spiral cracking and walner lines

updated fri 15 feb 02

Curtis Nelson on tue 12 feb 02

What are Wallner lines? Even Hamer and Hamer
doesn't say.

On page 16 in the book "Ceramic Faults and Their
Remedies" by Harry Fraser it says the following:

"If the clay is over-extended during the throwing
operation, the thrower trying to pull the clay upwards
or outwards at a rate the clay cannot tolerate, then
the strain introduced can cause spiral cracking to
occur along lines which follow the rotation of the
throwing wheel. The cracks are fine ones which usually
appear on drying but then open up during biscuit firing.
Examination of the fracture surface usually reveals a
grainy texture and WALLNER LINES or rib marks,
pointing to the direction from which the crack
originated, are sometimes present."

Any one know?

Curt Nelson

Tommy Humphries on tue 12 feb 02

Hey Curt,

Had to go dig out my copy to check...

Wallner lines are illustrated in fig.71 on p.84 . This drawing by itself is
not real clear as what is meant by "parabolic curves pointing to the origin
of the crack" For a much better illustration of this effect look on the
cover, the upper left hand picture shows this to great effect.

Tommy


----- Original Message -----
From: "Curtis Nelson"
To:
Sent: Tuesday, February 12, 2002 12:08 PM
Subject: Spiral Cracking and Walner Lines


> What are Wallner lines? Even Hamer and Hamer
> doesn't say.
>
> On page 16 in the book "Ceramic Faults and Their
> Remedies" by Harry Fraser it says the following:
>
> "If the clay is over-extended during the throwing
> operation, the thrower trying to pull the clay upwards
> or outwards at a rate the clay cannot tolerate, then
> the strain introduced can cause spiral cracking to
> occur along lines which follow the rotation of the
> throwing wheel. The cracks are fine ones which usually
> appear on drying but then open up during biscuit firing.
> Examination of the fracture surface usually reveals a
> grainy texture and WALLNER LINES or rib marks,
> pointing to the direction from which the crack
> originated, are sometimes present."
>
> Any one know?
>
> Curt Nelson
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
>

iandol on wed 13 feb 02

Dear Curtis Nelson=20

I suggest you write to Harry Fraser at potclays@btinternet.com. He is an =
approachable person.

Best regards,

Ivor Lewis.

Martin Howard on thu 14 feb 02

approachable person.>

Already done that and waiting for an answer.
Will let the list know when Harry responds.

Martin Howard
Webbs Cottage Pottery
Woolpits Road, Great Saling
BRAINTREE, Essex CM7 5DZ
01371 850 423
martin@webbscottage.co.uk
http://www.webbscottage.co.uk
Updated 8th February 2002