If you buy a tube of paint, you actually don't know what is in it. There might well be the correct pigment in it but has
it been adulterated with something and the other ingredients are covering it up? Maybe you need a paint with a pigment that
for some reason costs what you consider to be too much money or maybe you can't get that paint at all. These are all reasons
why you might want to make your own paint.
We'll cover:
Note, this is broken up into a number of pages so that it is easier to navigate on smaller devices. If you want to see it all in one page,
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Introduction
First of all, what does a paint need to do? A paint is applied to a surface so that it can modify the way that the
surface reflects light. In order to achieve that, we conventionally have a chemical compound of some sort that interacts
with light effectively glued to the surface. The conventional name for the compound that reacts with light is a 'pigment' and the
glue is called the 'medium' (because it holds the pigment to the layers underneath the paint - in effect, it is in the middle,
hence the name).
Ideally, the paint should have a life span that is appropriate for the purpose therefore the pigment and the medium should
both be stable against light and the chemical and physical environment that they find themselves in. Ideally, they shouldn't
react chemically with each other and additionally, the different pigments in the painting shouldn't react with each other
either, or if they do, they should be protected from each other by the medium or an additional layer that is painted
between them like a protective 'varnish'. Just consider just how long paintings are kept for.
So, what makes a good pigment? Essentially, all pigments are largely transparent. Even titanium white is - if you put it
into a liquid with the same refractive index (the amount by which it slows down light) then light will not be bend when it travels
into a grain of titanium dioxide or again on the way out therefore it will not reflect off the interfaces between the pigment and
the liquid. However, each pigment slows down light to a different extent and some chemicals even do it differently according to
the orientation of the crystal lattice and the angle of polarisation of the light.
Basically, the greater difference between the refractive index of the pigment and that of the medium, the more opaque the
pigment. If a pigment's refractive index is fairly close to that of the medium it will be fairly transparent.
What makes a good medium? The medium should:
- be transparent;
- be colourless;
- be dimensionally stable - not shrink or expand with changing humidity or other environmental circumstances;
- be able to wet the pigment;
- not react with the pigments;
- keep the pigment distributed evenly throughout it during storage;
- stick to the surface you are painting and to previous layers of itself;
- protect the pigment from the environment;
- set solid but with a bit of remaining flexibility;
- have a solvent that makes it mobile enough to paint with (or some other temporary way of making it mobile
such as heat in encaustic painting).
With water colour paints, the medium is usually acacia gum (gum arabic) to which a little honey is added as
a humectant - the honey allows better mixing with water and when the paint dries, being fructose, it sets solid.
With oils, linseed oil is the best as it has a high concentration of the oils that react with air to polymerise
which forms a flexible plastic that holds the pigment well. There are other so-called 'drying oils' such as walnut
oil, safflower oil and so on but they are not as strong.
Additionally, avoid oils that are washed with caustic soda or other alkalis because doing so starts off
something called saponification which essentially breaks down the oil to make a soap which will make it more susceptible
to reacting with some pigments. (You can make soap by heating an oil such as olive oil with a strong alkali such as
caustic soda (NaOH) solution and it will break down the oil into the sodium salt of the long-chain fatty acid and glycerol -
the former being what we call 'soap' but be careful if you want to have a go at this by making sure that all of the caustic
soda has been removed as it can cause serious burns. Essentially, don't do it.)
What makes a good solvent?The solvent should:
- be able to dissolve in the medium (or the pigment);
- not chemically react with the pigment (or the medium);
- make the viscosity of the resulting solution low enough to be easy to paint with;
- only contain volatile compounds - if there is something else in there, you will be adding that to your paint and that is
not a good thing. Some turpentines have non-volatile compounds in them and these will prolong drying times as they need to
oxidise themselves. Ideally, you should be able to add turpentine and if you add too much, you can let it evaporate to the
correct level. You can perform a basic test on a turpentine sample by putting a few drops onto a non-absorbent surface and
letting the volatiles evaporate - what is left ends up in your paint but ideally, there should be nothing left. If you evaporate
a few drops of double distilled English turpentine like this, there will be nothing left;
- Either be harmless or smell. A solvent that isn't harmless that is not particularly smelly can do you damage without you
knowing about it. For instance, if solvent fumes were at an elevated level and you could smell them, you would know that you
should open a window or take similar action to reduce the solvent you were breathing in. However, if you couldn't smell it,
you could be doing yourself harm without knowing about it until, potentially, damage could have been done.
So, ideally, you should be using just water for watercolours or using double distilled English turpentine for oils.
All images and original artwork Copyright ©2019 Paul Alan Grosse.