Bill Aycock on sun 8 aug 99
The properties of liquids and liquid-solid mixes that have to do with how
they react when deformed are called the rheological properties.
"Thixotropic" is the label put on one of them. A thixotropic mix gets
easier to deform (slump- flow- pull- whatever) the more you work it. It has
nothing to do with the pH, although changing the pH can make it look like a
change.
Quicksand is a thixotropic mix. Thats why they say- "dont struggle !"- in
other words, stop "working"the stuff. The thing that makes a
particle-liquid mix behave this way has to do with the range of the sizes
of the particles, and the amount of the various sizes. ( the study of that,
is called "micromeritics, as if you cared)
A clay body mix contains different ingredients, each having a range of
sizes. I cant tell you how to change the parts of the mix to change the
rheological properties (there are five different types)- but- one simple
step will usually help if the problem is Thixotropic-
stop and let the clay rest.
This helps because the property called thixotropic has to do with how much
"work" you put into the shape- but it also has to do with the time it takes
to do the work.
Bill- on Persimmon Hill- remembering that peanut butter is also
thixotropic, especially if mixed with jelly.
-
Bill Aycock --- Persimmon Hill
Woodville, Alabama, US 35776
(in the N.E. corner of the State)
W4BSG -- Grid EM64vr
baycock@HiWAAY.net
w4bsg@arrl.net
Michael Banks on mon 9 aug 99
I must disagree, Bill. The thixotropic symptoms Jeff Lawrence reported in
his original post (which started this thread) were consistent with incipient
deflocculation of the throwing body, caused by excess soluble soda and/or
excessively high pH.
Let's not get hung up on semantics here. Sure, thixotropy is a physical
condition exhibited by a range of substances ranging from mayonnaise to
liquid cement. And pH might be irrelevant in controlling thixotropic
behaviour in these two materials, along with quicksand and ketchup.
But unlike mayonnaise, clay-water mixtures are very sensitive to the
concentration of sodium ions which generally elicit particle repulsion
(deflocculation). To oppose deflocculation, acids are the most powerful
flocculents, as they donate protons which can crowd in on clay platelets,
very closely indeed. Adding enough acid will eventually overwhelm any amount
of soda and flocculate the clay. The measure of the concentration of protons
(hydrogen nuclei) in an aqueous phase is pH.
A throwing clay at its' optimum plasticity, is normally in a flocculated
state. However this optimum state can go into a decline in the presence of
minerals such as nepheline or soda feldspar which release sodium into the
pore water. It is my belief that there are degrees of flocculation and in
the absence of sodium ions, the forces of attraction are at a maximum. With
the introduction of sodium into the pore water, the attractive forces
progressively weaken, although the clay is still in a flocculated condition.
But with increasing sodium concentration, the solid clay exhibits increasing
sensitivity to losing cohesiveness in response to kinetic energy inputs
(working). Otherwise stated: Progressive thixotropy.
As anyone who has mixed up a casting slip knows, a stiff dough of
flocculated clay progressively deflocculates and liquefies as the defloc
agent (normally a sodium compound) is added, whilst stirring. But until the
right amount of defloc agent is added the "dough" might liquefy while being
stirred, but will immediately seize-up again, the moment you stop stirring.
The tendency to liquefy increases as sodium is added, i.e. thixotrpy
increases with sodium concentration. As more sodium is added, less stirring
(working) is progressively needed. Ultimately, a point is reached where the
mix is fully deflocculated and remains fluid without stirring. The process
can be driven backwards by adding acid (lowering the pH) in which case the
mix becomes increasingly stiff (flocculated) and decreasingly thixotropic.
In fact in industry, thixotropy in casting slips is adjusted by pH
adjustment! This is accomplished by varying the ratios of say, sodium
silicate/soda ash. Good casting requires a fine balance between optimum
fluidity and thixotropy. Adding acids or other flocculents would not allow
this fine adjustment and would tend to stiffen fluidity drastically. But
adjusting the ratios of two sodium compounds, both of which are
deflocculents, but each having a different pH, achieves this.
