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romancing the tone

Architect's memo 55: December 1998

Through its close links with the architectural profession, Resene is very aware of the importance of colour in the realisation of architects' concepts. Indeed, in many instances the durability of the concept's aesthetic appeal is at one with the durability of the colour scheme chosen.

Much has been written on the durability of colours and most of the basic information has been included in previous Architects Memos. However, our clients keep coming back with the question, "How long will the colour last?"

There is no precise answer to this question because no-one knows exactly what future weather patterns will be, however, certain things are predictable. For example there are some pigments that are absolutely lightfast (principally the oxide colours and black) and the appearance of colours based on these pigments (or mixtures of them) will only be affected by film erosion (or chalking).

Chalking is influenced by the inherent durability of the binder and the initial gloss level - onset of chalking will be a lot slower for a glossy, 100% acrylic than for a low sheen styrene/acrylic. Chalking can be virtually prevented by the use of overglazes with a corresponding increase in the long term appearance of colours based on the aforementioned pigments.

These pigments, however, are generally quite dull and use must be made of a range of organic pigments in order to achieve the full palette of light, bright and bold colours demanded by today's market. Some of these organic pigments (in the blue-green range) approach the oxides for durability, while many (particularly in the red, orange and yellow range) fall somewhat short. What then becomes crucial is the mixture of pigments chosen to produce a colour. A coat of Resene Bright Red will contain about 100 layers of particles of Pigment Red 112, a medium durability red.

As the top layer of colour is destroyed by the UV light the next row of particles comes to the fore without any changes in hue. In the case of a pink using, say, 5% of Red 112 and 95% Titanium Dioxide; the balance of red to white in the top layer of pigment changes dramatically as the red is destroyed, favouring the Titanium Dioxide. This results in an unacceptable fading. It is worth noting that if one cut through the top faded layer, the original pink would be restored.

Similarly if one made a purple by blending a Pigment Red 12 with a durable Phthalocyanine Blue, preferential destruction of the red element would see the changing of the desired purple to an unacceptable blue.

Wherever possible, therefore, colours should be made of blends of pigments of similar lightfastness. Better colour stability is achieved by a blend of two medium lightfast pigments than a blend of pigments with medium and high lightfast. Unfortunately this ideal is not always possible and many colour combinations will contain one weaker link.

This link is, however, predictable and, as a tool to assist specifiers, one can provide colours with this weaker link deliberately reduced. This technique can emulate what the colour would look like at some future date when all, or some, of that particular tint may have degraded.

It must be stressed that overglazes used to prevent chalking are typically transparent to UV light and will not protect sensitive pigments underneath them. There is also evidence to suggest that solventborne glazes can, in certain cases, 'sensitise' sensitive pigments even more and hasten this early demise. Ultra violet absorbers can be included in clear glazes which will afford some additional protection to sensitive pigments. The protection is in direct proportion to the level to which these expensive materials are added. S.P. factors for this approach are not known but are theoretically possible.

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The Resene architect's memo section provides technical information on a variety of topics relating to paints, finishes and coatings.

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