Neon-Cat on thu 2 oct 08
Here's how floating blue glaze and rutile blue glaze really work.
In the paragraphs below I'll explain how rutile mineral (not chemically
refined titanium dioxide powder) causes a blue color shift (iron appears
blue) and discuss other optical effects unique to rutile. A section on boron
in a floating blue glaze will show how it helps us see more of the special
effects created by rutile. Cobalt's role as a co-catalyst, color toner, and
colorant in a floating blue glaze will be briefly mentioned. Adsorption
principles and the role of impurities within a glaze or clay body that add
additional effects to glaze color will be presented. How different clay
bodies under a floating blue glaze or a rutile blue modify the effects of
the applied glazes will also be briefly discussed.
Rutile is one of the five forms of naturally occurring titanium dioxide.
Titanium oxides including rutile (TiO2, titanium dioxide) are composed of
tetragonal unit cells where Ti (4+) cations occupy corners and the center in
an octahedral polyhedral structure (Mn oxides, and Al, Mg, Fe, and Mn
hydroxides also form octahedral polyhedral structures). In rutile
(crystals), six oxygen ions in the form of an octahedron surround each
titanium. The TiO6 (8-) octahedron shares two edges with the adjacent
octahedra forming two types of ribbons that link over the remaining free
octahedral corners creating an infinite number of little tunnels (often
called channels) through the oxide or oxide complex. Various cations may be
accommodated within these channels. Because of the rutile's complex
structure it is classed as a Ditetragonal-Dipyramidal crystal. When rutile
is coordinated with the silica from the glaze ingredients and clay body
constituents, the silicon is always, within the parameters of our pottery
and ceramic kilns, in tetrahedral coordination. Titanium, an active
transition metal, has variable oxidation states (4+, 3+, and 2+) and can
form numerous stable compounds and complexes. When acting as a catalyst
titanium is successful because it can shift rapidly from one oxidation state
to another and form complexes. It should be noted that rutile, as single
crystals, can be oxidized and reduced.
Titanium, iron and other transition metal cations can catalyze chemical
reactions, including thermochemical reactions, some more strongly than
others. For example, for montmorillonite clay containing substituted
materials the catalytic abilities from best to worst are Ti > Fe > Cu > Mn >
Ca > quartz > talc. Titanium (rutile) is also a photocatalyst
(photo-reducer). Rutile is transparent and translucent on thin edges. In a
glaze rutile particles are translucent to transparent to visible light. Its
color ranges from blood red, brownish-red, brown, bluish, brownish-yellow,
golden-yellow, yellow, violet, grayish-black, to black. Its crystals are
prismatic (slender prisms) and acicular (needle-like). Its reflected light
color is gray with a bluish tint. Unmelted pure and substituted rutile
crystals may be seen in the glaze, lighter in color where impurities have
moved out or been exchanged for other cations of a lighter hue. Unlike the
chemical titanium dioxide, a homogeneous white powder often included in
glazes as an opacifier, rutile comes to us as crystals and boules (less
crystalline masses) that have high optical transmission and high
birefringency (a double-refraction phenomenon in which an unpolarized beam
of light is divided into two beams and refracted in different directions,
each with different wavelength velocities). Under the influence of
ultraviolet radiation (UV light) rutile (not the chemical TiO2) at
wavelengths between 290 - 400 nm causes a shift in color toward the blue
visible spectrum so that iron appears blue. Rutile is used in many
industries as a "color absorber" for this blue-shift effect so that colors
are enhanced or made more pleasing. Rutile is highly efficient at absorbing
UV light and once absorbed the ultraviolet light is converted to heat and
not reemitted. So while not often blue itself, rutile does help create the
color we perceive as blue. Exposure to UV radiation occurs in the kiln from
the flame itself in gas or wood firing or as heated pots glow and give off
light. Outside the kiln, post-firing, common sources of UV light are
sunlight and fluorescent lighting. When other metallic elements are present
as oxides or ions, the rutile helps to scatter light.