Why incipiently deflocculated clays should liquefy when worked (kinetic
energy added), i.e. exhibit thixotropy, is a moot point (kinetic energy
probably breaks tenuous attractive forces, similar to heat melting butter) .
But strongly flocculated kaolinitic clays usually don't liquefy no matter
how much kinetic energy is added. The smectites however, are notorious for
liquefying in earthquakes and these clays are known to form only in
relatively high pH conditions. So not only does pH affect thixotropy in
clay-water mixtures, it also controls the species of clay mineral formed in
the geologic environment in the first place.
Michael Banks,
Nelson,
New Zealand
----- Original Message -----
> ----------------------------Original message----------------------------
> The properties of liquids and liquid-solid mixes that have to do with how
> they react when deformed are called the rheological properties.
> "Thixotropic" is the label put on one of them. A thixotropic mix gets
> easier to deform (slump- flow- pull- whatever) the more you work it. It
has
> nothing to do with the pH, although changing the pH can make it look like
a
> change.
>
> Quicksand is a thixotropic mix. Thats why they say- "dont struggle !"- in
> other words, stop "working"the stuff. The thing that makes a
> particle-liquid mix behave this way has to do with the range of the sizes
> of the particles, and the amount of the various sizes. ( the study of
that,
> is called "micromeritics, as if you cared)
>
> A clay body mix contains different ingredients, each having a range of
> sizes. I cant tell you how to change the parts of the mix to change the
> rheological properties (there are five different types)- but- one simple
> step will usually help if the problem is Thixotropic-
>
> stop and let the clay rest.
>
> This helps because the property called thixotropic has to do with how much
> "work" you put into the shape- but it also has to do with the time it
takes
> to do the work.
>
> Bill- on Persimmon Hill- remembering that peanut butter is also
> thixotropic, especially if mixed with jelly.
>
>
> -
> Bill Aycock --- Persimmon Hill
> Woodville, Alabama, US 35776
> (in the N.E. corner of the State)
> W4BSG -- Grid EM64vr
> baycock@HiWAAY.net
> w4bsg@arrl.net
>
Bill Aycock on tue 10 aug 99
I must really thank Michael for one of the best short dissertations on the
properties of clay and slip I have read. I recommend that you save it. If
you dont need it now, you probably will later. It really is very good.
However, He and I have a symantic problem, that may be the core to
understanding the phenomenon Jeff ran into. It may well be that his problem
has to do with flocculation, and not (stricktly speaking) to thixotropic
changes. First- Thixotropic characteristics are time-dependant, and,
generally, are reversible. The change in flow characteristics in
thixotropic materials does not require a pH change, nor does it require one
to change back. If Jeffs changes occured while he was throwing, with only
water being added, there was little change in pH. I may not understand all
that happened to him, but it sounds like the physical changes due to
thixotropy, to me.
I have had glazes that had characteristics that appeared to be kin to
thixotropy, but were really ionic (electrical) in nature. They would clump,
and pour in blobs. After rigorous screening, they would pour OK, but **
with no work being done to them** they would clump into islands in the
bucket. pH changes would cure this. In this case, the clumping is due to
electrical attraction and repulsion between particles, and time is neede
for the particles to move. This is not the case in clay, with much lower
fluidity, and no (or limited) chances for spontaneous particle motion.
I will watch for a report from Jeff about the changes he made, and the
results.
This is interesting.
Bill- taking a break while repairing the porch on the studio.
At 11:04 AM 8/9/99 EDT, you wrote:
>----------------------------Original message----------------------------
>I must disagree, Bill. The thixotropic symptoms Jeff Lawrence reported in
>his original post (which started this thread) were consistent with incipient
>deflocculation of the throwing body, caused by excess soluble soda and/or
>excessively high pH.
>
>Let's not get hung up on semantics here.
**SNIP**
>
>Michael Banks,
>Nelson,
>New Zealand
-
Bill Aycock --- Persimmon Hill
Woodville, Alabama, US 35776
(in the N.E. corner of the State)
W4BSG -- Grid EM64vr
baycock@HiWAAY.net
w4bsg@arrl.net
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