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  • Writer's pictureH. Procter

Can spectrophotometry help you?

The world of the tanner is complicated, no more so than when the roles in the tannery can only be done by a very talented few (or in some cases, the one technician). Colour matching is often one of these roles. Having taught colour matching for many years, I have experienced every excuse in the book as to why the trainee feels that they will never be able to effectively colour match. The most common trait amongst trainees that prevents them from becoming an effective colour matcher is confidence. The difficulty that this brings to the table is that colour matching by eye is a subjective experience. Ask a 6-year old to comment on whether a microwave is better than an oven (a subjective assessment) and you will not be surprised that, due to their lack of experience and their lack of confidence about cooking, they will not really being able to comment. Colour assessment is one of those skills that requires experience and confidence to improve.

Colour characterisation, prediction, and formulation rely on the following skills:

· colour detection

· the ability to break hues apart into their components

· to be able to assess the fraction of hue present

· to translate the assessment into a formulation based on the dyes present (and their quantity)

· to understand how the colour of the substrate changes in relation to several variables (like pH, moisture content, surface loading or penetration).

If you reflect on these skills, you will immediately see that experience is the biggest help for all these skills. But, like a 19-year old trying to apply for a job, lack-of-experience can only be remedied by – experience.

So, a tannery should start somewhere and the age-old apprenticeship is one of the best ways to tackle a colour training programme. I would always practice shade characterisation first. Knowledge of how additive colour is achieved is the first step – the construction of the classical colour wheel using some of the tannery dyes (in stock) is a good start. Trainees need to have the knowledge that primary (single hue) colours, blend into secondary to create an addition colour consisting of a combination of the primary shades and the proportion of the primary blends determines how close to either of the primaries the result will be.

Leading on from the primary and secondary blending, the trainee then needs to acquire a knowledge on how the introduction of the third primary colour will result in the occurrence of subtractive colour matching. Subtractive colour matching can only be done once a learner is comfortable with additive colour matching. When we begin to use subtractive colouring, it is because we are trying to ‘kill’ a shade and the ‘kill’ is achieved by the addition of a colour on the opposite side of the colour circle, see Figure 1. As can be seen from the diagram, for transparent colourants the primary colours are paired with a secondary colour (on the opposite side), in other words if we want to reduce the redness of a leather we add a touch of green.

Figure 1. The standard colour matching ‘triangle’ or ‘circle’ representing colour space. The primary-secondary pairs for transparent colourants are shown.

During the process of learning about additive and subtractive colour matching the student gets to learn two very important lessons: tinctorial strength (the ability of a small amount of a dye to colourise the water); and particularly the tinctorial influence primaries have versus secondary colours. A small amount of primary will always change the colour faster than a secondary will. Tinctorial purity will also be an influential factor when shading – a blended dye will more than likely be a secondary colour, which will influence the chroma (dirtiness) of the colour. A higher chroma means cleaner colour. In Figure 2, you can see how I have taken a single dye colour and from #A to #B - all I did was change the hue (I added yellow – note how the lightness changed and the colour went slightly dirtier, not much). Then from #B to #C I added the third primary, blue. All three primaries will give a tertiary colour and the resulting colour will move to the centre (where all the dirty colours are). Shading is all about learning: How do I get different colours? How do I keep them clean? A resulting colour can never be purer than the starting colours.

For textiles, this is where we stop teaching shading. 2-Dimensional materials (the third dimension is negligible in cloth), are not greatly influenced by the penetration of the colourant into the inside of the material. However, leather is a whole different story. Penetration of the colourant away from the eye results in two massive changes:

1. The colourant is less saturated and the chroma is reduced (can be fixed by Top Dyeing)

2. Movement of the dye away from the surface results in chromatographic separation of the dye components (if a blended dye), so the hue can shift.

Now, to address the role of objective colour matching, why not let the human do it? Firstly, because humans make bigger mistakes. Secondly, because we want to resolve arguments independently/accurately. Ever had this argument: “It’s correct!”, “No – it’s wrong!”. Yip, the 6-year old circular disagreement between customer and tanner is more common than you think. It is much easier to look at two sets of empirical numbers (like those given in Figure 2) and easily see the presence of a difference or not. Colour difference equations can also take it to a new level.

Figure 2. The influence of primary addition on the colour co-ordinates. A. Starting colour (a pure red) B. Starting red with a little yellow C. Secondary orange, from #B with a little blue.

The use of metrics (numbers) means that a learner can see what the old colour matcher is talking about and can learn about the additive effects first, through watching the hue change. Subtractive effects can then be watched as the student sees how destructive dye shade ‘bombing’ (Yes – I have seen those dye formulations with five or more dyes, some of those dyes are added only to counter the mistakes in the selection of first three).

Are computers always correct? Of course, not. Can they predict dye additions (like humans) – given time and ‘training’, yes (like humans). Colour spectrophotometers will always have a few advantages over humans:

· Detector differences can be measured and minimised (standardised)

· They never get colour fatigue

· They are metrics - so prevent childish arguments

· Human colour matchers change companies, retire, or sometimes die

· Do not need good light to operate in

· Once trained, do not need more pay

They will not be able to easily replace a colour matcher, but they will go quite far, especially in a company that is lacking experience.

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