In minerals and gems rutile has the property of asterism, specifically
epiasterism; inclusions of rutile will form reflective areas where parallel
waves of light are seen in the shape of a six-rayed star. The stars are
actually white to yellowish-white translucent crystals of rutile. It is
quite possible that this effect is also produced, to a limited extent, in
floating blue glazes.
Oxides undergo isomorphic substitution that accounts for the impurities
reported in rutile: iron (up to 10%), chromium, vanadium, tantalum, niobium,
and tin. Rutile mineral, by definition, may contain up to 15% total
impurities. When iron substitutes in the rutile structure the rutile
composition may be complex as the iron content varies. With the iron in
rutile and any added iron oxides in the glaze recipe the color developed
depends on the Fe2+ and Fe3+ ratio. By themselves, Fe 2+ cations impart a
yellow color and Fe 3+ yields green color. Influenced by rutile, the iron
species appear light or dark blue. This complexity of composition and the
distribution of the various cations among structurally nonequivalent
positions are one of the causes for phase transformations at lower
temperatures within the kiln. Feldspars used in our glazes and our clay
bodies also undergo isomorphic substitution (a form of permanent, non-pH
driven adsorption). Titanium (Ti 4+) may substitute in the phyllosilcate
lattice structure for aluminum (Al 3+) providing the rutile has come apart
in the heat of the kiln enough to release titanium ions or chemical
reactions have occurred during the course of glazing and firing that have
allowed for Ti cation mobility. The melting point of rutile is 1825 - 1840 C
(3317 - 3344 F).
Rutile (or its Ti cations) may adsorb onto the edge surfaces of clay
particle groupings (tactoids) or be found within the interstitial layers
between the octahedral and tetrahedral layers of the clay minerals,
primarily clays with a 2:1 layer ratio - the pyrophyllite group of clays
(talc and pyrophyllite), smectites (montmorillonite, bentonites, etc.),
vermiculite, illites, and micas (muscovite, lepidolite, etc.). Kaolinite
(kaolin) does not commonly undergo isomorphic substitution but will adsorb
ions on its external edges and surfaces. If your glaze is acidic (unlikely)
or you've treated your clay body with vinegar the pH change improves the
exchangeability of kaolinites, especially calcined kaolins. Because charge
differences must continually be satisfied so that there is an overall
neutrality, cations may constantly exchange or group, often in coupled
substitutions, for example Ca (2+) exchanges with Na (+) while at the same
time Al (3+) exchanges with Si (4+). In stoneware bodies where mica might be
present as part of an included fireclay or in a recipe calling for ball clay
boron (B 3+) will substitute for silicon (Si 4+). Isomorphic substitution
(and other adsorption processes) may occur between glaze ingredients when
the glaze is mixed and between the glaze ingredients and the clay body
during glaze application to the clay body and during firing.
The number of metal ions adsorbed by clay generally shows a direct
correlation to the surface area of the clay minerals involved and their
cation exchange values (CEC) where humus > vermiculite > allophane >
smectite > chlorite > peat > kaolinite. Even organic materials (humus, etc.)
in clay bodies will adsorb metals. The firing of organic material gives rise
to exothermic chemical reactions raising the temperature of all sites
surrounding the organic material and this affects the reaction and
adsorption of other substances within the particle field. Iron, as an added
glaze ingredient or a constituent of rutile, will substitute for other
elements in our pottery materials: Fe 3+ replaces Mg 2+, Fe 2+ and 3+
replaces Al 3+, etc. Elements that are commonly involved in isomorphic
substitution are (in no special order) Cu, Zn, Cr, Co, As, Se, Ni, Pb, Cd,
Mn, Ca, Na, K, Al, Mg, Li, B, Fe, and Ti. At any rate, various shades of
dark and light blue can result from titanium and iron substitutions in the
layers of the clay minerals. Where saturation is poor, iron and titanium
will produce only gray hues. Perceived color may of course be affected by
those ions we cannot see, the ones tucked behind other atoms, or buried
between clay layers that collapse around the ions during firing. Recall,
too, clay body surfaces may have become, in various places, "stained" by the
adsorption of colorants through the processes described previously. A clay
surface stained brown in places beneath the color blue will result in a
deepening of perceived blue color in these areas. Adsorption is hindered by
rising kiln temperatures as water is lost and when temperatures are reached
that collapse clay lattices or transform oxides and other ceramic materials.
