Architects memo no. 110: February 2013
50 shades of !!!
Our last memo introduced a little pigment technology
and touched on some of the reasons why certain pigment
chemistries are inherently more durable than others. In
this memo I thought that we could look at how these
chemistries affect mixtures of these pigments, as found
in the vast majority of paint colours offered.
It is fair to say that in mixtures typically pigments
don’t interact with each other per se (in the sense that
there are beneficial or negative chemical reactions)
but that each sits within the film; each interacting
with its immediate environment – predominantly solar
radiation. It is differences in rates of these interactions
which creates the illusion of pigment incompatibility.
Before shedding some light on the above by way of
examples, let us re-state a basic rule: “because paint
pigments work by absorbing the majority of the visible
spectrum and only by reflecting in one specific colour
wavelength, colours achieved by pigment mixtures will
always be duller (less saturated) than the same colour
achieved by a single pigment (chromophore).
This is not necessarily a bad thing, as subtle complex
colours are created by such pigment mixtures, but it is a
golden rule nonetheless.
Every paint manufacturer’s tinting system will contain
as their main (and often only) green, a pigment based
on phthalocyanine chemistry. As long as the pigment is
purchased from a reputable supplier, such tinters should
be uniformly durable. Let us have a look at a target
colour which is a bright yellow/green, the main pigment
of which will be phthalo green. The desired shade may be reached by either (a) adding a significant amount
of yellow iron oxide (yellow ochre), (b) adding a small
amount of the mixed metal oxide bismuth vanadate, or
(c) adding a small amount of organic yellow PY74.
It is actually unlikely that the (a) option would obtain the
necessary brightness but, if the colour were accepted,
prolonged exposure would see the super durable
yellow iron oxide prevail, and the colour would become
yellower as the phthalo green eventually faded. Option
(b) would achieve the necessary brightness and, because
of the excellent durability of the mixed metal oxide, the
shade will be maintained for many years. Rotorua could
provide the exception – but that is another story!
Option (c) is the tricky one. In interior situations
this relatively cheap organic pigment will produce a
beautiful, durable colour, but, put outside, the yellow
will fade within the year, resulting in a return to the
basic, phthalo green shade.
There are many, much more obvious examples – greens
and bordeauxs that turn blue, pinks that turn white,
the list goes on. The golden technical rule is only blend
pigments of like durabilities. The major dilemma is –
how is the layman or (laywoman) supposed to know?
Frankly, I have no idea. I believe that it helps to know
that such problems can and do exist and that a simple
tonal colour match that seems to look the same tells
one nothing about covering power or durability of the
shade produced. My best advice is stick with what you
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