Architects memo no.
55: December 1998
romancing the tone
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
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
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
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
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.