Increditde strides in porcelain ware pro-
duction and firing equipment have oc-
curred in the last couple of decades. Ro-
botics and computer controllers have revo-
lutionized the whole ceramics industry.
However, clay bodies themselves have
tended toresist change. Understanding the
dynamics of powder, slurry, and wet ma-
terials processing, forming, drying, and
firing is not easy,

A pottery porcelain is actually just a vitri-
fied clay body with low Fe203 contami-
nation, A general porcelain recipe is fairly
easy to derive. The initial thought process
goes like this:

* The only common low-iron clay is Kao-
lin. Kaolins normally have low plasticity,
so some plastic ball clay having minimal
iron may be needed.

* These clays are refractory, so fluxes are
necessary to make them mature at a lower
temperature. Feldspar is effective and tn-
expensive,

* Adding flint reduces body expansion
making it easier lo fit glazes and it acts as
acheap filler and firing stabilizer.

The Universal 25 Porcelain recipe is a
product of this type of reasoning. It is
made from 25% each of ball clay, kaolin,
feldspar, and flint or more simply MI%
clay and 25% each of flint and feldspar.
Thousands of potters and companies use
this as is of alter it to accommodate spe-
cific materials or circumstances.

Before continuing, let's define the physi-
cal properties to look for in a porcelain
(other than price). Here are a few:

Fired translucency: You can compare
the translucency of fired porcelains by
firing very thin slices (2 mm thick), gluing
them on cardboard with holes in it, and
then holding it up to the light.
Consistency: Only quality control over
several years will demonstrate this. Dif-
ferent kaolin and ball clay companies dis-
play diverse commitments and attitudes
toward maintaining physical properties of
their matenals for the ceramics industry.
Plasticity: This can be deduced from dry-
ing shrinkage, and judged physically by
comparing workability,

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TECHNO TIPS
Formulating a Porcelain

Drying Performance: Use an accelerated
drying gradient lest.

Fired and Dry Strength: Make bars and
break them in a strength tester,

Speck Development: Observe in fired
ware, especially in reduction burning.
Solubles: Calcium and magnesium sul-
phates migrate to the surface with water
during drying and leave visible surface-
scum after firing. Do a FORESIGHT
SOLU test to determine if a problem.

Thermal Shock Resistance: Refers to the
fired clay’s ability to withstand thermal
gradients. Compare bodies using a flame
or ice water/boiling water immersion test.
Ease of Glaze Fit: Determine if normal
glazes craze when stressed.

Fired Whiteness: Compare fired samples
in bright light.

Fired Volatility: How wide o range of
temperatures will the porcelain perform
well at? Will it sag or warp if slightly
overfired or be dry if slightly underfired?
Surface Character: Is the surface dry,
amooth, glossy, or pebbly?

Fired Colour: [s it white, off-white, bluwe-
white, grey-white or yellow-white?

Porcelains can be compared in all of the
above areas. Logically, you cannot have
the best of all of ther. There are always
trade-offs and compromises. These physi-
cal properties can all be measured using
simple equipment, methods and observa-
tions as described in the FORESIGHT
Ceramic Database software test suite.

Recipes & Strategies

Most people have noted that 25 Porcelain
recipe has flaws that can be corrected for
individual situations and materials.

A starting cone 10 recipe using materials
commonly available in North America:

204% Flint

10%  CusterFebdspar
10% NephelineSyenite
16% Tibet Kaolin

16% Pioneer Kaolin
16% EPR

5% NP Blend Ball Clay

5% OM#4 Ball Clay
2% Bentonite

Peatere Coald of Britich Moburmbhia

The basic method to formulate a porcelain
starting from the 25 Porcelain recipe is:
* Start with two kaolins, ball clays, and
feldspars.

* Remove or add feldspar at the expense of
ball clay to achieve fired maturity,

* Remove flint and add kaolin until your
glaze does not fit, then back off. Alter the
glaze if possible.

* Reduce ball clay and add kaolin until the
plasticity is just tolerable.

* Add bentonite if it can be tolerated,

* Test the drying performance and fired
properties thoroughly and adjust feldspar
if necessary.

A start-from-seratch method would he:
* Start with a diversified 75 kaolin-25
feldspar mix.

* Add Mint at the expense of kaolin till the
glaze fits.

* Add or remove feldspar to adjust fired
maturity.

* Add ball clay or bentonite at the expense
of kaolin oll the workability is tolerable.

+ Fine tune maturity using feldspar.

If you need to formulate a casting perce-
lain, remember that much lower plasticity
is needed and it is more important to use
kaolins of large particle size, so water can
easily be drawn out by the plaster mould.
The cleanest materials are also the least
plastic; thus casting porcelains can achieve
much whiter and more translucent effects
than their plastic counterparts. Also, if you
intend to pour slip, i ts imperative that you
understand the principles of ‘defloculation'
so that the amount of water in the slurry
can be minimized and proper mould re-
lease and casting time can be achieved, If
you have not witnessed the magic of add-
ing a few drops of adispersant into.a mixer
where a hopelessly thick clay-water mix
defies agitation, you have not lived!

If you are making a plastic porcelain for
use in modeling, throwing or machine
forming, pay careful attention to its drying
properties. Since porcelains are fine-
grained, they don't usually dry well, thus
plasuic porcelains are even worse. It is
important to have a good test to rate and
compare drying performance (PORE-
SIGHT ships with such atest predefined).

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