Bottoms becoming rounded in firing

Hi all!

Been away for a while, but now I am back! I got a kiln for my birthday -
just a little, used one, but it works a treat!

I make these round little things
http://ceramics.nsen.ch/slides/20060101-j-12.html
http://ceramics.nsen.ch/slides/20060101-j-13.html
and am having trouble with the bottoms.

I form them in a half-globe plaster form, so the basic shape is a ball, then
I flatten the bottom, and it stays flat all through drying, but then
remembers it was round when it gets fired. Lately, I have been pushing the
middle of the bottom up a little into the pot, but that isn't a very
satisfactory sollution for me. Any ideas on what I can do to keep the
bottoms flat through firing?

Marianne

Marianne,

I've been hobby-potting for only about 2 years now, so my
experience is limited.

The technique I use is to leave a slightly thicker bottom
wall on the form, and when it is leather hard, use a piece
of abrasive cloth (find in the drywall section of the
hardware store) to rub the bottom flat.

Hope that's useful.

Robert


Bubbles wrote:
Hi all!

Been away for a while, but now I am back! I got a kiln for my birthday -
just a little, used one, but it works a treat!

I make these round little things
http://ceramics.nsen.ch/slides/20060101-j-12.html
http://ceramics.nsen.ch/slides/20060101-j-13.html
and am having trouble with the bottoms.

I form them in a half-globe plaster form, so the basic shape is a ball, then
I flatten the bottom, and it stays flat all through drying, but then
remembers it was round when it gets fired. Lately, I have been pushing the
middle of the bottom up a little into the pot, but that isn't a very
satisfactory sollution for me. Any ideas on what I can do to keep the
bottoms flat through firing?

Marianne

"Robert Lackie" wrote in message
.. .
Marianne,

I've been hobby-potting for only about 2 years now, so my
experience is limited.

The technique I use is to leave a slightly thicker bottom
wall on the form, and when it is leather hard, use a piece
of abrasive cloth (find in the drywall section of the
hardware store) to rub the bottom flat.


Thanks, Robert. I was thinking maybe that would help. I tried making them a
little concave at early leather-hard, but that didn't seem to help. Cutting
or rubbing might well help. Will try this.

Marianne

"DKat" wrote in message
...
Those are lovely Marianne! There was a recent post I found interesting on
clayart where they were talking about putting feet on mugs. It seems that
you could use this method to 'indent' the center of the bottom of your pot
that would give you a 'foot'. I would use a wooden ball to tamp in the
center of the bottom of you piece


Thanks, D!

Thing is, I want the bottom to be flat on the table all the way round. Thus,
making those 3 dents will make "holes", right? I guess I could try doing it
a little further in - or maybe even just that the hitting will convince the
clay to forget? ;-)

Hubby also suggested I make convex "lenses" that I set the pots on to dry
and to fire, so that gravity will assist a little.

Marianne

"Bubbles" wrote in message
...

"DKat" wrote in message
...
Those are lovely Marianne! There was a recent post I found interesting
on
clayart where they were talking about putting feet on mugs. It seems
that
you could use this method to 'indent' the center of the bottom of your
pot
that would give you a 'foot'. I would use a wooden ball to tamp in the
center of the bottom of you piece


Thanks, D!

Thing is, I want the bottom to be flat on the table all the way round.
Thus,
making those 3 dents will make "holes", right? I guess I could try doing
it
a little further in - or maybe even just that the hitting will convince
the
clay to forget? ;-)

Hubby also suggested I make convex "lenses" that I set the pots on to dry
and to fire, so that gravity will assist a little.

Marianne

is it happening during final firing or bisquing? if the latter, you can take
a board and use spray adhesive to stick a piece of 220 grit sandpaper to it,
then use it to sand the bottom of the items. use a dust mask.

if the former, you could use some sort of lap wheel. there are small stained
glass laps, bit larger ones for lapidary uses, and really large ones for
doing rims of bowls, for example.

here's one i made that i use for both flattening the bottoms of ceramic
pieces, and rims of glass bowls:

http://groups.msn.com/chaniarts/wood...to&PhotoID=430

regards,
charlie
http://glassartists.org/chaniarts

"Charles Spitzer" wrote in message
...
is it happening during final firing or bisquing? if the latter, you can
take
a board and use spray adhesive to stick a piece of 220 grit sandpaper to
it,
then use it to sand the bottom of the items. use a dust mask.


Most often during bisque firing. Thing is, I am picky - and sanding leaves
traces, in addition to often "rubbing out" my "stamp" on the bottom.


if the former, you could use some sort of lap wheel. there are small
stained
glass laps, bit larger ones for lapidary uses, and really large ones for
doing rims of bowls, for example.

here's one i made that i use for both flattening the bottoms of ceramic
pieces, and rims of glass bowls:

http://groups.msn.com/chaniarts/wood...to&PhotoID=430


Looks good, but would be a "desperate measure" for me, as my bottoms are
done before bisque.

Marianne

Hi Bubbles, how about using a damp chamois leather stretched over a
board instead of sandpaper.
Andy

That wasn't exactly what I had in mind. Imagine a round ball of clay, take
a hard round object that is much smaller than the ball of clay and tap it
into the clay. You get a slight concaved indentation in the ball which now
makes that part of the ball no longer round but on the edges 'flat'. I just
posted what they had said to share the information and to show where I got
the idea from. They are doing it on a flat mug around the edge that already
exists to give 'feet' of a short. What I'm suggesting is creating something
of a edge (rounded though it may be).

"Bubbles" wrote in message
...

"DKat" wrote in message
...
Those are lovely Marianne! There was a recent post I found interesting
on
clayart where they were talking about putting feet on mugs. It seems
that
you could use this method to 'indent' the center of the bottom of your
pot
that would give you a 'foot'. I would use a wooden ball to tamp in the
center of the bottom of you piece


Thanks, D!

Thing is, I want the bottom to be flat on the table all the way round.
Thus,
making those 3 dents will make "holes", right? I guess I could try doing
it
a little further in - or maybe even just that the hitting will convince
the
clay to forget? ;-)

Hubby also suggested I make convex "lenses" that I set the pots on to dry
and to fire, so that gravity will assist a little.

Marianne

"DKat" wrote in message
...
That wasn't exactly what I had in mind. Imagine a round ball of clay,
take
a hard round object that is much smaller than the ball of clay and tap it
into the clay. You get a slight concaved indentation in the ball which
now
makes that part of the ball no longer round but on the edges 'flat'. I
just
posted what they had said to share the information and to show where I got
the idea from. They are doing it on a flat mug around the edge that
already
exists to give 'feet' of a short. What I'm suggesting is creating
something
of a edge (rounded though it may be).


Ahhh - I see what you mean, now. But I have been concaving the bottoms when
they are still flexible, but that doesn't seem to help much. Do you think
thumping them at leather-hard will be more effective?

Marianne

"plodder" wrote in message
ups.com...
Hi Bubbles, how about using a damp chamois leather stretched over a
board instead of sandpaper.
Andy


Good idea, Andy - or at least after sandpapering to finish off with a
slightly damp chamois. Must be careful about creating wet areas when they
are that dry.

Marianne