Titanium substitution for aluminum in a floating blue glaze might be likened
to the process that creates blue sapphires from the clear mineral corundum.
Quantities of associated Fe2+ and Fe3+ in daylight-blue sapphires may be as
low as 0.005% to 0.8%. The trace impurities in rutile impart their own color
to the glaze or subtly influence glaze color through their own color shifts
(an optical effect). For example there will be yellow and gold and green
color shifts with iron; purple shifts from vanadium, and pink shifts from
chromium. When high amounts of rutile (15% or more) are utilized in a glaze
it can become light sensitive much like 'color shift' sapphires. High rutile
glazes may darken in bright light and lighten and pale slightly in darkness.
The type of inside light under which a floating blue or rutile blue glaze is
viewed - incandescent verses fluorescent -- may also affect high rutile
glazes (purples become blue, pink become green).
So with this brief explanation we now more fully understand how crystalline
rutile (not the powdered chemical titanium dioxide) creates its magical
effects in floating blue or rutile blue glazes - everything from a
variegated look to a mixed effect combining a crystalline appearance with
speckled, streaked, glittery, mottled, color shifting, and floating blue
effects.
High temperature rutile blue glazes (low in silica, boron, and alumina,
while high in calcium) get many of their colorful effects created by rutile
through the same principles discussed above. In general though, the rutile
effects are not as pronounced as in the lower temperature floating blue
glazes where the optical effects from blue shift and color variegation are
seen because of the glaze depth and transparency and the inclusion of
borosilicate glass in the glaze.
Boron, a non-metal or semi-metal, acts as a flux and glassmaker in a glaze
recipe. In floating blue glazes borosilicate glass in fired glazes allow us
to better see the effects that rutile creates. Borosilicate itself is
vitreous and clear. Like silica glass it has no crystalline structure. We
all know that silica, SiO2, in our glazes is a glass former par excellence.
However its high melting point (1723°C or 3133°F) and its high viscosity in
the liquid state make it difficult glaze material to work with and melt.
Hence our use of fluxing materials. Boron, in the form of ceramic oxides
(boric oxide, B2O3), may be fluxed or itself act as a flux. When melted
boric oxide readily forms a vitreous network of borosilicate glass at
temperatures below those need to form silica glass.
For glazes utilizing soda ash (sodium carbonate, Na2CO3.10H20) or soda
feldspar (12% or less Na2O) or Nepheline Syenite (10% or less Na2O) or
Gerstley Borate or Gillespie Borate (approximately 4% Na2O), boron will
react with sodium forming sodium tetra borate (Na2B4O7.10H2O) beginning
around 800 - 900 C (1472 - 1652 F). Borosilicate and alkali borosilicate
glasses are subject to defects much like ordinary silica glass. Defects in
the glaze network translate into slight abnormalities involving light
transmission and absorption. Boron (3+) may substitute for Si (4+) and other
substitutions among cations and anions may occur (available anions come into
play when charge balance is needed). Areas of bubbles, concentrations of
color, and unfluxed material occur in borosilicate glasses as in silica
glass.
