Architects memo no. 105: October 2011
little red hiding good
Anyone who has applied an oil-based varnish to timber
knows that it goes on as a, more or less, transparent amber
liquid and stays that way throughout the drying process
until the solid film is formed. Waterborne acrylic varnishes
behave very differently however. These materials, both
in the can and on application, are distinctly white milky
liquids which, initially, obliterate significant amounts of
the timber grain. On drying, this ‘wet’ opacity is lost and
a clear film results.
The reason for this is straightforward* but the purpose of
the example is to illustrate the differences between ‘wet’
and ‘dry’ hiding. This difference still exists when pigments
are added to these varnishes to make paints. An oil-based,
solventborne white enamel will behave in the same way
the varnish does, that is, there will be little change to
opacity during the drying process. This is pretty reassuring
when one is painting as, if one’s application looks pretty
good in the wet state, one can be pretty sure that it will
look fine when it dries.
An equivalent paint based on a waterborne system,
however, contains the same ‘temporary’ hiding that our
acrylic varnish has which, again, is lost on drying. This
phenomenon (plus some almost inevitable flocculation
of the white pigment) leads to a significant loss in hiding
during the drying process. Application which seems to
be satisfactory from a hiding perspective can disappoint
when the job dries.
The same phenomenon affects colour depth between wet
and dry. Again, with solventborne, oil-based enamels, what
you see is, pretty much, what you get. With waterborne
systems, however, that transient ‘milkiness’ present in the
wet resin always makes the wet colour look paler than
when the film dries fully.
There is an exception to the above and this is when paints
are specifically designed to have air voids in the dried
paint film. Air voids, just like the foam on a glass of D.B.
Export Dry, can have an opacifying effect. Typically, air
voids reduce the strength of the film so this strategy is
mostly used in ceiling paints. As the air voids are full of
water during the wet state, they do not contribute to any
‘wet’ hiding. However, as the film dries, water evaporates
out of these voids and extra ‘dry’ hiding develops as air
fills these voids. Any colour shade also ‘lightens’ during
the drying of such paints.
This leads me, tenuously I’ll admit, to the relative hiding
power of colours.
Let us firstly look at the widespread notion that white
paints, tinted to pale yellow shades, actually lose hiding power. We have investigated such propositions
(complaints) thousands (well hundreds) of times and have
never been able to confirm this. Our test is to apply paints,
at a standard film thickness, over a black and white test
card and measure any differences over the two areas.
Although your scribe, bending over backwards with head
almost touching floor, could argue a possible logic to the
anecdotal stories, on balance they are probably best filed
under ‘Urban Legend’.
What is absolutely certain, however, is that certain
coloured pigments have inherently lower hiding power
than others. Lead chromates can produce brilliant shades
of strong, bright yellow with hiding power as good as, or
even better than, a good quality white. Lead chromate
can be matched for shade by, for example, Hansa yellow.
The obliteration, however, is abysmal. Apply it over a
pure white background and the colour will look stunning
– apply it over a black background and the colour will
appear, well, black!
The hiding power of pigments is determined by two
factors which influence how the pigments interact with
light. The first of these factors is particle size, which can
be manipulated by the pigment manufacturer but the
second factor, refractive index, is absolutely inherent in
the pigment and cannot be altered. If the refractive index
of the pigment is close to that of the binder, light passes
through it as if it were not there. Massively increasing
the levels of such a pigment will have little effect on the
There is another property of pigments, which affects the
depth and opacity of colour. This is tinting strength. Tinting
strength means the amount that the pigment can be
‘diluted’ with other, high hiding pigments with little loss of
their colour purity. Phthalocyanine blue is such a pigment
which, while quite transparent in itself, can produce deep
hues even in the presence of significant amounts of high
hiding, titanium dioxide.
While the search for new brilliant, high opacity
chromophores continues, it would be fair to paraphrase
the old ‘Speights’ ad “She’s a hard road finding the perfect
colour pigment but!”
*Acrylic emulsions are suspensions of sub-micron, plastic particles
suspended in water. The refractive index of the particle is higher than
that of the water it is suspended in and hence interferes with light passing
through it – appearing milky. As the water disappears via drying, the
‘difference’ then disappears and the acrylic achieves its own transparency.
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