Richard Jeffery on fri 23 nov 01
you can calculate weight per cubic centimetre, and refer to some standards,
but I'm not convinced - wood structure varies both across and up the trunk.
Maybe it's fine for an engineer (no discourtesy intended) working in
construction, with quick grown softwood or some tropical species which are
fairly uniform - but it's not much of a guide if you're using hardwoods for
furniture and the like. It can give you an indication, but only between
broad limits - you can rarely have a standard that matches the actual bit of
tree in front of you.
you're more likely to use a moisture/humidity meter - usually there are two
prongs which you push into the wood - the gizmo measure a small current
passed between these prongs. even then, you can only measure as far as you
can reach, which is up to about 1" into the board. If you have a 6" beam,
you still won't know how dry the centre is, except that it is probably less
dry that the outside.
perhaps more to the point, is deciding just how accurate a measure you need
to work with - in the case of timber, there is no point being too accurate -
wood will re-absorb a certain amount of moisture from the atmosphere, and
lose it again, through the seasons. you have to design around the
dimensional changes that brings about. Maybe it's the same with clay - you
could probably measure moisture content by weight loss for a particular clay
body fairly accurately - but maybe you would then end up spending longer
drying things out than you really need, and where a slower start to biscuit
might overcome any residual problem. you can get the humidity levels
accurate, but after a certain point, so what. In most cases, where is the
real benefit?
Richard
Bournemouth UK
www.TheEleventhHour.co.uk
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of David Hewitt
Sent: 23 November 2001 11:55
To: CLAYART@LSV.CERAMICS.ORG
Subject: Water content of clay and drying out pots for biscuit firing
At some time or another I am sure that we have all failed to dry out a
pot sufficiently before biscuit firing and so the item has burst.
A question, posed to me by an Engineer, was, can't you tell by the
weight? He was used to dealing with timber and this, apparently is the
standard method of telling if timber has been dried out enough for
whatever purpose it is intended.
It is a thought that, if we took the weight of clay used for throwing
the items and then checked the % reduction in weight as they dried, we
might get close to telling if the non chemically combined moisture had
been removed. I appreciate that a certain amount of the clay is 'lost'
in the throwing process from the finished item, but it is a small
amount.
If so, what % reduction could one expect? Might this be as much as 30%?
Has anyone pursued this avenue or are you all like me, and go by the
change in colour and the feel and, if in doubt, leave a bit longer.
If you do use such a method I would be interested in knowing more about
it.
David
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP18 3DQ, UK. Tel:- +44 (0) 1633 420647
FAX:- +44 (0) 870 1617274
Web site http://www.dhpot.demon.co.uk
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David Hewitt on fri 23 nov 01
At some time or another I am sure that we have all failed to dry out a
pot sufficiently before biscuit firing and so the item has burst.
A question, posed to me by an Engineer, was, can't you tell by the
weight? He was used to dealing with timber and this, apparently is the
standard method of telling if timber has been dried out enough for
whatever purpose it is intended.
It is a thought that, if we took the weight of clay used for throwing
the items and then checked the % reduction in weight as they dried, we
might get close to telling if the non chemically combined moisture had
been removed. I appreciate that a certain amount of the clay is 'lost'
in the throwing process from the finished item, but it is a small
amount.
If so, what % reduction could one expect? Might this be as much as 30%?
Has anyone pursued this avenue or are you all like me, and go by the
change in colour and the feel and, if in doubt, leave a bit longer.
If you do use such a method I would be interested in knowing more about
it.
David
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP18 3DQ, UK. Tel:- +44 (0) 1633 420647
FAX:- +44 (0) 870 1617274
Web site http://www.dhpot.demon.co.uk
vince pitelka on fri 23 nov 01
> Has anyone pursued this avenue or are you all like me, and go by the
> change in colour and the feel and, if in doubt, leave a bit longer.
> If you do use such a method I would be interested in knowing more about
> it.
David -
This is an interesting idea, but it seems you would have to know the exact
weight of the piece at the time it was thrown in order to have a basis for
comparison. Even knowing the weight of the original lump would not be
enough, since we remove a good part of that weight in slurry and by trimming
the bottom, etc., and we add water to the clay in throwing. It seems that
you would have to weigh the pot right after throwing, and I cannot see how
that would be worth the effort. Feeling the surface of the pot is a good
way to judge, to see if it is still cool from the evaporation of moisture.
But you can fire by a schedule where it will not matter if the pots are
still damp. At the Craft Center our kiln schedules are set up so that a
bisque-fire takes a 48 hour slot. If someone wants to fire faster that is
up to them. When we use the 48-hour format we load the kiln, and leave it
on overnight with just the bottom element on low, and with the lid propped
open on a piece of softbrick (to avoid damaging the kiln and lid surfaces).
The next morning, we close the lid and turn the middle and top elements to
low, two hours later all elements to medium, two hours later all elements to
high. The kiln is unloaded the following morning.
If I am firing large flat items, especially if they are still slightly damp,
I place each piece on three small softbrick shims (3/8" thick, the size of a
quarter) to allow air circulation beneath them, so that they dry out and
heat up evenly. I also use this system to fire multiple plates or bowls on
top of each other, placing three softbrick shims in the same location on
each successive plate or bowl, up to four on top of each other. I never
have any problems with large plates, bowls, or tiles cracking in the bisque
when I use this method.
You can put things in the bisque firing when they are still quite damp, as
long as they are forms which can stand quick-drying. The other day I threw
a series of cider jugs for a woodfiring we are doing next week. When I put
them in the bisque kiln the upper portion and the handle was starting to
bleach (change color from drying), but the lower portion was still very
damp. I set each of them on three softbrick shims. I preheated and fired
as described above, and had no problems.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Des & Jan Howard on sun 25 nov 01
David
David Hewitt wrote:
> At some time or another I am sure that we have all failed to dry out a
> pot sufficiently before biscuit firing and so the item has burst.
>
Been there, done that!
>
> A question, posed to me by an Engineer, was, can't you tell by the
> weight? He was used to dealing with timber and this, apparently is the
> standard method of telling if timber has been dried out enough for
> whatever purpose it is intended.
>
> It is a thought that, if we took the weight of clay used for throwing
> the items and then checked the % reduction in weight as they dried, we
> might get close to telling if the non chemically combined moisture had
> been removed. I appreciate that a certain amount of the clay is 'lost'
> in the throwing process from the finished item, but it is a small
> amount.
>
It would be interesting to try, ,just as an exercise, not practicable on a
production basis for us. Also all of our pots are turned, to a greater or lesser
degree.
>
> If so, what % reduction could one expect? Might this be as much as 30%?
>
> Has anyone pursued this avenue or are you all like me, and go by the
> change in colour and the feel and, if in doubt, leave a bit longer.
> If you do use such a method I would be interested in knowing more about
> it.
We have a dry, (usually), climate, summer & winter.
Evapourative cooling chills drying pots, when pots reach
workshop ambient temperature they're ready for bisquing.
Des
--
Des & Jan Howard
Lue Pottery
LUE NSW 2850
Australia
Ph/Fax 02 6373 6419
http://www.luepottery.hwy.com.au
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