Porcelain is a clay body that draws in many a potter because of its bright white color, translucency, and the way glazes look oh so fabulous on it. But it’s a fussy little clay body susceptible to collapsing during the forming process and cracking during the firing. Plus it has a memory like an elephant – jiggle it the wrong way during the forming process and there’s a good chance it will remember your mistake during the firing resulting in a warped pot.

But it’s so pretty and I, for one, still like to take my chances with it. With practice, you can learn learn how to work with this persnickity clay and minimize your loss rate. Today, Gwendolyn Yoppolo explains what porcelain will put up with from the wet phase to the bone dry phase. Plus, don’t miss the March/April issue of Pottery Making Illustrated in which Gwendolyn explains how to make her sweet little juicers (like the one shown here!). – Jennifer Harnetty, editor.


Porcelain commands us to be attentive in our touch and
responsive to its needs. Beyond the basic technical demands that clay bodies
all have in common, porcelain also needs to be treated properly to avoid warping
and cracking during drying and firing. One of the most important things to
remember is to watch your timing-this is best learned through experience.

Building onto a form that’s too soft causes slumping. Adding
softer clay onto a form that is too dry results in cracking. Altering and/or
bending a form that’s too dry or leather hard causes warpage and cracking. In
general, join only pieces of similar dryness and reinforce all joints with
extra clay and compress them together with a rib.

Slow and even drying is critical. Periods of rest, where the
pieces are wrapped in an airtight chamber to slow drying and redistribute
moisture, do help. The clay has a chance to get used to its new form at each
phase, without having one part dry too quickly for the rest of the piece.
Another valuable technique is to restrict movement of the piece during the
drying phase.

Here is a loose guideline and timeline for when to do what
while working with porcelain. The phases are not distinct, but are separated
out from the continuum of the entire process for the purposes of discussion. In
fact, they blend together in many ways, especially the “cheese” sections.
Because porcelain is thixotropic, it has a nice way of resoftening once it has
reached the hard cheese stage, so you can actually go back and perform some
soft cheese processes. Porcelain also rehydrates locally to some extent, so you
can go back in a concentrated area. These guidelines are designed as a starting
point for you to figure out your own way to achieve success.



Don’t forget to download your free copy of Successful Tips for Buying and Using Pottery Clay: How to Select the Right Clay, Estimate your Clay Needs, and Test Clays for Better Results to figure out which clay body is right for you!


Phase: wet clay            

forming (additive)

Processes Supported:

  • throwing on the wheel
  • hand building
  • molding elements

Things to Remember:

  • pay attention to the space inside of your vessel  -  you are shaping this receptive space first, and will make the walls around it match

Phase: soft cheese

altering and building (additive)

 

Processes Supported:

  • changing the form’s shape
  • adding onto the form
  • other additions (handles, knobs)
  • texturing surface

Things to Remember:

  • slip and score all joinings
  • compress joints with a metal rib or wooden tool
  • perform any bending of the walls or altering of curves

Phase: hard cheese

trimming and refining (subtractive)

Processes Supported:

  • trimming
  • rasping away areas of form
  • cutting away clay
  • carving patterns

Things to Remember:

  • basic form should not be altered
  • perform subtractive processes to lighten form or add aesthetic elements

 

Phase: stale cheese

dry shaping (subtractive)

Processes Supported:

  • clean surface up
  • lighten form further
  • soften edges
  • trimming or scraping with rib
Things to Remember:
  • just before the piece is bone dry, it responds very well to having its surface scraped or trimmed
  • if the work has become bone dry, you can sponge it down to do some of these processes

Phase: bone dry

erosive action (subtractive)

Processes Supported:

  • sponging
  • some light carving

Things to Remember:

  • sponging the form down reduces sanding, erases unwanted marks, and softens edges
  • don’t add too much water!

To learn more about Gwendolyn Yoppolo or to see more of her work, please visit www.gwendolynyoppolo.com.
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