Titanium is another of those materials that is both complex and not yet fully understood.
Its most common form is TiO2, with a valency of +4. But it is also found with valencies of 3 (Ti2O3), 2 (TiO) and -1.
One would think that TiO2 is a glass former, as this is the same form as SiO2. But there's more to it than that in deciding what is a glass former, and Ti fails on a number of the rules people have come up with to determine glass formers:
- Goldschmidt says that the ratio of diameter of the metal ion to the oxygen ion must be in the range of 0.2 to 0.4, to allow tetrahedra to form, whereas for Ti4+ it is 0.6, increasing to 0.8 for Ti3+
- Stanworth says that the electronegativity must be in the range of 1.90 - 2.20, whereas Ti is 1.54
- Sun gives the strength of the oxygen bond as the criterion, with bond strengths >80kcal/mol being glass formers, <60 network formers, and 60-80 as intermediates. For titanium, the bond strength is 73. This was extended by Rawson and then Boubata to include the melting point and the specific heat capacity, but titanium still fails the test
Although it cannot form a glass on its own, when mixed with something like potassium a glass will be formed, even though neither potassium nor titanium are glass formers.
For additions of up to 10% molar of TiO2, the Ti replaces Si in the glaze matrix. Above tht (at least to 12%, the limit of the experiments), TiO5 polyhedra are formed. This increases the melting point, stiffness and hardness, but makes the glaze more brittle and prone to cracking.
In a borosilicate glaze, in at least some instances, adding Ti significantly hardens and toughens the glaze.
Non-crystaline TiO2 is pretty well transparent. Whilst about 8-10% by weight can be dissolved into the glaze at high temperature, on cooling this drops to 5%. The anatase form of TiO2 is an excellent opacifier, except that at above about 850°C it changes to rutile. Rutile crystals then start growing, and they absorb UV light very close to the visible band, giving a cream colour.
Small amounts of Ti (<2% molar) can have strong effects on the colour. This is most noticeable in reduction and in some boron and phosphor glazes, where the TiO2 reduces to Ti2O3, giving violet-purple-blue colours.
Titanium can have a strong effect on the behaviour of other colourants. In the case of iron, it is mostly present as Fe3+, taking the role of an almost colourless network modifier, with a small amount acting as a glass former, giving the amber yellow colour. Adding titanium enables more of the iron to act as a glass former, deepening the colour.
It has the same effect on copper, moving the Cu2+ from network modifier to glass former, and shifting the colour from blue to green or brown. And on uranium, moving U6+ from modifier to former, the uranyl changing to uranate, and creating strong yellow colours.
When used with small amounts of manganese, the colour shifts from a weak yellow to a strong amber, or orange if cerium is also present.
Small amounts of Ti (3 - 5% by weight from one source, upwards of 2.5% molar from another) will act as a seed, encouraging other materials to crystallise. Note that in many cases the crystals would normally just form on the surface, whereas in the presence of a seed the crystals form throughout the thickness of the glaze.
Adding 10% or more will result in the formation of long, thin rutile crystals.
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