Persnickety Porcelain: How to Minimize Your Loss Rate With this Gorgeous, Yet Difficult, Material

The main thing I take from this article is to make sure you keep the drying time even. and a lot of other advice recommends a dusting of Aulumina Hydrate on the kiln shelf where the piece is going to sit to allow it to slide as it expands and cools (makes sense)  – also the reduction in temperature from 1186-1220 cone 4-6 to cool should be as slow as possible.

February 9, 2010

Persnickety Porcelain: How to Minimize Your Loss Rate With this Gorgeous, Yet Difficult, Material

by Gwendolyn Yoppolo Read Comments (30)

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.

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.

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!

» Persnickety Porcelain: How to Minimize Your Loss Rate With this Gorgeous, Yet Difficult, Material.

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