|Co-efficient of Linear Expansion
|Frit Softening Point
-Titania by itself is quite refractory. But when other oxides are present in the melt TiO2 becomes much more complex: because it opacifies, variegates, and crystallizes glazes. It also modifies existing colors from metals like Cr, Mn, Fe, Co, Ni, Cu.
-In amounts below 1% titania can dissolve completely in a glaze melt. In slightly greater amounts it can give a bluish-white flush to transparent glazes (depending on their amount of alumina).
-Above 2% it begins to significantly alter the glaze surface and light reflectance properties through the creation of minute crystals. This crystal mechanism gives soft colors and pleasant opacity, and breaks up and mottles the surface. In the 2-6% range, it increasingly variegates the glaze surface. Many potters add titania to their glazes or paint on overglaze titania washes for this purpose.
-As amounts increase above 5% the opacity and matteness accelerates. As much as 25% can be absorbed by some lead glazes. Up to 0.8 molar can be used to effect crystal melts in glossy glazes.
-Although titania will form a glass by itself, it is not highly soluble in silica melts. However, it is considered by some as a glass former in certain circumstances since it can stiffen the melt and stabilize the fired glass against leaching (i.e. it is used in lead frits to lessen the solubility of the lead).
-Titania can act as a modifier and within a narrow range it will combine with fluxes to make a glass. It can also act in a flux-like way in very high silica melts.
-Minute amounts (e.g. 0.1%) can be used to intensify and stabilize colors (i.e. iron can be altered to produce yellow and orange). In small amounts (e.g. 1%) it can alter and intensify existing color and opacity in a glaze.
-Titania can be reduced to produce colors in keeping with the elements present. If highly reduced it can yield a red, with iron the color could be yellow, brown or green. Other combinations can yield blues, greens, yellows. Titania is oxygen-hungry and will quickly oxidize from its reduced state if given the chance.
-Glazes containing titania are phototropic and can change color slightly by the action of light. They can also be thermotropic in that they can change color (i.e. toward yellow) when heated.
-Titanium frits are used to opacify and whiten engobes in the production of ceramic tile.
-Some have chosen to treat TiO2 as an 'inert' with respect to the chemistry of the glaze. However, a phase diagram of Al2O3 and TiO2 shows a eutectic at 80% Al2O3 at 1705C demonstrating that TiO2 does 'react' with the second most important ceramic oxide.
-TiO2 is considered an impurity in ball clays and kaolins used to make porcelain because it can react with any iron present to form rutile crystals which detrimentally affect body color and tranlucency.
All common traditional ceramic base glazes are made from only a dozen elements (plus oxygen). Materials decompose when glazes melt, sourcing these elements in oxide form. The kiln builds the glaze from these, it does not care what material sources what oxide (assuming, of course, that all materials do melt or dissolve completely into the melt to release those oxides). Each of these oxides contributes specific properties to the glass. So, you can look at a formula and make a good prediction of the properties of the fired glaze. And know what specific oxide to increase or decrease to move a property in a given direction (e.g. melting behavior, hardness, durability, thermal expansion, color, gloss, crystallization). And know about how they interact (affecting each other). This is powerful. And it is simpler than looking at glazes as recipes of hundreds of different materials (each sources multiple oxides so adjusting it affects multiple properties).
A raw TiO2-containing mineral used in ceramics to color and variegate glaze surfaces.
A super white powder used in ceramic glazes to variegate, opacify and moderate color.
|In high fire matte glazes, iron oxide and titanium can produce red colors.
|TiO2 is used to control the way crystals grow in classic crsytalline glazes.
|Additions of 5-10% titanium to many types of glazes produces yellow and light tan coloration, the surface effect is usually crystalline in nature. Lead greatly enhances the yellow at low temperatures.
|Titania in significant amounts (+5%) will almost always contribute to a glaze opacity of mottled and variegated character.
|Titania is a classic addition to produce mottled and variegated effects in all sorts of glazes. The more you use the greater the effect (up to 10%).
|By Tony Hansen
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