The white tiled surfaces of the sail-like shells are one of the great architectural triumphs of the Opera House. The glistening forms respond lyrically to the changing Sydney sun; the intricate arrangement of matt and gloss tiles helps to define the curves of the roof and create a living surface, the whiteness of which contrasts with the darker colours of the surrounding city.
Utzon had been inspired by the domes of Islamic mosques, and tiles he had seen in China and Japan, which he described in the Red Book as 'the homelands of the art of ceramics'. Equally, he was influenced by the durability and weather resistance of glazed tiles in the extreme Australian climate. Utzon was unable to find a standard product that met his requirements so he set out to produce a special tile that met his very personal brief: a tile that, when applied over the huge areas of the Opera House shells, would 'produce colour, surface texture and pattern required by me'.
Utzon had worked before with the Swedish firm of Hoganas, located some 20km from Halsingborg, and they were the natural choice to develop the tile. They experimented with a range of materials, finishes and shapes, and built a full-scale mock up of a corner of a shell which showed that the most appropriate layout on the double-curved structure was a square tile laid diagonally. Tests revealed that the normal pressed tile gave a very dull effect, while extruded tiles gave a livelier reflection. Despite the fact that Hoganas spent nearly two years developing the tiles, SOHEC insisted, for political reasons, that the contract should go out to tender. Utzon was relieved when Hoganas's price came in cheaper than the two local companies who were also invited to submit.
Utzon alighted on a tile with a white transparent glaze and an uneven texture for the main areas. The texture provided a diffused and softer reflection of the sun than the hard image created by a standard glaze finish.