Rimas VisGirda on fri 22 jan 10
All the talk about platelets is interesting, but no one has addresses what =
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actually allows two pieces of clay to be joined=3DE2=3D80=3DA6 The ability =
of cla=3D
y to stick to itself is an inverse function of 2 variables: f(moisture, pre=
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ssure) -that is moisture (content) and pressure; a third factor that comes =
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into play as the clay starts to get leather hard is the moisture gradient. =
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I did some experimentation when I started teaching and this is what I found=
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.=3D0A=3D0AIf you take 2 pieces of (wet) plastic clay and push them togethe=
r VE=3D
RY firmly (OK, you can do a little wiggle) they will stick together. If you=
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don't use enough force (pressure) they will pop apart.=3D0A=3D0AAs the 2 p=
iece=3D
s get drier, it takes more force to get them to remain stuck.=3D0A=3D0AAs y=
ou s=3D
tart to get to leather hard it becomes virtually impossible to exert enough=
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pressure to get the clay to stick to itself -this is where scoring and sli=
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pping can come into play.=3D0A=3D0AScoring and slipping creates a moisture =
grad=3D
ient within the clay wall. We all know that if you take 2 pieces of leather=
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hard and just add some H2O to the area to be joined, they clay will initia=
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lly stick but will pop apart as it dries. That is because the H20 makes the=
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SURFACES to be connected very wet so it takes little pressure to make them=
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stick, BUT it also wets the area of contact and the clay expands. As it re=
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-dries it shrinks and causes a stress at the joint and pops apart.=3D0A=3D0=
ASco=3D
ring the clay puts furrows into the clay wall; adding slip to the furrows w=
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ets them quickly and results in a slurry. BUT it also provides a moisture g=
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radient into the wall. As you stick the 2nd piece onto the scored and slipp=
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ed area, the added moisture migrates into both pieces of clay resulting in =
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a moisture gradient as well as evaporating into the air. The stresses over =
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this expanded area are much less, consequently the pieces stick together ir=
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regardless of whether the platelets are aligned or not. Also irregardless i=
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f you use H20, magic water, apple vinegar, etc -I prefer rice vinegar, it h=
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as a nice zip to it=3DE2=3D80=3DA6 Think of what happens as a tall cylinder=
is dr=3D
ying, the top can be bone dry while the bottom can be still wet. But the pi=
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ece does not crack, that is because of the moisture gradient, in this case =
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from top to bottom that allows for the evaporation of H2O in a STEADY GRADU=
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AL rate=3DE2=3D80=3DA6=3D0A=3D0AFYI. My method for attaching handles is as =
follows -I=3D
typically make a pulled handle that is half a heart shape after applicatio=
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n or a circle or ellipse (email me if you want to see a picture). I try to =
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catch my mugs, etc at early leather hard, I score 2 horizontal needle troug=
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hs at the areas to be joined, I try to gauge the depth to be about 1/3 to 1=
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/2 the wall thickness, with a small brush I add some H2O to the troughs, fi=
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lling them with H2O, I make a few small vertical scores to the area (this m=
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akes my 'slip'), supporting the wall on the inside I press the top of the h=
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andle to the area allowing the bottom to hang free, then form the appropria=
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te curve and repeat for the bottom of the handle=3DE2=3D80=3DA6 I think wha=
t I ha=3D
ve done here is initiated somewhat of a moisture gradient within the wall o=
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f the pot (which is drier than my handle. The handle is still deformable an=
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d 'wet' and will stick to the contact area; it will also draw some moisture=
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into itself, as it
is drier than the contact area. As all this equalizes, we have a moisture =
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gradient in place from the interior of the wall through the wetter stages o=
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f junction and into the drier handle. As the piece dries the moisture can d=
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issipate in a controlled rate, much like the illustration of the drying cyl=
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inder, and alleviate stresses at the joint thereby allowing them to stay to=
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gether=3DE2=3D80=3DA6=3D0A=3D0AWorks for me, -Rimas=3D0A=3D0A
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