Porcelain Shrinkage Rates

The shrinkage rates of all clays sold on Valentine Clays (the biggest distributor of clay in the UK and overseas) can all be found on the following PDF file. Please click on the following link to be directed to the PDF stating all this information.

technical-information

Take note of the porcelain shrinkage rate which is at an astonishing 18% when fired at the recommended 1200 degrees in a kiln. This is a factor I must consider seriously when producing my own work as almost 20% of whatever I produce will be lost. To combat this I must test the specific batch of Ming Porcelain I’ll be using to model with and compare it to the suggested 18% shrinkage rate to see where it stands in this spectrum. To do this I will cut a slice of the clay into a ten centimetre square, then measure it throughout the various stages of drying. Once its final firing has been completed, I will use a ruler to see just how much the slab of clay has shrank. With this information I will then know how much bigger to originally model my work to allow shrinkage and end up with the desired size once fired.

 

 

Porcelain Tips

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.

 

All information in this post is a direct copy from the website stated below:

http://ceramicartsdaily.org/ceramic-supplies/pottery-clay/persnickety-porcelain-how-to-minimize-your-loss-rate-with-this-gorgeous-yet-difficult-material/

Shrinking Facts

It is important to have in depth understanding of the physical and chemical process clay goes through during drying and kiln firing. Having this understanding will help in avoiding clay and firing defects with your project and can help troubleshoot many of the problems you experience with your pieces such as cracking, breaking, exploding and glaze defects.

Shrinking

Clay shrinks both in drying and in firing. Different clay bodies shrink at different rates which can be as little as 4%, or as much as 15% for some clay bodies. Even one percentage point can make a difference in the final product depending on your need for precision. Your design, therefore, needs to account for the shrinking for the temperature to which you fire. For example of the clays we use at Lakeside Pottery is Standard’s Baldwin 192 and it is specified to shrink 11% for cone 6 firing temperature. Our testing showed that about 5% shrink during the drying process, 1/2% during bisque firing (cone 06) and 5.5% during glaze firing (cone 6). The total of 11% is shown in the lantern picture below (before and after).

We also found throughout after years of firing that even with the same clay body, shrinkage can vary 1-2% from batch to batch. Where accuracy is important, we run a batch test before proceeding with a project or tell our commission customers to expect +/- 3% variance.

 

Why is it important to let clay work dry slow?

Be aware that the larger to piece is, the more movement of shrinkage will take place. For a example, if clay shrinks 5% during drying, a piece of clay that is 5″ long, will shrink 1/4″ and a piece that is 20″ long, will shrink 1″ during the drying. This is why it is beneficial to know that the larger the piece is, the more important it becomes to let it dry evenly (slow drying will yield even drying). See next chapter about Air Drying. If one part drys faster than the other part, the dryer part will shrink more on wetter part which will create stresses on the clay, thus, cracking could occur.

 

Air Drying

Wet clay contains a large amount of water, a minimum of 25% water. When clay starts to dry, water evaporates from it. As this happens, the particles of clay are drawn closer together resulting in shrinkage. Many problems with clay are formed by uneven rates of drying, which create stresses in the clay. Sometimes these stress show up right away as cracks or warpage, other times not until during or even after firing. So it is important to ensure drying is even. This is done by ensuring uniform thicknesses throughout the piece, drying slowly, and even slowing down the drying of certain parts.

Clays which have very fine particle sizes will shrink more than clays with larger particle sizes. Porcelain clay has very fine particle sizes which makes it very plastic and also shrinks the most. These bodies have the most strength in the dry state. Groggy clays such as sculpture bodies shrink the least. (Grog is clay which as already been fired and then ground to various particle sizes.) These bodies shrink less because they have lower water content to start with, and also provide channels through which moisture can escape toward the surface. These are called “open bodies”.

When the water has evaporated form between the clay particles, and all the remaining clay particles are in contact, drying shrinkage is complete. This is called the leather hard stage. The particles themselves are still damp, but their drying will not cause any additional shrinkage.

