Tom Buck on sat 28 nov 98
Caryl W:
Ron Roy noted your two glazes (Butterscotch, Seafoam Blue) are
"balanced" for C6 which is good. I'd like add data on colour effects, info
I had prepared earlier and sent offline to another Clayarter.
Why the different colours, and why do they change when the order
of application is changed? Well, one cannot give you a precise
explanation without chemically analyzing the fired glaze on a pot.
However, the likely culprit is the level of Titanium oxide in the
combo, espy at the surface.
Parmelee/Harmon note in "Ceramic Glazes" that titanium oxide will
yield a range of colours in many alumino-silicate melts. Here is a bit of
what they and others say (I wrote this earlier, $ are Canadian):
Veronica:
I have prepared for you the following summary of titanium
materials and their glaze effects.
Titanium "raw" materials
There are available commerically three varieties/forms of
materials containing titanium dioxide:
1) Ilmenite, a natural crystalline mineral that combines
titanium dioxide and iron oxide, reported as 62% TiO2 + 26% Fe2O3
plus small amounts of alumina and silica. Although the composition
(ultimate analysis) of ilmenite is reported as above, the crystal
is actually repeating units of TiO2 and FeO. It is chemically
very stable. Ilmenite is sold in two grades (particle sizes),
namely granular and milled, and costs approx. $4/kilo in small
amounts, $1.60 when a 44 kg bag is purchased. It is a black
powder, granular staying on an 80-mesh screen, milled grade
goes through this screen.
2) Rutile, a natural crystalline mineral whose colour will vary
between red, reddish brown, to black, is also made from ilmenite to become
the chief feed to chemical plants producing high-purity titanium dioxide
for the paint industry. Rutile ultimate analysis is typically 93% TiO2 +
2.0 - 2.3% Fe2O3 in the FeO form. It should be noted that some rutile of
commerce comes from plants making TiO2 pigments; the rutile is often an
upgrade from cruder ilmenite ore. Rutile's behaviour in a glaze recipe
follows that of titanium dioxide since the FeO content is relatively low.
Rutile is sold in two grades, granular and ceramic (fine powder), and
costs $7+/kg for 5+ kg.
3) Titanium dioxide is usually a bright white high-purity
powder containing 98% TiO2, pigment grade (but some cruder TiO2
powders are also made). TiO2 is produced chemically from one of
titanium-containing ores, mostly ilmenite or rutile. It costs
$10/kg (5 kg) and $7/kg (22.7 kg).
Titanium compounds in Glazes
Here is what Parmelee (1st edition) had to say about
Titanium 50 years ago:
"Titanium acts somewhat similarly to silicon and is added
to glazes both for its influence on colour and its tendency to
produce opacity since it is only slightly soluble in the melt.
The coefficient of thermal expansion factor of the oxide (TiO2)
is about midway in the list of common [essential] oxides...
"Titanium is used in glazes [rutile or TiO2] to develop
colour or textural effects. It is also an important reagent for
the development of crystalline glazes...
Parmelee says, depending on the base glaze, adding TiO2
or rutile to the glaze will produce a range of colours, brown,
green, blue, even red. He noted that TiO2 may be reduced by
carbon monoxide to form a different oxide, one that appears red
like a reduced copper glaze. Also, sometimes an intense blue is
obtained in reduction, which he attributed to the formation of
titanium nitrides. He also said that rutile + cobalt oxide
would yield a green (yellow + blue optically blended).
In a glaze containing zinc oxide (Bristol glaze) fired in
oxidation to Cone 6, the actual colour yielded by iron oxide
(0.09 moles) will go from an "intense brown" to an "intense
green" when 0.10 moles of titanium dioxide (0.10 moles) is added
to the glaze mix.
Here is what Rhodes wrote many years ago in Clay and Glazes
for the Potter:
[The ore] rutile is used in glazes as a source of
titanium provided the colour [from] iron is acceptable.
Rutile gives tan or brown colour to glazes. Its tinting power
however is weak, so more often rutile is used for textural
effects, namely, broken colour or mottled colour. [One adds]
3-5% to produce such effects.
