THE COLOURING OF TITANIUM: AN OPTICAL APPEARANCE
In nature, metals are not coloured, with the only exceptions of gold and copper; all the other sixty or so metals are more or less light shades of grey. When their surface appears coloured, this is due to the presence of a thin transparent patina capable of giving rise to the phenomenon of light interference that generates the different colours.
These thin films can be obtained by deposition from the gas phase, by corrosion, by reaction with other elements, particularly sulphur and iodine, and above all, and this is what interests us, by oxidation at high temperature.
This is what happens to the cooking surface of Tritania pans and pots when heated in the air.
When the heating temperature is low, the oxide film on the surface of the titanium is almost transparent and therefore impossible to detect by the naked eye. When the temperature increases, the oxide film on the pot gradually thickens and interferes with light. Thus, to our eyes, the inner surface will show different shades of colour, bluish or iridescent. For example, like the iridescent colours of soap bubbles or oil stains on wet asphalt.
Research into the colouring of titanium tells us that after heating a titanium surface for half an hour at 200° C, its colour is silver, at 400° it turns yellow, at 500° blue and at 600° red.
THE ADVANTAGES OF OXIDISING TITANIUM
This chemical-physical phenomenon, called 'interference colouring', has no harmful effect. On the contrary, the oxide layer protects the titanium surface and greatly increases its resistance to corrosion by food, thus further improving its healthiness and performance (nonstickiness).
This particular colouring also proves that the cooking surface of TRITANIA cookware is really made of pure titanium.
It is therefore not necessary to remove the oxide film from your pot, and indeed it would be better not to do so, for the reasons we have explained.
However, if you really don't like this colour effect aesthetically, you can rub the surface of your pot vigorously with an abrasive sponge and a cream detergent until it is removed.
It should be remembered that the titanium surface does not lose its properties in any way, not even with a rather vigorous washing. The small scratches that may appear are absolutely superficial (more of an optical effect than real scratches), because the titanium surface is not scratched in any way and cannot be removed , as happens with non-stick coatings.
IF YOU WANT TO KNOW MORE
From, Pietro Pedeferri, Titaniocromia, Interlinea Edizioni, 1999, pages 321-34
But let's see how the interference staining mechanism works. We are helped by Pietro Pedeferri, an Italian chemical engineer who has taught Electrochemistry and Corrosion and Material Protection at the Politecnico di Milano.
When we observe a titanium surface covered by an oxide film, our eye is hit by two superimposed light waves: one reflected from the upper face of the film in contact with the atmosphere and the other from the lower face in contact with the metal.
The second light wave crosses the dioxide film twice more than the first one and reaches our eye later.
This causes the elimination of a colour component and therefore the appearance of the complementary colour, which ultimately depends on the thickness of the film.
The oxide film formed on the firing surface by heating in air has nothing to do with titanium dioxide (chemical formula TiO2).
Titanium dioxide is a chemical compound in the form of a colourless crystalline powder, tending to white, used as a colouring or matting agent in paints, plastics, construction cement (E171), as well as in cosmetics (identified by Colour Index CI77891), and as a sun filter, identified by INCI: TITANIUM DIOXIDE.
We will talk about this in another article in this blog.
The colours of titanium
These are the colours that titanium can assume due to optical interference phenomena, depending on the thickness of the oxide film.
It was Leopoldo Nobili, an Italian physicist and inventor of 'metallocromy', who first studied the phenomenon of titanium colouring by oxidation in 1827; he identified a scale of 44 different colours that he called 'electro-chemical appearances'. This colour scale follows a natural order, as it also appears in nature in the rainbow and in the colours assumed by clouds at dawn and dusk.
Leopoldo Nobili, Memorie e osservazioni edite e inedite, David Passigli, FiRENZE, 1834.
After identifying the principles underlying the achievement of colour, Pedeferri noted that titanium can retain the memory of the colours it has taken on. The result was an intense activity of experimentation of the expressive potential of metallocromy in which science and art have been intertwined.
Works on display at the Pietro Pedeferri Exhibition. Titaniocromie. Between science and poetry the light of colours, Villa Monastero in Varenna, 2010.