Maurice Weitman on sat 23 sep 06
Where's Ivor when you need him?
I've been reading about tapping to center.
It's a fine technique, and fun, too... impresses the babes.
But this bit about the vacuum holding it in place bugs me.
In no particular order,
Mel said:
>if you are going to trim/turn a ring foot to a
>a perfectly leather hard pot...the water allows
>you to tap to center, tap the top of the pot
>and have the pot stay in place with vacuum.
EP offered:
>You have to have water on the wheel head in contact
>with the rim of whatever you are tapcentering to make
>a vacuum under the piece and to slide the lip.
and in another post:
>[...] a light coat of water makes a vacuum and [...]
Neal chimed in with:
>[...] When I had eight mugs to
>trim, I used a bit of water--just enough
>and not too much. Just as Mel wrote, the
>water helps keep a vacuum.
WHAT VACUUM? What force makes a vacuum here?
Isn't it just the wet rim sticking to the bat/wheelhead?
And why hasn't Ivor jumped all over this?
Huh?
Regards,
Maurice, in Fairfax, CA, where we're under a "Red Flag" warning...
no, not terrorists, just very low humidity, now in the 20s, much
better than yesterday when it dipped below 10%. Combine that with
quite warm temps (80s) and windier than breezy air, and no rain for
the past four or five months, and you have a scary condition.
Nothing like the Santa Ana winds down south, but enough for us
woods-dwellers to be noivous.
Ivor and Olive Lewis on sun 24 sep 06
Dear Maurice Weitman,=20
Just thought to keep out of this mess for the time being and let the =
kids have their squabble.
Vacuum. This presumed state of nature has the same validity as Aether =
(also spelled Ether, from Ethereal), another imaginary concept of an =
earlier age. Similar, no doubt, to Ectoplasm.
Pleased to know you have been thinking.
Having been instructed, as long ago as Mel was a young shaver under =
tuition with Mr Uchiada, that the best way to trim the base of a =
cylindrical form to leave a narrow rim it in pristine condition was to =
use a chuck (which lasts for ever...). This is placed on a damp wheel =
head after having its base dampened and tapped to centre. I suppose it =
is held in place not by suction or vacuum or air pressure but by the =
electronic attractions between clay, water and metal.
Ah, yes, there is a counterpart to the chuck used in the same way, for =
long necked forms called the "Chum". Much better and more precise for =
long production runs than my Gee Gee. This I reserve for singletons and =
special occasions.
What disturbs me is that people are speaking of placing such things as =
mugs directly onto the wheelhead, or a bat on the wheel, without =
internal support, to do their turning. I had a quick look through the =
standard textbooks relating to throwing and I cannot find one =
illustration of this method. Lots about Chucks and Chums with good =
illustrations and cross cultural consistency. Yes, variations in style =
but not in principle. Make me wonder about the nature and quality of =
instruction received by some when they are learning the basics. What has =
been described in you examples also raises issues of Design and Taste.
Good of you to ask.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Bruce Girrell on mon 25 sep 06
Ivor wrote:
> Vacuum. This presumed state of nature has the same validity as Aether ...
True, but the "vacuum" being discussed simply relates only to a reduction in
pressure on the interior of a vessel compared to atmospheric pressure on the
outside. Because of the large surface area of even a small pot, a small
reduction in pressure on the interior would be very effective in keeping a
pot firmly in place on the wheelhead.
Alas, I don't believe that even this tiny reduction in pressure occurs. The
idea is that a whack on the (relatively) pliable exterior of the pot deforms
the pot slightly, expelling some air. When the pot returns to its original
shape after the whack, a small pressure differential is created.
There are two problems that I see with this explanation.
1) Clay behaves as a plastic material, not an elastic one. Therefore,
deformations in the surface of the rim, the curvature of the bottom or
wherever else that may occur when the pot is whacked will tend to be
retained and will not spring back to the original shape. Any air expelled
would simply be expelled, after which the interior volume would remain
essentially unchanged and no pressure differential would be created.
2) The wheelhead is wet. If air is expelled it would reveal itself as a
bubble. We can take a straw poll to find out others' observations, but I do
use this technique and I have never observed any bubbles being expelled as
the pot is "set" with a tap.
An easy test would be to drill a hole in the side of a pot so that no
pressure differential could possibly be created and compare the tenacity of
the adhesion of this pot to a pot that has no hole. My guess is that the two
will behave essentially identically.
None of this negates the usefulness of the method. It's one of those things
that falls into mel's category of "I don't care how it works. It works.
That's all I need to know to make my pots."
Bruce Girrell
Taylor Hendrix on mon 25 sep 06
Let me throw a monkey wrench into things, Bruce.
I am suspecious of the 'pressure differential' explaination. Could it
just be adhesion (friction?) that holds the pot to the wheelhead? If
you expect a bubble to form, you might have used a bit too much water
I believe, hehe.
As with many of our techniques, timing, and not deep understanding of
how something works, is most important. Man this stuff is fun.
And your test is direct and to the point. Wish I had some leather
hard so I could quickly test it...drat.
Taylor, in Rockport TX
On 9/25/06, Bruce Girrell wrote:
>> There are two problems that I see with this explanation.
> 1) Clay behaves as a plastic material, not an elastic one. Therefore,
> deformations in the surface of the rim, the curvature of the bottom or
> wherever else that may occur when the pot is whacked will tend to be
> retained and will not spring back to the original shape. Any air expelled
> would simply be expelled, after which the interior volume would remain
> essentially unchanged and no pressure differential would be created.
>
> 2) The wheelhead is wet. If air is expelled it would reveal itself as a
> bubble. We can take a straw poll to find out others' observations, but I do
> use this technique and I have never observed any bubbles being expelled as
> the pot is "set" with a tap.
>
> An easy test would be to drill a hole in the side of a pot so that no
> pressure differential could possibly be created and compare the tenacity of
> the adhesion of this pot to a pot that has no hole. My guess is that the two
> will behave essentially identically.
Ivor and Olive Lewis on tue 26 sep 06
We seem to be thinking along similar lines Bruce.
I can offer one possible explanation for the reduction of pressure in =
the enclosed cavity.
If there is some degree of hot air inside the cylinder before it is =
inverted and placed on to your wheelhead, cooling will decrease the =
volume of gas in relation to the volume of space, allowing the 15 psi to =
exert itself on the whole exterior of the pot.
On the other hand if the clay has dried to leather hard it will have =
changed from a plastic state to an elastic state and your rebound theory =
may hold true ! ! !
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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