In glazes containing boria (B2O3), rutile may cause
pronounced streaks or spots, particulalrly in those glazes
which are opaque and highly tinted with some other colourant,
such as copper or iron.
Rutile may also increase opacity when included at 5%
or more in the glaze mix.
[The ore] ilmenite with its higher iron content behaves
more like iron than titanium but the larger particle sizes (80
mesh or larger) causes specking/spotting. 1-3% is sufficient.
Often these spots show a "halo" effect.
Crystalline glazes, with their very low alumina content,
and hence very fluid, are often made with zinc oxide (0.3 moles)
and rutile, the latter helping the formation of crystals.
And Frank Hamer wrote this about rutile [Potters Dictionary]:
Rutile. TiO2. Natural titanium [IV] oxide (aka dioxide)
with a small amount of iron [oxide] which gives it a yellow
colour ...[which] is retained when [it] is used in glazes,
therefore some bright creams, yellows and oranges can be produced
in association with tin oxide, cobalt oxide, and red iron oxide.
...Amounts of rutile, up to 10% in a glaze, give a broken
colouring effect due to crystallization. In association with tin
oxide a whole variety of streaked and mottled effects are
obtainable...
Rutile readily produces a crystal structure in a glass on
cooling and therefore opacity and a matt surface due to crystals
can be produced by introducing up to 25% rutile in a glaze.
Glazes containg 15% or more of rutile, or rutile and titania,
are subject to phototrophy after firing. Phototrophy is the ability
to change colour by the absorption of light [photons]. Such glazes
darken with light and grow pale with the lack of it. This reversible
proceess can continue indefintely.
Rutile containing over 25% iron oxide, as FeO [iron II oxide,
black iron oxide], is usually called by another mineral name, ilmenite,
and this ore has a colour resonse more akin to iron oxide instead of
titania.
Here is what Rhodes wrote many years ago in Clay and Glazes
for the Potter:
[The ore] rutile is used in glazes as a source of
titanium provided the colour [from] iron is acceptable.
Rutile gives tan or brown colour to glazes. Its tinting power
however is weak, so more often rutile is used for textural
effects, namely, broken colour or mottled colour. [One adds]
3-5% to produce such effects.
In glazes containing boria (B2O3), rutile may cause
pronounced streaks or spots, particulalrly in those glazes
which are opaque and highly tinted with some other colourant,
such as copper or iron.
Rutile may also increase opacity when included at 5%
or more in the glaze mix.
[The ore] ilmenite with its higher iron content behaves
more like iron than titanium but the larger particle sizes (80
mesh or larger) causes specking/spotting. 1-3% is sufficient.
Often these spots show a "halo" effect.
Crystalline glazes, with their very low alumina content,
and hence very fluid, are often made with zinc oxide (0.3 moles)
and rutile, the latter helping the formation of crystals.
And Frank Hamer wrote this about rutile [Potters Dictionary]:
Rutile. TiO2. Natural titanium [IV] oxide (aka dioxide)
with a small amount of iron [oxide] which gives it a yellow
colour ...[which] is retained when [it] is used in glazes,
therefor some bright creams, yellows and oranges can be produced
in association with tin oxide, cobalt oxide, and red iron oxide.
...Amounts of rutile, up to 10% in a glaze, give a broken
colouring effect due to crystallization. In association with tin
oxide a whole variety of streaked and mottled effects are
obtainable...
Rutile readily produces a crystal structure in a glass on
cooling and therefore opacity and a matt surface due to crystals
can be produced by introducing up to 25% rutile in a glaze.
Glazes containg 15% or more of rutile, or rutile and titania,
are subjec to phototrophy after firing. Phototrophy is the ability
to change colour by the absorption of light [photons]. Such glazes
darken with light and grow pale with the lack of it. This reversible
proceess can continue indefintely.
Rutile containing over 25% iron oxide, as FeO [iron II oxide,
black iron oxide], is usually called by another mineral name, ilmenite,
and this ore has a colour response more akin to iron oxide instead of
titania.
-------
I hope this makes some sense to you. Til later. BFN.
Tom Buck ) tel: 905-389-2339
& snailmail: 373 East 43rd St. Hamilton ON L8T 3E1 Canada
(westend Lake Ontario, province of Ontario, Canada).
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