mixing it up with bluebird

updated mon 14 aug 06

Lili Krakowski on mon 7 aug 06

How do you define clay mixing? I never have used my old, beloved, trusty,
reliable Bluebird 440 to mix clay from dry materials.

BUT I use it all the time to mix too-hard-to-throw-but-not-leatherhard clay
with softer clay,even small amounts of slurry, and
to add grog or sand when needed.

For that it works perfectly. And, yes, sometimes I have to put the same
pugs through several times to get where I want the clay to be.....but it
does a great job.







Lili Krakowski
Be of good courage

Alan P on tue 8 aug 06

Hello Lili,

<> Mixing is the process to change a
heterogeneous arrangement into one that is homogeneous.

Providing the components are not immiscible the vigorous agitation of
fluids is the most effective mixing. Although the use of pugs to attempt
mixing is not unknown this does not mean they are good for the role, and
the lack of mixing is most evident with large differences in the feed, be
it with the materials or water contents. The shredding and subsequent
compaction of the feed can deceive that mixing is occurring but apart from
a minor redistribution the net effect is simply reconstitution into a non-
homogeneous extrusion. With a minor redistribution occurring multiple
passes does increase the effect but nevertheless it is not possible to mix
a mass of material by forcing it down a barrel.

Lack of mixing gives a non-homogeneous product, and in pottery this can
have serious adverse effects: considering just water content this will mean
differential shrinkage and hence drying cracks.

Recognising this phenomenon is far from new; it has been demonstrated, it
has been published, and dedicated clay mixers have existed for hundreds of
years.

Individual choices are based on a range of factors but being aware of
consequences allows these to be informed. The reason for my initial post
was simply to highlight a fallacy that is still being promoted.


Kind regards,


Alan

Wayne Seidl on tue 8 aug 06

I'm sorry, Ms Lili and all, I should have been more specific.
The Bluebird model (24S) of which I was speaking is a mixer only, not a pug
mill.
You can see the picture on the Axner website: http://www.axner.com
It was not until after I posted that I realized Bluebird also makes pug
mills.
My apologies for any confusion.
Best,
Wayne Seidl

-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Lili Krakowski
Sent: Monday, August 07, 2006 7:02 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Mixing it up with Bluebird

How do you define clay mixing? I never have used my old, beloved, trusty,
reliable Bluebird 440 to mix clay from dry materials.

BUT I use it all the time to mix too-hard-to-throw-but-not-leatherhard clay
with softer clay,even small amounts of slurry, and
to add grog or sand when needed.

For that it works perfectly. And, yes, sometimes I have to put the same
pugs through several times to get where I want the clay to be.....but it
does a great job.

Lili Krakowski
Be of good courage

Kathi LeSueur on tue 8 aug 06

Alan P wrote:

>.............
>Providing the components are not immiscible the vigorous agitation of
>fluids is the most effective mixing. Although the use of pugs to attempt
>mixing is not unknown this does not mean they are good for the role, and
>the lack of mixing is most evident with large differences in the feed, be
>it with the materials or water contents. The shredding and subsequent
>compaction of the feed can deceive that mixing is occurring but apart from
>a minor redistribution the net effect is simply reconstitution into a non-
>homogeneous extrusion. With a minor redistribution occurring multiple
>passes does increase the effect but nevertheless it is not possible to mix
>a mass of material by forcing it down a barrel.
>
>Lack of mixing gives a non-homogeneous product, and in pottery this can
>have serious adverse effects: considering just water content this will mean
>differential shrinkage and hence drying cracks.........>>>>>>
>

This is all pretty technical language to me. All I know is that I take
stiff clay, put it in my Bluebird pugmill with some soft clay or even
some slurry from clay I'm recycling and out the other end comes
excellent, consistant clay. Once in a while I run it through the machine
twice, but usually I can use it after one run through. That's the best
evidence I have that the pugmill works to mix scrap clay.

I buy 5 tons of mixed clay a year. Without my pugmill much of the scrap
would be discarded. I've often extruded my scrap through the pugmill to
make business card holders. That machine has paid for itself over and
over with just that one item made from scrapl

Kathi

>
>
>
>
>
>

Bonnie Staffel on wed 9 aug 06

So how do you explain the fact that if one changes clay color while =
working
the Bluebird there is a spiral of the new clay on the inside which
eventually disperses on further pugging to make the clay "homogenous". =20

" Lack of mixing gives a non-homogeneous product, and in pottery this =
can
have serious adverse effects: considering just water content this will =
mean
differential shrinkage and hence drying cracks."

