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The Eternal Pigment

  When the ancient Egyptians came to decorate the works of art that they buried with their Pharaohs, they used a blue pigment of such brilliance that it would enhance the quality and value of any object. The pigment was derived from the semi precious mineral, Lapis Lazuli, and it continued to be used for many centuries.

  The quality of the blue produced by this method was never in question. But the scarce availability and difficulty of extracting the pigment meant that the cost was high and the pigment could only be used in the most prestigious applications. It was clear that a rich prize awaited anyone who could devise a synthetic route to manufacture this pigment, which is now known as Ultramarine Blue.

  While Ultramarine continued to be used both in artists’ colours and laundry products, it was the birth of the plastics industry which stimulated the development of the Ultramarine pigment that we know today. Ultramarine had been perfect for use in artists’colours due to its unique shade with unrivalled brightness and its excellent lightfastness. It was non-toxic and non-irritant, ideal for an application like laundry with its inevitable skin contact. But add to these virtues its excellent heat stability, an obvious advantage in a high temperature application such as plastics, and it is no wonder that Ultramarine soon became regarded as an indispensable plastics colourant.

  But how does Ultramarine come to possess such a combination of admirable properties?

  The answer is in its production process – which is itself a fascinating story.

  The chromophore responsible for the blue colour in Ultramarine is based on sulphur and has the formula S3-. It is strange to think of a yellow chemical like sulphur giving a blue colour, but in this particular form it does.

  However, the discovery of the chromophore was only one step in the development of a synthetic process to manufacture Ultramarine. The problem is that the chromophore is unstable and is readily oxidised to non-pigmentary sulphate.

  The chromophore, therefore, needs to be protected from these chemical reactions and this is achieved by stabilising it within the protective cage structure of a clay (kaolin) lattice.

  The production process involves heating a mixture of sulphur, clay and sodium carbonate to a temperature of almost 800oC in a kiln, excluding air to prevent premature oxidation of the sulphur before the reaction temperature is reached. After sufficient residence time at this peak temperature to form the initial chromophore, the mixture is allowed to cool before air is slowly allowed into the kiln to complete the reaction with an oxidation stage. This high temperature process takes more than two weeks to complete.

  With this knowledge of the production process we can already see the reason for some of the desirable qualities of Ultramarine. The chromophore is very unusual, leading to a unique shade which cannot be matched by other pigments. Being transparent, Ultramarine adds an attractive blue undertone to transparent polymers as well as neutralising unwanted yellowness in opaque white plastics. The high temperature manufacturing process provides the Ultramarine pigment with an inbuilt stability to heat. In fact, Ultramarine blue pigments are stable to more than 350oC , high enough for use in all common plastics. Finally, the raw materials are all quite benign - no heavy metals are used in the production of Ultramarine and organic compounds would not survive the high manufacturing temperature. Ultramarine is one of the safest known pigments by virtue of its raw materials and production process.

  This high temperature process produces the ‘raw’ Ultramarine, but the process is far from complete. After this ‘dry’ process it is necessary to refine the raw Ultramarine, a process which starts by slurrying the raw Ultramarine with water. This second stage is commonly called the ‘wet’ process. In this stage the impurities present within the raw Ultramarine are removed. The most important of these is unreacted sulphur, which would otherwise give rise to an odour when used in high temperature applications like plastics. Soluble material, primarily sodium sulphate, is also removed.

  Finally the pure Ultramarine pigment is ground to its final particle size then separated from the water. The grinding process leaves a broad spread of particle sizes with a range from less than 1 up

  to 3 microns. By a process known as classification, these particles are separated into a number of discrete fractions. Each fraction is dried to produce a fine powder.

  Why is a fine particle size so important?

  The properties of an Ultramarine pigment depend primarily on its particle size.

  Finer particles are stronger in tinting power, they are brighter and also greener in undertone than the coarser particles produced at the same time. Ultramarines intended for technical applications range from less than 1 to 3 microns, Ultramarines coarser than this are normally confined to low quality applications such as laundry powders (see Figure 1). The correct product can be selected from the particle size range available based on tint strength, undertone and brightness.

  Once again knowledge of the Ultramarine manufacturing process provides another indication of the final property of the Ultramarine. Pigment particles of 1 to 3 microns are relatively large and as one might expect, Ultramarine disperses very easily.

  Ultramarine has been the blue pigment of choice for thousands of years. Despite more choice in the blue area of the spectrum the properties which made Ultramarine popular so long ago are still valued just as much today. And probably thousands of years in the future our descendants will still be marvelling at the brilliant colour of Ultramarine, the eternal pigment.


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