|Monthly Tech-Tip |
Alternate Names: TiO2
TiO2 occurs in many silicates in nature, accounting for over 1% of the earth's crust. Thus it is manufactured using a variety of materials and processes. Titanium dioxide power is very fine-grained and it agglomerates, so glazes containing it need to be sieved to break down the small lumps (even a high speed propeller mixer often won't do it).
Although titanium is the strongest white pigment known for many uses, in ceramics the whiteness (and opacity) it imparts to glazes is due to its tendency to crystallize during cooling. While titanium dioxide is used in glazes as an opacifier, it is not as effective and easy-to-use as tin oxide or zircon. It can be used as an additive to enliven (variegate, crystallize) the color and texture of glazes by introducing crystallization. Rutile works in a similar manner, typically both become saturated in the melt beyond about 5-6%, producing a dry and unstable glaze surface. In moderate amounts it encourages strong melts, durable surfaces and rich visual textures.
Titanium is available both as raw and surface treated products. Non-pigmentary grades flow more freely in the dry state. Self opacified enamels are made by adding titanium during smelting to super saturation. Upon firing the enamel, the titanium crystallizes or precipitates to produce the opacity. Titania is also used in dry process enameling on cast iron appliances for its effect on acid resistance, color and texture. In glass, non-pigmentary titanium dioxide increases refractive index, intensifies color.
These are porcelain and stoneware mugs. The glaze is G2571D (based on the fritted G2571B version of G2571A). This project started with calculations to source boron from a frit (instead of Gerstley Borate), MgO from talc and a frit (instead of dolomite). These moves enabled eliminating raw silica from the recipe. This produced a finer silky texture and better melt fluidity (for hosting colors). Adding rutile and zircopax produced a great bamboo. But what about variegating using titanium instead of rutile (it contains no iron so colors should be brighter)? Look what happens with this red stain! The titanium has done it's job even a little too well (creating a slightly more abrasive surface). Cobalt and titanium also worked. Imagine if other stains would produce dramatic results like this, each with the amount of titanium tuned to it!
The rustic and variegated effect have been achieved. However, notice that the drips from drainage during glazing are visible. This is because this type of variegation is produced by variations in the glaze thickness. These can be intended, like the thinning that occurs on the edge of contours, but also unintended (like drips and runs).
The body is Plainsman M390. Firing is the cone 6 PLC6DS schedule. Each horizontal row is a commonly-used base glaze. The top one is an MgO matte, the next one down is a calcium matte, row 3 is G2926B glossy and row 4 is Ravenscrag Slip+frit. The two mattes behave very differently from each other with the additions of opacifier. Thickly applying an opacified glaze will obviously affect visual character (column 4). Tin oxide fires whiter than zircon (e.g. Zircopax). If you like the G2934 recipe, consider the G2934Y variant for better melting.
The glaze is G1214Z cone 6 base calcium matte. 5% titanium dioxide has been added. This Plainsman M390 tile was fired at cone 6 using the PLC6DS firing schedule. Titanium can create reactive glazes, like rutile, even with matte surfaces (provided the glaze has good melt fluidity). Calcium mattes host crystallization and work particularly well. Because titanium dioxide does not contain iron oxide lighter colors and better blues are possible than with rutile. Like rutile, the effects are dependent on the cooling rate of the firing, faster cools produce less reactivity.
This is a lithium glaze fired at cone 6. It has 6% titanium. This effect will also work in other types of transparent recipes. There should be more blue with slower cooling. Mixing some rutile in (e.g. 4 titanium, 2 rutile) should enhance also. 6% is pushing the edge of how much titanium should be in a recipe. Any more, or cooling too slow, could transform the surface into a mass of white crystals (which would be rough and non-functional). It is best to manually program firings, the up and down schedules? That would make the result consistent no matter how heavily or lightly loaded a kiln is.
The base glaze (inside and out) is GA6-D Alberta Slip glaze fired at cone 6 on a buff stoneware. However on the outside the dried glaze was over-sprayed with a very thin layer of titanium. The dramatic effect is a real testament to the variegating power of TiO2. An advantage of this technique is the source: Titanium dioxide. It is a more consistent source of TiO2 than the often-troublesome rutile.
|Oxides||TiO2 - Titanium Dioxide, Titania|
Titanium dioxide at Wikipedia
Interesting deposit in South America
Glaze opacity refers to the degree to which it is opaque. There is more than meets to eye to the subject of opacity control.
Opacifiers are powders that turn transparent glazes opaque by various chemical and physical mechanisms (and combinations of mechanisms).
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Titanium Dioxide Toxicology
|Frit Softening Point||1830C|
|Density (Specific Gravity)||4.26|
|Glaze Crystallization||TiO2 dissolves into the melt during firing but normally re-crystallizes (or acts as a crystallization catalyst) during cooling (with rutile structure).|
|Glaze Matteness||Titanium can be used in glazes to produce a matte surface with increasing amounts of crystallization in amounts up to 25%. The effect works in most stoneware glazes and is better when the glaze is slow cooled.|
|Glaze Opacifier||Titanium is a crystalline mineral and encourages crystal development during cooling and freezing of the glaze melt. This generally produces opacity. However, titanium opacified glazes have a much different character than zircon or tin types. The latter produces a much more even and bright white coloration. When used as an opacifier the batch amount can range to 10% or more of the recipe.|
|Glaze Variegation||Smaller amounts of titanium dioxide (i.e. 5%) added to colored or opacified recipes can variegate the surface and make it more interesting (e.g. it alters the shape of crystals, shade of colors).|