct 98
Dana,
When you add coils, do you join them in the same place? If you have the ends of
What kind of clay do you use? That could be a clue.
claudia
Dana Henson on wed 21 oct 98
Claudia,
No, I don't necessarily join the coils in the same place...sometimes it
happens that way but is not planned.
The clay body is a cone 10 white stoneware with sand added and is actually
quite plastic.
Are you saying that if you join the ends of the coils each time you add a
layer as opposed to just coiling continuously that this could cause vertical
cracks in the pot?
Dana Henson
John Guerin on thu 22 oct 98
In a message dated 10/21/98 8:59:20 AM PST, G_HENSON@VENUS.TWU.EDU writes:
<< Claudia,
No, I don't necessarily join the coils in the same place...sometimes it
happens that way but is not planned.
The clay body is a cone 10 white stoneware with sand added and is actually
quite plastic.
Are you saying that if you join the ends of the coils each time you add a
layer as opposed to just coiling continuously that this could cause vertical
cracks in the pot?
Dana Henson
>>
When you say "with sand added", do you literally mean sand? Sand is not the
best temper (grog). Because of the way sand is produced, it has rounded
edges. Even prehistoric potters realized this through trial-and-error and
switched to ground up volcanic rock. The sharp edges of the volcanic rock
between the microscopic platelets of clay provide a better temper. Of course
the down side of grinding up your own rocks is that you aren't sure what other
minerals you may have included in your homemade temper. The native americans
of today solve this problem by grinding up the old pot shards they find laying
around on the ground. Out here in the Southwest there are lots of those from
prehistoric and historic pottery. This type of temper is very safe because it
has already been fired once and will not produce any adverse reactions in the
new pottery. The second advantage to this type temper is that since they
believe the old pottery possesses a spirit that is a combination of 'Mother
earth' and the original potter, they are including the spirit of their
ancestors in the new pottery with their own spirit. However collecting the
old pottery shards by non-Indians is illegal on state or federal land and even
on private land without proper permission, I purchase my temper from a ceramic
supply house. They call it Grog and it comes in Coarse (30 mesh), Medium (60
mesh) and Fine (150 mesh). The Grog is just ceramic material that has been
fired and ground up and screened to a particular mesh. Using such a
commercial Grog could help eliminate your vertical cracks. I tell my students
that horizontal cracks means they haven't properly blended their coils, but
vertical cracks means they didn't use enough temper. When I take my new
students out to dig clay we find it in veins where it is pure clay and have to
add up to 30% temper to it to provide a good "body" that will stand up to hand
building and will not crack during drying and firing.
John Guerin
Tucson, AZ
| |
|
