Karl P. Platt on mon 29 mar 99
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Let me chime in on this...I need to shift gears a bit anyway.
There are a lot of physical phenonmina described in the
attachments/type-of-slip thread I collided with which may not necessarily
have to do with the chemistry of deflocculation. And while I missed the
thread leading up to this it seems that the topic has become
extraordinarily complicated.
=3E Sodium ions are of course a powerful deflocculent.
They can also serve as a flocculent depending on who they are in kind
with in the slip. This isn't to deny or ignore the usually poor
(bizarre?) effects of variably soluble salts in our raw materials.
Consider Ghastly Borate as the extreme case.
=3EWhy doesn't this happen to handles on slip-cast pieces (which are full
=3Eof sodium ions)? I would guess that casting slips are formulated to
=3Ehave lower drying shrinkages than throwing bodies, so the drying
=3Estresses are much less.
Hold on thar', buud. Most slip cast handles are integral to the casting
in the first place. There is no process of attachment. Whether or not
drying/firing shrinkage causes breakage depends on net shrinkage, the
design of the article, casting technique, and the manner of heating --
drying is most seriously slighted. Most casting slips used for artware
(Ball Clay/Talc 50/50-ish) have net shrinkages much much higher than
throwing bodies. This is simply necessary to the larger volume of water
occupying the spaces between the particles making-up the slip. Other
factors, like particle size, clay/non-plastic proportions, type of clay,
whether or not the clay was peed on, and so on all can have their day,
too. Obviously deflocculents serve in any casting slip to minimize the
volume of water and, therefore, net shrinkage of the body, but in
virtually all cases pertaining to artware, the net shrinkage of a slip
cast article will be higher than a thrown article of the same clay body
-- made-up dryer to throw, of course.
=22Slip=22 for attaching handles is usually just watered down trim from
throwing, having not been treated with deflocculents in any way. One
imagines that there could be a number of cases in which it may be very
desirablel to employ them in making a little more refined =22slip=22 for
attachments -- certainly they gave it a lot of thought at Meissen. Using
water into which suitably tiny amounts of deflocculent were added to make
=22slip=22, instead of what ever water was around, could/should be very
advantageous in minimizing shrinkage differences within the joint.
=3EAs for the superior =22wetting=22 strength of acids applied to clay. I
=3Ewould put this down to the likelyhood that in the presence of acid, clay
=3Eplatelets in their haste to flock together must entrap a lot of water
=3Emolecules into the gaps in the boxwork. This would constitute a strong
=3Ewetting effect. The water in effect, gets dragged along for the ride.
If you search the archives you'll find a 4 year old post which
specifically addresses the influence of the pH of the medium as given by
various ions likely to be met in clayware slip on the nature of the
=22water=22 film adhering to the clay in a slip -- micelles would be a good
word to search on to cull the net for further info on the behavior of
colloids -- which is what one toys with when using slip. In brief,
acidity lends to improved plasticity of clays. This is because the water
film on the clay particles is given to be a bit thicker. This effect is
why you hear stories about this or that potter in a remote place or long
ago who buried their clay and peed on it for long periods of years. Not
to be crass, but peeing in glaze slip to thicken it up in a hurry was
entirely common up and down the Ohio Valley -- I learned this useful
trick it from an old guy back there in '82.
Changes in the fluid properties of a slip over time, or ageing, is
likewise related to changes in the pH of the watery part of the slip by
reaction with the solids in the slip -- stuff dissolves in it. Usually we
deal with the alkalies coming into solution, but the influence of weird
organic materials and bacteria can't be ignored --- often bacteria won't
let us ignore it. Who knows, Ceramists may well have founded any number
of new life forms in funky old slip. Of course, they are all beneficial
bacteria. Some clays popular in potting also contain not small amounts of
sulphur which, to the contrary of alkalies coming out of, say, a
feldspar, tend to make the slip more acidic.
The best most widely tangible textbook description of all of this
behavior in the ceramic literature is in Salmang's (old) book. It equates
theory with practical reality in a very elegant fashion and you needn't
have an Sc. D.'s enthusiasm to get it.
I might add that a good place to find literature relating to the
suspension of solids in water is the detergent industry. In fact, the
best suspending agents are often not sodium silicate/soda ash, but the
phosphates. Nuttin' like .01=25 TSPP by wt of the solids to stabilize a
slip -- in most water.
While I'm free associating in front of 3,000 people I might as well toss
in an admonition about water being the most important raw material in a
slip.
This all assumes that the mechanical nature of the attachment was well
crafted. .....Can't just stick the thang on with slip. And I see that
it's now 10:23 and 11 =BAC or something quite close to Laphroaig:30. Which
is to say that I'm going for a nightcap and see if I can get that pretty
lesbian to talk to me.
KPP
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