Bruce Girrell on tue 18 jun 02
I have a clay body that has decent workability but I need to amend it to
achieve other needs, e.g., thermal shock resistance. The clay body is
porcelainous, so plasticity is already an issue. The ingredients that
provide the thermal shock resistance are all non-plastic - pyrophyllite,
talc, molochite, etc. After adding the non-plastic ingredients to the clay
body workability/plasticity drops to an unacceptable level.
1) Is it better to use ball clay or bentonite (or some other clay) to
restore plasticity?
2) Is there any rule of thumb to use as a starting point for how much
plastic material to add, e.g., however much non-plastic material you added,
add 10% of that amount in bentonite* or 25% of that amount in ball clay?
3) Does either bentonite or ball clay lead to any other particular
unpleasant characteristic, such as a tendency of the wet clay to slump after
being formed?
Thanks for your help.
Bruce "if I said you had a beautiful body would you hold it against me?"
Girrell
*I'm adding a total of about 10% non-plastic materials so if this rule of
thumb were true, I would add 10% of 10% = 1% bentonite in terms of the
entire batch
BVCuma on wed 19 jun 02
>>Does either bentonite or ball clay lead to any other particular
unpleasant characteristic, such as a tendency of the wet clay to slump =
after
being formed?<<
___________________
Hi Bruce ; )
In the begining I figured all I had to do was
add bentonite to rectify short clay and make it plastic.
I had to add so much it made the clay into a weird
thixotropic material that did just that, slump while being formed...
worst stuff I have ever handled. When too much ball clay goes in
my problem has not been workability but crackability while drying.
I think for using bentonite the issue is iron contamination
so the porcelainous body you use, loss of whiteness maybe a concern.
Check with your supplier for a white burning bentonite.
2% is generally the upper limit in your body composition.
I believe macaloid is an expensive but good alternative.
I think the most critical aspect to building a porcelainous body
is to start with a good plastic kaolin... I do not have one here
and my porcelain experiments were dropped soon.
For thermal shock kyanite maybe a consideration
as it forms mullite crystals that can strengthen the body.
Talc can actually create thermal shock problems
rather than the perception that it reduces it..
so I have read in regards to raku body formulating.
I think it 25% ball clay in the total batch is approaching the upper =
limits.
Try wedging in a very small amount of shave cream from the tube
and age for a week to see if that helps or hinders...
(still wondering what are the essential lubricants used in them)
The best plasticizer as of late for me is too recycle my slip
used for throwing after it gets well "composted"
after aging the well mixed batch it makes all the difference.
Thats about all I can suggest
Goodluck,
Bruce ; )
BVCuma on thu 20 jun 02
>>I prefer to use a good white firing ball clay to induce or restore =
plasticity to tired reclaimed clay<<
_________________________
Very interesting post Ivor..
I must read it a few times more.
I find my reclaimed clay is much more plastic than before...
is this tired or ...what?
Something is growing in there...
and makes things rather slick.
All the best,
Bruce
BVCuma on sun 23 jun 02
>>I find my reclaimed clay is much more plastic than before...
is this tired or ...what?
Something is growing in there...
and makes things rather slick.<<
_____________________________
In fact I am noticing that I take good care of my recycled clay..
much like I do with plants.
Or more specifically like a bacteria culture...=20
for example yogurt.
Keep it alive under proper growth conditions.
save some for the next batch so it can work its wonders again.
Bruce
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