All information on this post is a direct copy from the the website stated below:

http://www.lakesidepottery.com/HTML%20Text/Tips/Clay%20drying%20and%20firing%20process.htm

 

Facts about porcelain according to ‘Ceramics for Beginners- Why I Use Porcelain Clay for Pottery’

• The more platelets there are in your clay the further you’re going to be able to stretch your clay out.
• If you drop it after a firing you’re less likely to break it.
• Porcelain has a vivid white colour to it before firing and after firing, even with some glazes applied over the top compared to other clays.
• It’s ‘kinda like a bad kid’ as it tends to do what you don’t want it to do whilst throwing it.

 

Overall, it has a prettier finish than most clays but is significantly more difficult to sculpt or throw.

Special Techniques

“Pate sur Pate” Fawn Zeller discovered a technique
called “Pate sur Pate” which
changed the way she sculpted in porcelain forever.
Tiny detail in her small work was often applied with wet slip,
to which white vinegar
had been added to thicken it to a buildup consistency.
Note: Fawn Zeller used very little vinegar to thicken slip
that she was applying to
her sculpt in liquid form. Other artists needing larger pieces
of clay while working in more traditional additive processes
might pour a bit of
slip onto a plate of hard, dry plaster to draw excess water out.
Fawn started sculpting in the customary manner by manipulating
porcelain clay in a moist,
leather hard stage – always keeping it moist throughout
the sculpting process but was
frustrated by the difficulties of keeping her small
sculpts consistently wet.

Porcelain doll maker technique:

Direct sculpting can be started before the greenware is even leather hard!
While doll artists are taught to be EXTRA careful when handling greenware
for larger dolls, miniature doll artists have found that
they can really abuse their greenware and get a startlingly good finished
product! To keep greenware from splitting and cracking in the kiln,
you often have to prop larger pieces and handle the wet greenware
VERY carefully … this isn’t a problem with miniatures (those dolls 7″ and under).

They regularly take miniature hands, cut the fingers apart and reposition
to articulate the hands before the greenware is even leather hard!!!
With a sharp scalpel (be careful), cut apart the fingers …
don’t try to cut all at once, but gently score through.
Then, using a sable brush dipped in distilled water, you can refine
the cut edges and even move the digits to make a pointed finger,
curled hand, fist, or anything that your heart desires. Set aside
the greenware, let dry as usual, clean and fire to bisque!
The same method can also be used to reposition heads! Experiment!

All information on this post is a direct copy from the website found below:

https://kathyoconnellsart.wordpress.com/2011/07/22/porcelain-clay/

Tips for working in Porcelain

Keep it Moist!
Early NIADA artist, Fawn Zeller, started sculpting
in the customary manner by manipulating
porcelain clay in a moist, leather hard stage
– always keeping it moist throughout the sculpting process.
While working in this traditional manner,
she was frustrated because her clay sculpt would
change from day to day due to an uneven drying
even though it was being kept in a damp enclosure.
Daily, she needed to re-wet it,
manipulate it;
or add wet slip to smooth out distortions.
Fawn said, “It would take at least three weeks of
manipulation until the clay itself
finally seemed to give up and the sculptor,
at last, was in charge”.

Molds
If molds are used at all, these castings typically
have minimal detail so most of the
sculpting is done via direct clay manipulation.
The cast clay is promptly removed from the mold
before it is even leather-hard. In
this very wet and extremely fragile state,
they start to sculpt with complete sculpting
freedom yet do not have to worry about
hollowing out later!

Make it hollow
Most clay must be hollow before it is fired
so that all the clay is a consistent
thickness in hopes that it will all vitrify at the
same rate without any distortion. Fawn Zeller hollowed
her pieces in the traditional
manner by cutting them in half,
removed the excess clay in the center,
then carefully rejoined the backs to the fronts.
Marlaine Verhelst starts her sculpts by laying
her clay down over a Styrofoam form which
when removed leaves a hollow consistently
thick sculpt ready to fire. As noted above,
others start off with a set of simple hollow
Castings.

Safety note: Inhaled porcelain dust can be fatal over time.
Protect your lungs when
cleaning, sanding or direct sculpting porcelain!

All information on this post is a direct copy from the following website:

https://kathyoconnellsart.wordpress.com/2011/07/22/porcelain-clay/

Drying phases & Supported Processes

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: 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!

 

All information in this post is a direct copy from the following website:

https://kathyoconnellsart.wordpress.com/2011/07/22/porcelain-clay/