Colour Management - Part Four

Ever since the beginning of mankind human being knows how to use the colouring material to decorate surroundings or to attract the other person or as a defending mechanism. Cave walls were painted with mineral colouring material, such as  iron ore by pre-historic men and women use to paint their bodies. Colours were also used as protection mechanism from wild animals. Dyed  flax fibres have been found in the Republic of Georgia dated back in a prehistoric cave to 36,000 BC. Archaeological evidence shows that, particularly in India and Phoenicia, dyeing has been extensively carried out  for over 5000 years. The dyes were obtained from animal, vegetable, or of mineral origin, with no or very little processing. By far the greatest source of dyes has been from the plants, notably roots, berries, bark, leaves, and wood, but only a few have ever been used on a commercial scale.

Figure 1 - Painted cave and a woman with make-up 

When light falls on any object some wavelengths of light are reflected and some are absorbed, and in some cases transmitted, depending on the nature of the object. The colour of the object is the result  of selective absorption and emitting of wavelengths. This  new reflected or transmitted light spectrum creates the appearance of a colour. Ultramarine reflects blue light, and  absorbs other colours. This mechanism is dependent on the chemical structure and chemical bonds in the object material. This process differs from fluorescence, phosphorescence, and other forms of luminescence, in which the material emits light. 

Many materials selectively absorb certain wavelengths of light. Materials that humans have chosen and developed for use as pigments usually have special properties that make them ideal for colouring other materials. A pigment must have a high tinting strength relative to the materials  it  is applied on. It must be stable in solid  form at ambient temperatures. For industrial applications, as well as in the arts, permanence and stability are desirable properties. Pigments that are not permanent are called fugitive. Fugitive pigments fade over time, or with exposure to light, while some eventually become dark.

 

 

Figure 2  -  Light refletion from pigment (left) and dye (right)

Pigments are used for colouring paint, ink, plastic, fabric, metal, cosmetics, food,  and other materials. Most pigments used in manufacturing and the visual arts are dry colorants ground into a fine powder. This powder is added to a vehicle (or binder), a relatively neutral or colourless material that suspends the pigment and gives the pigment its adhesion properties. 

Pigment is insoluble in the vehicle and is suspended in it whereas a dye, which either is itself a liquid or is soluble in its vehicle, resulting in a solution. A colorant can be both a pigment and  a  dye depending  on  the  vehicle it is used  in. In some  cases, a pigment can be manufactured from a dye by precipitating a soluble dye with a metallic salt. The resulting pigment is called a lake pigment. In contrast with a dye, a pigment generally is insoluble, and has no affinity for the substrate. 

The appearance of pigments or dye is intimately connected to the colour of the source light under which it is viewed, see part three. Sunlight has a high colour temperature, and a fairly uniform spectrum, and is considered a standard for white light. Artificial light sources tend to have great peaks in some parts of their spectrum, and deep valleys in others. Viewed under these  conditions, pigments or dyes appear to have different colours. Colour systems to represent colours must specify the light source used to view or measure colours. 

Other properties of a colour, such as its saturation or lightness, may be determined by the other substances that accompany pigments or dyes. Binders and fillers added to pure pigment chemicals also have their own reflection and absorption patterns, which can affect the final colour. Likewise, in pigment / binder mixtures, individual rays of light may not encounter pigment molecules, and may be reflected as is. These stray rays of source light contribute to the saturation of the colour. Pure pigment allows very little white light to escape, producing a highly saturated colour. A small quantity of pigment mixed with a lot of white binder will appear pale in colour. 

Pigments

Pigments and dyes are the most important components of printing inks formulation. Pigments are molecular crystalline structures produced to an optimum particle and size distribution. When applied in a vehicle to a substrate remains on the surface as well as fill the voids in substrate. There are hundreds of different types of pigments produced. Some are found in nature in mineral or vegetable form, but many are synthetic produced from a variety of chemicals. A simple classification would be organic and inorganic pigments. From the large variety of pigments a very few are suitable for the printing inks due to special demands of printing processes. Many are not economical to produce, some cannot give required tinctorial strength, particle size, cost, and specific combination of properties for printing applications. 

Pigments are grouped into colours and within the same colour group various pigments have different properties and are used for different printing processes or applications. These properties are derived from the chemical structure of the pigments. For printing applications these pigments are of 140 types and can be grouped according to spectrum colours as follows. 

Pigment                               Number of Pigments

Red                                                   49

Orange                                             12 

Yellow                                               30 

Green                                                4 

Blue                                                  13 

Violet                                                11 

Brown                                                6 

Black                                                  5 

White                                               10

 

Dyes

Dyes are mainly used for liquid inks, coatings, and lacquers. Special disperse dyes are used in both liquid and paste inks for heat transfer printing. Dyes are also extensively used in ink-jet digital printing inks. Out of eighteen verities of dyes the suitable ones for the printing inks are acid dyes, basic dyes, disperse dyes, and solvent dyes. 

Acid dyes are anionic soluble in water and organic solvents and some are soluble in alcohol, ketones, and esters. They give very bright hues and light fastness varies from poor to very good. Acid dyes are used in security inks, double tone, and invisible inks. 

Basic dyes were produced from coal tar and are cationic in nature. These give brilliant shades and high tinctorial strength, however its light fastness is very poor. These are soluble in water and alcohol but low solubility in organic solvents. 

Solvent dyes are soluble in organic solvents. These give extremely bright colours with fair light fastness. Disperse dyes are mainly used in heat transfer printing where it is transferred by sublimation process.

 

Dyes            Acid Dyes          Basic Dyes        Solvent Dyes     Disperse Dyes

Red                  9                          5                          1                           2 

Orange            5                         3                            -                           - 

Yellow            12                        5                            2                          1 

Green               4                          2                           -                          - 

Blue                11                       10                          1                          1 

Violet                5                          5                           -                           - 

Brown              9                         1                            -                           - 

Black                3                           -                          2                           - 

White               -                            -                           -                            - 

 

Other ingredients in printing inks that can affect the colour are : Oils, resins, solvents, plasticisers, waxes, driers, additives. 

Some of these ingredients affect the colours directly whereas others affect the colour as a result of its functional properties. 

The details on printing inks presented in this article are to build background information for ‘colour management’ as many factors not visible to the printer result in failure of its implementation. Special pigments, such as fluorescentt, phosphorous, luminescence, etc. pigments will be discussed in future articles.