The significant difference in properties between vitreous silica and
vitreous B2O3 is the change in coordination in borates between a three-fold
triangular coordination to a four-fold tetrahedral coordination depending on
the conditions. Four fold coordination increasing the connectivity of the
network. The result is an increase in glass transition temperature and a
decrease in the thermal expansion coefficient. At higher alkali oxide
concentrations in borate glass these trends reverse due to the formation of
non-bridging oxygen atoms. Similar additions into silica glass have the
opposite effect. When both silica and boron form glass in a glaze the boron
is present as a vitreous alkali borate glass dispersed in the silicate glass
matrix, often as small isolated droplets distributed through the silica-rich
matrix. The phase separation is on such a fine scale that both the silica
and borosilicate glasses appear uniformly transparent. The durability of a
glaze made with boron is determined by the rate of dissolution of the silica
glaze host since any solution or material will not come into contact with
the low durability alkali borate phase until the surrounding silica glaze is
removed.
One other very important difference between borosilicate glass and silica
glass in our floating blue glaze and one of the keys to how a floating blue
glaze works is that borosilicate glass transmits near-UV light (in part of
the UVB range, from 290 -- 320 nm) while silica glass does not transmit any
UVB (the UV light associated with sunburns). Both silica and borosilica
glass transmit UVA (320 - 400 nm) but not UVC (between about 260 - 320 nm).
So to more fully see and appreciate the optical effects of rutile (blue
shift, etc.), borosilicate glass is an important piece of the floating blue
glaze magic. Blue shift will be apparent with or without the borosilicate
but the boron in the glaze will give us a greater depth of color affects
than we normally see with silica glass glazes. It should also be noted that
the total transmittance of light waves in either a silica or borosilica
glaze depends on the thickness of the glaze. Even very thin glazes will not
produce 100% transmittance due to the way in which light is reflected on the
glaze at different places.
To reiterate - borosilicate glass and silica glass as created in our glazes
are clear and neither are responsible for the blue color in floating blue
glazes - that's accomplished by the rutile.
Cobalt, when added to a floating blue glaze may contribute its own color
effects or it may act as a color toner. In our floating blue glazes, cobalt,
as a co-catalyst (a transition metal property), helps to modify or enhance
the catalytic performance of rutile. To some extent, zinc and manganese, if
present, can also act as a co-catalyst with rutile to modify the catalyst
performance of rutile.
To summarize:
1. Rutile cause blue color shift (iron appears blue) and other optical
effects;
2. Boron glass in a floating blue glaze helps us see more of the special
effects created by rutile;
3. Cobalt acts with rutile as a co-catalyst and may impart its own color to
a glaze or act as a color toner within the glaze;
4. Impurities cause additional effects in glaze color and other properties;
5. The clay body covered by a rutile glaze matters - different types of clay
adsorb cations differently, contributing to an over all variety of effects
between different clay bodies glazed with the same glaze.
Sorry for the length of the post - there's no way to explain some things
without getting a little wordy and the question of just how a floating blue
glaze and its cousin the rutile blue glaze work has been interesting. I've
never seen a floating blue glaze or a rutile blue in person but I'll be on
the lookout for both now. In the meantime I can try one of the cone 10
rutile blue glazes from John Britt's book and experiment with Mel's rutile
wash so I don't feel left out.
Happy glazing rutile fans!
Marian
Neon-Cat Ceramics
Ivor and Olive Lewis on sat 4 oct 08
Nigel Wood presents a survey of opinions about the origins of blue
glazes and cites the work of Prof. David Kingery and Dr Pamela Vadiver
in his text "Chinese Glazes". He also cites Dr Robert Tichane who, in
"Celadon Blues", presents graphic evidence or images and X ray
analyses of a variety of glazes. In addition Nigel Wood speaks of
Prof. Chen Xianqiu of Shanghai and his work on the RO2 (SiO2+TiO2) to
R2O3 (Al2O3+Fe2O3+P2O3) ratio as a determinant of Liquid-Liquid phase
separation at stoneware temperatures.
For those who are looking for a practical approach to the problems of
"Floating Glaze" of Chapell or modifications that are termed "Floating
Blue" I can recommend both books as being worthwhile reading.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ps....Putting those ratios into Glaze Calculation programmes could
provide useful information ! (il)
| |
|
