search  current discussion  categories  glazes - misc 

> unglazed work

updated fri 28 feb 97

 

The Shelfords on tue 25 feb 97

Hi Leslie -

Re: >I'm interested in initiating a discussion on the different
>>ways of handling functional work that isn't glazed.

My experience has been, that clay that isn't glazed has, shall we say,
limited function. Terra cotta in flower pots and such makes use of the fact
that it is porous, so in anything where porosity is an advantage (flower
pots, clay bakers, wine or water coolers, etc.) unglazed clay, particularly
terra cotta, is functional. Higher fire stuff, that matures ideally at a
non-porous state, will still be susceptible to surface staining and
absorption of oil. And if the firing is less than to an ideal maturity -
i.e. most studio work - there will be that much more staining or potential
porousness. I've seen unglazed butter dishes that were a really bad idea
from a functional point of view - no matter how often the owner washed them
in boiling water, they always had a smell of rancid oil. I saw some
high-fire porcelain agate-ware vases in Britain which the maker swore were
perfectly water-tight, but I've had no experience with that. My own
agate-ware (stoneware), when unglazed on the inside, is always identified as
"semi-functional" - ok for holding fruit or anything dry, but not for wet or
oily things, and of course, mixed clay tends to be more susceptible to
thermal shock. But if glazed on the inside, it is much more functional,
though still not able to withstand thermal shock.

Re: > forms made of two colors of clay thrown together. What
>>have others done when they want the clay's appearance and pure form to
>>speak for the piece?
My first real "hook" into clay work was a photograph of a mixed clay coiled
and burnished pot by Ann Harris, UK, printed in "The Potter's Manual" by
Kenneth Clark (pg. 15). It set me to dreaming of the possibilities... And
Robin Hopper has done a lot of agate-ware or mixed clay work - the detail of
a plate on the cover of his "Ceramic Spectrum" is a case in point, as well
as a discussion in chapter 19 of same.

I have spent a few years experimenting with various technniques and effects
of mixed clay, and it has WONDERFUL possibilities, but I have found it very
time-consuming and there is usually a high percentage of loss. These are
steps involved in the type of work I have done:
1. Testing your various clays for shrinkage, and how they take colour from
stains and oxides. And if you are using oxides, you need to test how much
you can safely add before they begin to flux the clay and lower the firing
range, or perhaps boil, spit, fume, etc. into the surrounding clay. These
may or may not be effects that you want.
2. Mixing stains or oxides into the clay takes time whether you are talking
small amounts or large. You can avoid this if you are content to work with
natural clays. I have done several series of wood-grain-like pieces (not
consciously copying wood, but that was the effect) which were very
effective, using a white, a tan, a medium-red, a dark red, and a black clay.
3. Experimenting with patterns of putting the clay together before throwing
(or handbuilding - but I haven't done that with agate).
4. Throwing is slow and takes practice, as you are working with unwedged
clay, where there are BOUND to be air bubbles trapped between the layers of
colour. Also, unless you are content with variations on the barber pole or
candy cane, you will need to experiment a lot with throwing techniques to
vary the patterns. An ability to throw with the wheel going in either
direction can be very useful.
5. If you are going to use Robin Hopper's technique, i.e. faceting or
fluting to produce semi-repetitive patterns, you probably won't need to trim
the rest of the surface as well. Or, if you want a softer, fuzzier line of
colour or pattern, you can trim once over the entire surface, then burnish
lightly to increase the "impressionist" effect and to smooth out the
trimming lines. Otherwise, trimming is done over the entire unglazed
surface, and has to be done at least a couple of times, usually more, during
the drying period. I find I run through trimming tools at the rate of about
3 largish pots to a tool, because the final trimming is done when the pot is
much dryer than normal. This is to avoid smudging the colours, as well as
to remove the earlier trimming lines, which become exaggerated as the pot
dries. It is easy to lose pieces at this stage, as the vibration stress on
the walls is considerable.
6. If you are going to glaze the inside (which I usually do) either glaze
VERY carefully with NO drips, or else wax the entire unglazed surface. This
is for 2 reasons - if you get even the smallest amount of glaze on the
outside, you won't be able toremove it fully, because a trimmed surface has
open pores, and the glaze gets in there and WON'T get out. And secondly,
you can't have just a rim of wax, or you will have a very visible line on
the unglazed fired surface, where the wax protected against fuming.
7. Figure that agate will probably fire a cone or so below where the
individual clays would do. I'm not sure why - probably the fluxing action
of the oxides - maybe that won't be the case with stains, I don't know,
haven't used them.
8. I usually sand the piece all over the unglazed surface after final
firing, but that will depend on what you are trying to do. Maybe roughness
is part of the character of the piece.

Sorry - this seems to have gone on and on, but it hardly scratches the
surface of the subject. Although each piece takes a lot of time, each one
is unique and can be priced accordingly, so it all evens out. I love the
unique effects, and the natural shades and patterns.

I hope others will share their experience as well - I'm looking forward to
getting back into this when my studio is up and running again, and maybe I
can branch out into new directions. Thank you for raising the subject!

Veronica on Thetis Island in the Gulf of Georgia in BC
e-mail: shelford@island.net

Peggy Heer on wed 26 feb 97

Hi Veronica...I just want to add one more point to your excellent list of
how-to....

