John Britt on sat 12 apr 08
Bruce,
Don't know what physics property causes the effect, although Ivor may?!
I have seen the same type of patterning in deflocculated slips. It is a
dead give away when you see the spirling patterns.
It may be in the reflection from the variations in particle alignments?
John Britt
Bruce Girrell on sat 12 apr 08
They look a lot like the effect that you get when combing through contrasti=
ng colors of material, except the slip is thoroughly homogenized; there are=
no differing colors to contrast. So what makes the patterns?
You can see images of what I'm talking about here:
http://farm3.static.flickr.com/2401/2408959690_54b451b77e_o.jpg
http://farm4.static.flickr.com/3240/2408959614_116bf490bc_o.jpg
For scale, the plastic container is a standard hospital emesis basin, 9" x =
11" with a medium sized kitchen spatula.
I would guess that most everyone who has played with slip has seen patterns=
like these. I have seen this sort of pattern every time I make up some sli=
p. Every motion makes marks; every bubble that pops leaves a makr.
In this case, I mixed the slip by adding a bunch of trimming scraps to wate=
r in a blender (poor thing probably thinks that it has died and gone to kit=
chen appliance hell). The blended slip was passed through a 60 mesh screen =
and is in the process of drying in the basin. It is currently about the con=
sistency of heavy cream or kefir.
The patterns were created by swishing the spatula back and forth across the=
basin as I was keeping the slip mixed up to even its consistency.
Clearly, the patterns are a result of the motion of the spatula, but the st=
uff I'm stirring is about as homogeneous as you can get. I considered that =
it might be an optical aeffect, resulting from light interacting with edges=
and flats of clay platelets, but I have difficulty believing that explanat=
ion because 1) I would expect to see some degree of scintillation when movi=
ng relative to a light source and there is none and 2) I don't believe that=
platelets could organize and arrange themselves so quickly to line up with=
the motion of the spatula.
So..... What am I looking at here? Any ideas?
Bruce Girrell
Vince Pitelka on sun 13 apr 08
John Britt wrote:
"Don't know what physics property causes the effect, although Ivor may?!
I have seen the same type of patterning in deflocculated slips. It is a
dead give away when you see the spirling patterns.
It may be in the reflection from the variations in particle alignments?"
John -
I think that's exactly it. When I make terra sigillata and get down to the
final product, there are beautiful swirl patterns in the surface when it is
stirred. They are almost iridescent, and I have always assumed that they
could only be from the reflective surfaces of the kaolinite crystals - the
same thing that causes the shine on terra sig when it is applied to the
surface of a pot. That happens because the refined terra sig contains only
the finest platelets, and they tend to lay down flat on the surface,
reflecting light. It makes perfect sense that the same thing would happen
in a slip when you align the platelets by disrupting the surface by
stirring.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
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