I don't have any drying crack problem. I believe that you have to know =
what
your clay does and how to handle it throughout the whole process from =
wet to
dry to final firing. =20

Regards,

Bonnie Staffel

http://webpages.charter.net/bstaffel/
DVD Throwing with Coils and Slabs
DVD Beginning Processes
Charter Member Potters Council
http://vasefinder.com/bstaffelgallery1.html

Alan P on wed 9 aug 06

Hello Kathi,

It=92s good that you have found a processing method with which are happy,
however pugging a feed of stiff clay and slip is not advisable if a
homogeneous extrusion is required. Further processing following your
pugging, even just hand wedging, would help give a material of more uniform
water content and so reduce any problems associated with differential
shrinkage

Regards,


Alan

Kathi LeSueur on thu 10 aug 06

Alan P wrote:

>Hello Bonnie,
>
>You asked <>while working the Bluebird there is a spiral of the new clay on the inside
>which eventually disperses on further pugging to make the clay homogenous>>
>
>I do not fully understand your questions. Your post appears to describe the
>change over between bodies when feeding a new body down a barrel, which in
>the most basic terms is the mechanics of using a pug, will displace the
>previous contents. This does not give intimate mixing>>>>
>

Again, while all of this technical stuff is interesting, personal
experience is what counts when it comes to clay. There may be all kinds
of reasons why the mixing I'm doing in my pugmill shouldn't work. but,
the fact of the matter is that it does. I get good extrusions that don't
warp or crack. My throwing clay comes out just like I like it.
Consistant, no lumps. Bluebird makes a great machine for my purposes.
I've had this one for over ten years with no problems . Others my have
different experiences.

I've had the same comments about my chimneyless gas kiln. (the one made
famous by J.T. Abernathy). He, I, and many others with this design kiln
have been told all of the technical reasons why it isn't a good design
and won't fire properly. All we can tell you is that it does. It is very
even, reduction is even. Easy and relatively inexpensive to fire, and
cheaper to build than a kiln with bag walls and a chimney. I know, it
won't work. But it does.

Kathi

>
>

Alan P on thu 10 aug 06

Hello Bonnie,

You asked <while working the Bluebird there is a spiral of the new clay on the inside
which eventually disperses on further pugging to make the clay homogenous>>

I do not fully understand your questions. Your post appears to describe the
change over between bodies when feeding a new body down a barrel, which in
the most basic terms is the mechanics of using a pug, will displace the
previous contents. This does not give intimate mixing

Although I cant pass comment about the specifics of your processes without
examining them I can about 1) my own experiences, 2) the experiences of
others I have spoken to including manufacturers, and 3) recognised
phenomena.

<> Whilst you are fortunate not to
others have, and some have been identified as being due to moisture
gradients resulting from insufficiently mixed bodies

I have noted below some journal references that may be of interest

Kind regards,


Alan



<insufficient to allow unmixed raw materials and water to be fed into the
pug mill to produce well-mixed, homogeneous bodies by the time they exit
the production die. Even less mixing occurs in piston extruders. Pug mills
are designed to chop filter press blanks and other feed materials so they
can flow into the auger(s). Although this looks like mixing, the primary
function of extruders is extruding - not mixing. Use mixers for mixing,
extruders for extruding, and delaminators for delaminating. Yes - each of
these devices will perform some of the other functions, but they typically
won't be highly efficient when performing the other functions. If you want
to mix and delaminate and extrude, then put all three devices in series
(i.e., mixer feeds delaminator which feeds extruder.) >> Ceramic Processing
E-zine. Volume 3 Number 9. Dinger D. 2005



In a chapter dedicated to factors influencing strength Searle & Grimshaw
note: <on the strength of products. If, as is often the case, the mixing is
incomplete, the strength of the articles will vary in differing parts of
their structure. The process of pugging is far less effective method of
mixing than tempering the material in a edge running mill with revolving
pan>> The Chemistry and Physics of Clays and Other Ceramic materials. 3rd
edition. Searle A., Grimshaw R. Ernest Benn. 1959