If you put the balls of different colored/textured clays together at least
2 to 4 weeks ahead of the time (store in double plastic bags) you are
going to use them, you will find that you will not get the cracking or
breakage that you would get if you just mix the clays and use right away.
If the clays have time to get to know each other, take on the moisture and
consistancy of the others, get 'married', you will find that the throwing
is much easier and the cracking will be at a minimum.
This was one of the first techn. I tried in the early 70's and it was by
accident that I found this little but most important tid bit. I never had
any cracking thereafter.
Thanks for bringing back memories of a great techn.
As Always in Clay and half way through gutting my studio. House is packed.
;>}}}}}}} Peggy




>----------------------------Original message----------------------------
>Hi Leslie -
>
>Re: >I'm interested in initiating a discussion on the different
>>>ways of handling functional work that isn't glazed.
>
>My experience has been, that clay that isn't glazed has, shall we say,
>limited function. Terra cotta in flower pots and such makes use of the fact
>that it is porous, so in anything where porosity is an advantage (flower
>pots, clay bakers, wine or water coolers, etc.) unglazed clay, particularly
>terra cotta, is functional. Higher fire stuff, that matures ideally at a
>non-porous state, will still be susceptible to surface staining and
>absorption of oil. And if the firing is less than to an ideal maturity -
>i.e. most studio work - there will be that much more staining or potential
>porousness. I've seen unglazed butter dishes that were a really bad idea
>from a functional point of view - no matter how often the owner washed them
>in boiling water, they always had a smell of rancid oil. I saw some
>high-fire porcelain agate-ware vases in Britain which the maker swore were
>perfectly water-tight, but I've had no experience with that. My own
>agate-ware (stoneware), when unglazed on the inside, is always identified as
>"semi-functional" - ok for holding fruit or anything dry, but not for wet or
>oily things, and of course, mixed clay tends to be more susceptible to
>thermal shock. But if glazed on the inside, it is much more functional,
>though still not able to withstand thermal shock.
>
>Re: > forms made of two colors of clay thrown together. What
>>>have others done when they want the clay's appearance and pure form to
>>>speak for the piece?
>My first real "hook" into clay work was a photograph of a mixed clay coiled
>and burnished pot by Ann Harris, UK, printed in "The Potter's Manual" by
>Kenneth Clark (pg. 15). It set me to dreaming of the possibilities... And
>Robin Hopper has done a lot of agate-ware or mixed clay work - the detail of
>a plate on the cover of his "Ceramic Spectrum" is a case in point, as well
>as a discussion in chapter 19 of same.
>
>I have spent a few years experimenting with various technniques and effects
>of mixed clay, and it has WONDERFUL possibilities, but I have found it very
>time-consuming and there is usually a high percentage of loss. These are
>steps involved in the type of work I have done:
>1. Testing your various clays for shrinkage, and how they take colour from
>stains and oxides. And if you are using oxides, you need to test how much
>you can safely add before they begin to flux the clay and lower the firing
>range, or perhaps boil, spit, fume, etc. into the surrounding clay. These
>may or may not be effects that you want.
>2. Mixing stains or oxides into the clay takes time whether you are talking
>small amounts or large. You can avoid this if you are content to work with
>natural clays. I have done several series of wood-grain-like pieces (not
>consciously copying wood, but that was the effect) which were very
>effective, using a white, a tan, a medium-red, a dark red, and a black clay.
>3. Experimenting with patterns of putting the clay together before throwing
>(or handbuilding - but I haven't done that with agate).
>4. Throwing is slow and takes practice, as you are working with unwedged
>clay, where there are BOUND to be air bubbles trapped between the layers of
>colour. Also, unless you are content with variations on the barber pole or
>candy cane, you will need to experiment a lot with throwing techniques to
>vary the patterns. An ability to throw with the wheel going in either
>direction can be very useful.
>5. If you are going to use Robin Hopper's technique, i.e. faceting or
>fluting to produce semi-repetitive patterns, you probably won't need to trim
>the rest of the surface as well. Or, if you want a softer, fuzzier line of
>colour or pattern, you can trim once over the entire surface, then burnish
>lightly to increase the "impressionist" effect and to smooth out the
>trimming lines. Otherwise, trimming is done over the entire unglazed
>surface, and has to be done at least a couple of times, usually more, during
>the drying period. I find I run through trimming tools at the rate of about
>3 largish pots to a tool, because the final trimming is done when the pot is
>much dryer than normal. This is to avoid smudging the colours, as well as
>to remove the earlier trimming lines, which become exaggerated as the pot
>dries. It is easy to lose pieces at this stage, as the vibration stress on
>the walls is considerable.
>6. If you are going to glaze the inside (which I usually do) either glaze
>VERY carefully with NO drips, or else wax the entire unglazed surface. This
>is for 2 reasons - if you get even the smallest amount of glaze on the
>outside, you won't be able toremove it fully, because a trimmed surface has
>open pores, and the glaze gets in there and WON'T get out. And secondly,
>you can't have just a rim of wax, or you will have a very visible line on
>the unglazed fired surface, where the wax protected against fuming.
>7. Figure that agate will probably fire a cone or so below where the
>individual clays would do. I'm not sure why - probably the fluxing action
>of the oxides - maybe that won't be the case with stains, I don't know,
>haven't used them.
>8. I usually sand the piece all over the unglazed surface after final
>firing, but that will depend on what you are trying to do. Maybe roughness
>is part of the character of the piece.
>
>Sorry - this seems to have gone on and on, but it hardly scratches the
>surface of the subject. Although each piece takes a lot of time, each one
>is unique and can be priced accordingly, so it all evens out. I love the
>unique effects, and the natural shades and patterns.
>
>I hope others will share their experience as well - I'm looking forward to
>getting back into this when my studio is up and running again, and maybe I
>can branch out into new directions. Thank you for raising the subject!
>
>Veronica on Thetis Island in the Gulf of Georgia in BC
>e-mail: shelford@island.net

Peggy Heer / Heer Pottery E-Mail p4337@connect.ab.ca
Edmonton AB, Canada
http://www.ffa.ucalgary.ca/artists/pheer/