[Clayart] scoring

[Vince Pitelka]

One thing that has not been addressed (or else I missed it) is the
importance of using a technique appropriate to the type and condition of the
clay.  When joining wet clay to wet clay, almost anything works as long as
you jiggle the connection to get the platelets to intermesh.  That's what
we're trying to do when we join clay, and anything that accomplishes that is
okay, but wetting the surfaces and jiggling the attachment won't work when
the clay is leather hard.  I do a lot of stiff-slab construction, often when
the slabs are fairly hard-leather-hard, and I have never found a better
method than scoring and using slurry.  

Sharp-pointed tools work well for scoring only with a very light touch.  Any
more pressure and they make a deep, narrow slit in the surface that slurry
cannot penetrate, leaving a network of small bubbles across the joint.
That's about the last thing you want.  For clay that is still on the soft
side, a toothbrush works great, using water, slurry, vinegar, or magic water
as the medium. My favorite scoring tool is a serrated ribs, and I love the
Mudtools serrated stainless steel ribs. They create a network of shallow
grooves that are is easily filled with slurry, and when both surfaces are
scored, well-slurried, and pressed together, the excess slurry expels all
air bubbles and the platelets intermingle thoroughly.

At the handbuilding conference up in Philadelphia in September there was a
nice discussion of why vinegar and magic water both work for joining, even
though one flocculates and the other deflocculates. The explanation is
simple.  Vinegar aids in joining because it makes the particles attract -
they are stickier.  Magic water aids in joining because it makes the
particles repel, which opens up a microscopically-thin layer of platelets on
the surface, creating the ideal situation for intermingling and intermeshing
of the platelets.  

In coil-construction I never score and slurry unless I have left the piece
to stiffen up for a while, and then I score and slurry before adding the
next coil.  Otherwise I use the Nigerian method of smearing each coil into
the previous one, which gives the strongest possible joint.  When done
properly, this method produces vessels or sculpture as strong as by any
other method.  There's an easy test you can do.  Take too small balls of
plastic clay and press them directly together as hard as you can. You will
easily be able to pull them apart along the same joint.  Now take two balls
of clay and smear them back and forth as you press them together. You will
not be able to pull them apart along the joint. Obviously this does not work
if the clay has stiffened at all, but for soft clay, smearing is probably
the most effective means of joining.  Unfortunately it is only applicable in
some situations.  You don't want to ruin a handle you have pulled by
attempting to smear the upper attachment into the surface of a soft mug.
- Vince

Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka at dtccom.net   
http://iweb.tntech.edu/wpitelka/