I.Lewis on sat 5 feb 00
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These notes follow on from ideas of what happens when raw materials selected=
to
make white translucent pottery are heated to maturity.
Translucency is caused, to a greater or lesser extent, by scattering light =
as it
passes unabsorbed through a substance. It is a measurable quality, =
intermediate
between opacity and transparency though the intensity which is observed from
transmission may diminish to nothing with increasing thickness. The effect =
may
be present though we do not see and appreciate it. Though we may see a =
surface
as white, this does not mean the substance is opaque and white in the same =
sense
that frosted Silver is opaque and white. Substance which are opaque to light
absorb the energy.
Porcelain fired to maturity so that all of the chemical an physical =
reactions
and changes which can occur have happened will consist of two or more =
phases.
One should be Mullite crystals which are elongated needle like e prisms,
uniformly scattered through the material. The other main phase is a vitreous
glass which forms when a silicate solution is supercooled. Any other =
crystals
will be dependent on the original recipe, the firing temperature and the =
cooling
rate. There may be free quartz and cristobalite. Voids, often called =
porosity,
derived from air spaces in the dried clay should be minimal.
Mullite is possibly transparent and colourless. Silicate glass is often
transparent and colourless. Quartz is frequently colourless and transparent.
Cristobalite is said to be colourless and translucent. Voids are =
transparent.
Optical properties of all these constituents have an important bearing on =
what
we see when we examine a sample of fired porcelain by both transmitted and
reflected light.
Light is a remarkable form of energy. When it impinges on surfaces or
interfaces, several well know effects are possible.
1 Transmission. Light passes through a material and emerges seemingly =
unchanged.
Examples=3B Window glass, clear air.
2 Reflection. Light bounces away from an interface or surface. Example=3B =
Mirrors,
internal reflection in periscope prisms and reflex camera systems.
3 Refraction. Light changes it=92s direction as it passes from one =
transparent
medium to another. Examples=3B Lenses, apparent shallow water.
4 Dispersion. Light separates into its component colours or wavelengths.
Examples=3B raindrops and the rainbow, faceted gemstones.
5 Diffraction. Light bends as it passes between narrow gaps and can separate
into its component colours. Example=3B the diffraction grating used in
spectroscopes, perhaps iridescence of feathers.
6 Interference. Light which has been separated into its component =
wavelengths
can recombine or cancel. Example=3B Thin oil films on water, Newton=92s =
Rings, soap
bubbles.
7 Diffusion. The profound scattering of reflections. Examples=3B White =
paper,
clouds, foam and porosity.
These effects can be seen in many transparent minerals and vitreous =
materials.
But because the crystals in fired clay are so small, an effect can occur =
which
gives the impression of whiteness. Myriads of minute crystals and remaining
voids cause millions of reflections as light moves deeper into the material.
Light which emerges having reversed its direction will appear white and
diffused. If this effect is strong the material appears to be opaque. =
Looking
from the opposite side, if no light falls on the surface, the material will
appear opaque and dark. It seems possible that refraction and reflection are=
the
most important of these factors in determining the degree of translucency of
porcelain.
There may be people who believe, because the material appears opaque, that =
it is
not optically active in a way which we would desire when attempting to =
achieve
translucency. Appearances may be deceptive. Look at thick porcelain which is
covered by a transparent or near transparent coloured glaze. Light shines on=
the
surface. Some is immediately reflected. But a proportion which is =
transmitted in
from the surface is refracted and reflected back to the viewer from inside =
the
fabric of the pot. This additional light enhances and amplifies the colour =
of
the glaze. Not only is light seen from the surface of the glaze because of =
it=92s
own reflective quality. Light which has passed through it is now coloured =
and is
reflected back as that colour. This intensifies the effect. Colour appears =
to
come from within the pot so that the pot seems to have body colour, as if
stained. It may be argued that this is no different from using a white =
stoneware
or a white slip as a base for decoration. But stoneware is contaminated by
materials which absorb light or change it=92s colour and white slips may be =
based
on opacifiers with high reflectivity. For the most part, pots made from
porcelain are viewed with frontal illumination. Even when the material is
exquisitely thin, back lighting plays almost no part in our appreciation of =
the
medium unless this is contrived by gallery director or photographer or a
deliberate design intention of an artist.
These notes, which have been kept as clear and a simple as possible, show =
that
pottery does not need to appear translucent for that quality to exist in a =
pot
and have a profound influence on our aesthetic appreciation. Bearing these
things in mind as work is conceived may prompt incorporation of design =
elements
which enhance translucency effects. Carving, as in examples shown by Earl
Brunner, provides edges where light can penetrate through the porcelain. We =
may
ignore information which is known, but ignorance limits our creative =
potential.
Ivor Lewis. Inquisitive. Searching. Suggesting that if there a is need for
elaboration of the basic principle which influences translucency, without
resorting to abstract symbolism of mathematical representation or scientific
jargon, then ask questions. I would do my best to keep it simple. Or perhaps
someone else may make a better fist of it than I.
Michael Banks on sun 6 feb 00
Just a few added comments to Ivor's on translucency:
The amount of scattering of incident light inside a porcelain body is
significantly controlled by the contrast in the refraction between the
mineral phases present.
Mullite may well be colourless and transparent (although many natural
mullite crystals are coloured and contain iron & titanium), but it's
refractive index (~1.65) is higher than silica glass (~1.46) and light rays
travel at different speeds through these two materials -resulting in
scattering.
In my opinion mullite will therefore have a detrimental effect on
translucency. This loss of translucency must be balanced by mullites
positive effects on body rigidity (when hot in the kiln) and contributions
to ware toughness. I've noticed that the most translucent porcelains have
high silica/alumina ratios. I believe this is to maximise the formation of
silica glass and keep mullite levels to a bare minimum level (commesurate
with hot strength considerations). A trade-off between translucency and
slumping resistance.
Refractive index contrasts explain why titanium has a negative effect on
translucency. This is because rutile needles crystallize inside the
porcelain. Rutiles refractive index is a whopping 2.9 (even transparent
rutile) which causes severe incident light scattering and resultant
transmission losses. In addition rutile scavenges any iron present which
also raises its R.I. further, as well as colouring the cystal needles (pale
yellow-brown) -absorbing even more incident light. This absorbsion colour
of rutile (which is almost impossible to totally eliminate) gives some
porcelain an attractive honey colour. But if more than about 0.5% TiO2 ( as
well as higher iron -which can turn rutile almost black) is present, the
ware becomes an opaque grey, as a high percentage of incident light is lost
by absorbsion and internal refraction among the forrest of rutile needles.
Michael Banks,
Nelson,
New Zealand
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